Tascam M-___ Story...
- Thread starter sweetbeats
- Start date
sweetbeats
Reel deep thoughts...
I like certain kinds of puzzles. Sometimes helping somebody find the piece of gear for their situation is a fun kind of puzzle. Fstrat76's epic "Need new smaller mixer" thread has caused me, on numerous occasions since he started that thread, to search and research what is and has been available in the area of mixing consoles. I keep coming up dry...nothing that exactly seems to fit the bill...always some sort of compromise to be made at least in my estimation relative to his specific parameters (such as number of inputs, number of outputs, inline monitoring and switching features, footprint size, age, price, and not least of all sonic performance). Overall it seems there are more vintage offerings that meet more of the parameters than something new or more recent, and that means a greater likelihood of having to "go through" the mixer...repair, clean, upgrade/replace caps, etc...unless somebody else has done that already. Anyway, in my inability to find fstrat the perfect mixer, I've found myself pondering what those parameters are for me? If I could have any mixer (caveats being any mixer for which I had the technical and financial ability to maintain, that would fit in my space and that I could reasonably afford to purchase), what would it be and what would be the determining features/characteristics identifying that mixer?
I keep coming back to the M-__.
Not because it meets all those identified parameters. No.
It doesn't have the preferred number of input modules...and bear with me, some of these missed marks are just going to sound silly...the overall build is not up to par with the big boys (i.e. the PC boards are phenolic resin instead of glass fiber, the chassis support for the main board on the modules is non-existent and the existing module chassis build is thinner gauge electro-galvanized formed steel, the push buttons are non-illuminated...for instance...), line-level inputs/outputs are are unbalanced, the audio power rails are standard +/-15V, and I'm not jazzed about the EQ.
But I keep coming back to the M-__ because, by this point, quite simply we're old friends. We know each other pretty well...I simply like it, and that compels me to look past the facets that fall short of my wish-list. Yes...there are and have to be enough of those parameters met for the relationship to work...it's a good sounding mixer, is very unique and has a lot of very cool features...so feature-wise it hits a critical mass, but really what it comes down to is I subjectively and intrinsically like it, and we're old friends. I've invested a lot in the relationship, and though it's inanimate I feel like it's come through for me...it has drawn other human friends into the project that have helped me get it working and figure it out...so by its very unique nature it sort of HAS invested in the relationship through the help and involvement of others. So I'll figure out how to work around the points that fall short and compromise for the sake of the friendship.
And then I can't help thinking that's a pretty good standard for person-to-person relationships too.
I keep coming back to the M-__.
Not because it meets all those identified parameters. No.
It doesn't have the preferred number of input modules...and bear with me, some of these missed marks are just going to sound silly...the overall build is not up to par with the big boys (i.e. the PC boards are phenolic resin instead of glass fiber, the chassis support for the main board on the modules is non-existent and the existing module chassis build is thinner gauge electro-galvanized formed steel, the push buttons are non-illuminated...for instance...), line-level inputs/outputs are are unbalanced, the audio power rails are standard +/-15V, and I'm not jazzed about the EQ.
But I keep coming back to the M-__ because, by this point, quite simply we're old friends. We know each other pretty well...I simply like it, and that compels me to look past the facets that fall short of my wish-list. Yes...there are and have to be enough of those parameters met for the relationship to work...it's a good sounding mixer, is very unique and has a lot of very cool features...so feature-wise it hits a critical mass, but really what it comes down to is I subjectively and intrinsically like it, and we're old friends. I've invested a lot in the relationship, and though it's inanimate I feel like it's come through for me...it has drawn other human friends into the project that have helped me get it working and figure it out...so by its very unique nature it sort of HAS invested in the relationship through the help and involvement of others. So I'll figure out how to work around the points that fall short and compromise for the sake of the friendship.
And then I can't help thinking that's a pretty good standard for person-to-person relationships too.
sweetbeats
Reel deep thoughts...
Thaaaaaat's better. 
sweetbeats
Reel deep thoughts...
Mounted the power supply back in the rack tonight. It's symbolic of the power supply being done. I honestly don't remember when the last time it was the power supply was assembled and in the rack...I was reviewing some notes today and I've gone 10 rounds with that thing it seems, thanks almost entirely to some indecision on my part, some faulty install technique, a number of mistaken assumptions, and I think some issues with my diagnostic equipment. All this is to say it really wasn't ever a problem with the power supply per se.
Tore apart the Control Module (master section) for the umpteenth time tonight to chase down why one channel of the Control Room buss wasn't making it to the headphone amp. Long story short I'm not certain what the problem was, but it is working now. I suspect there's a weak solder joint in there or maybe there was a stray piece of solder or something. The good news is if it comes back it was pretty easy for me to identify where the Control Room buss branches to the headphones and to the Control Room out jacks, and then inspect and test that section from the branch to the headphone source select switch. If/when the problem comes back I'll be able to find the source...there are no components in that section of the circuit except for a resistor in each channel, but there are a number of jumpers and a small buss PCB so the issue is in there somewhere. But it is working and working well...and I identified a few other things I want to address while it's opened up. So, it's good.
You ever find yourself just listening to the same song over and over again for a half hour just because you're listening to it through your mixer? Playing with all the ways you can get the signal from the inputs to your monitors? Lots of ways to do this on this mixer.
Tore apart the Control Module (master section) for the umpteenth time tonight to chase down why one channel of the Control Room buss wasn't making it to the headphone amp. Long story short I'm not certain what the problem was, but it is working now. I suspect there's a weak solder joint in there or maybe there was a stray piece of solder or something. The good news is if it comes back it was pretty easy for me to identify where the Control Room buss branches to the headphones and to the Control Room out jacks, and then inspect and test that section from the branch to the headphone source select switch. If/when the problem comes back I'll be able to find the source...there are no components in that section of the circuit except for a resistor in each channel, but there are a number of jumpers and a small buss PCB so the issue is in there somewhere. But it is working and working well...and I identified a few other things I want to address while it's opened up. So, it's good.
You ever find yourself just listening to the same song over and over again for a half hour just because you're listening to it through your mixer? Playing with all the ways you can get the signal from the inputs to your monitors? Lots of ways to do this on this mixer.
sweetbeats
Reel deep thoughts...
So last night I finished all the upgrades/modifications/repairs to the Control Module I've had on my list for a loooooong time, and temporarily reassembled it to test it. Everything is working except for the 64Hz tone setting in the oscillator...I had to dig down into the way-back machine to refresh my memory on this one as I've dealt with it before...I found the solution to the last time I had this issue back in post #338. I sent the Control Module home with evm1024 and he had it on his bench for a bit and found the issue had to do with the feedback loop cap C3 on the oscillator driver. He replaced the cap and it worked. I'm not sure if he used a new part or a spare used part or what, but I'm so thankful he took the time to identify the problem...its the first place I'm going to look. Other than that one little problem absolutely every feature and function of the control module is working, and working well. Its a joy to have everything else working without popping/fizzing/noise/intermittence...its the first time ever since I've had the console this is the case. The cool thing is with the modifications, and on its own merit, its sounding good, particularly the critical STEREO buss output...and particularly-particularly the balanced outs which use the ultra-high headroom semi-discrete +/-35V powered line amp. Like, its distinctly different than the other outputs, which sound great, but the balanced stereo out sounds particularly...beefy...quick and powerful and dynamic. Its neat.
So I've got to apply some DeoxIT to a couple level setting switches for the "EXTRA IN" inputs, and resolve the oscillator issue, but then the Control Module gets fully reassembled and put back in the frame and hopefully stays there for a long while, and then I'll turn to implementing mods to the I/O modules and re-testing the functions of those as I go.
In the wee hours last night I captured a 16 minute video detailing the changes to the Control Module. I mainly did it as a quick way to document what's been done, what's what, etc. before putting the module back together, because I guarantee I'll forget what's been done and later down the road will need to know...it was quicker to capture a short video than try and write it all down...probably should write it down too...and draw up a schematic of the power supply that includes all the changes there too...and...and..and...
Anyway, here's the video if anybody is interested. Please keep in mind it was really late, I was a bit tired, and its pretty dry subject matter but I think it should be part of this thread:
YouTube
So I've got to apply some DeoxIT to a couple level setting switches for the "EXTRA IN" inputs, and resolve the oscillator issue, but then the Control Module gets fully reassembled and put back in the frame and hopefully stays there for a long while, and then I'll turn to implementing mods to the I/O modules and re-testing the functions of those as I go.
In the wee hours last night I captured a 16 minute video detailing the changes to the Control Module. I mainly did it as a quick way to document what's been done, what's what, etc. before putting the module back together, because I guarantee I'll forget what's been done and later down the road will need to know...it was quicker to capture a short video than try and write it all down...probably should write it down too...and draw up a schematic of the power supply that includes all the changes there too...and...and..and...
Anyway, here's the video if anybody is interested. Please keep in mind it was really late, I was a bit tired, and its pretty dry subject matter but I think it should be part of this thread:
YouTube
sweetbeats
Reel deep thoughts...
Interesting...working on the oscillator issue (64Hz tone not working)...
So when evm1024 replaced the 4,700pF cap C3 (oscillator interstage driver feedback cap), he put in a used part. First of all I noticed the solder lands were kinda roached. I removed the cap, exposed some new trace on each land for better continuity...measured the cap evm1024 installed and it measured about 4,900pF...put everything back together and it still wasn't working but it was *better*. And if I placed my finger on the tails of the cap it *would* work. I thought "hm...needs more capacitance?"
I strapped a 390pF cap in parallel with C3, and put everything back together and...now the 64Hz, 400Hz and 1kHz tones weren't working. More is not better.
So C3 is the feedback cap for output #1 of the dual opamp U3, the first amp stage...there is another 4,700pF cap in place for the feedback cap C5 of output #2, the second stage. Out of curiosity I pulled that off the board and measured that. Well...! 8,600pF. And there is one more identical part C1 which straps the non-inverting input of the first stage to ground...pulled that and measured and sure enough it *also* is measuring 8,600pF.
Bad mylar caps.
Now digging through stuff to see if I can find three 4,700pF mylar caps.
So when evm1024 replaced the 4,700pF cap C3 (oscillator interstage driver feedback cap), he put in a used part. First of all I noticed the solder lands were kinda roached. I removed the cap, exposed some new trace on each land for better continuity...measured the cap evm1024 installed and it measured about 4,900pF...put everything back together and it still wasn't working but it was *better*. And if I placed my finger on the tails of the cap it *would* work. I thought "hm...needs more capacitance?"
I strapped a 390pF cap in parallel with C3, and put everything back together and...now the 64Hz, 400Hz and 1kHz tones weren't working. More is not better.
So C3 is the feedback cap for output #1 of the dual opamp U3, the first amp stage...there is another 4,700pF cap in place for the feedback cap C5 of output #2, the second stage. Out of curiosity I pulled that off the board and measured that. Well...! 8,600pF. And there is one more identical part C1 which straps the non-inverting input of the first stage to ground...pulled that and measured and sure enough it *also* is measuring 8,600pF.
Bad mylar caps.
Now digging through stuff to see if I can find three 4,700pF mylar caps.
sweetbeats
Reel deep thoughts...
Well I found some. I think the parts are supposed to be 5% tolerance. The parts I found are about 7.5% greater than spec. Don't know if that's the cause of my trouble or not, as with those replacement parts at C1, C3 and C5 everything is working except for the 6.4kHz tone. Go figure.
sweetbeats
Reel deep thoughts...
Okay this is interesting...more clues...
Before when the 64Hz tone wasn't working I could force it to work by putting my finger across the lands of any of the feedback caps or the resistors...now with the 6.4kHz tone not working I can still force it to work but only by putting my finger across one or two of the resistors in the network...not sure which one or ones yet, but here's the other thing...it starts working just fine after the console has been powered up about 10 minutes...!
So, it works, it just takes awhile to get going. This should help in figuring out what's causing the problem.
Before when the 64Hz tone wasn't working I could force it to work by putting my finger across the lands of any of the feedback caps or the resistors...now with the 6.4kHz tone not working I can still force it to work but only by putting my finger across one or two of the resistors in the network...not sure which one or ones yet, but here's the other thing...it starts working just fine after the console has been powered up about 10 minutes...!
So, it works, it just takes awhile to get going. This should help in figuring out what's causing the problem.
sweetbeats
Reel deep thoughts...
Worked on the oscillator issue again this eve...no solutions yet but the problem is consistent...I captured a video of the issue...might help it make more sense:
[url=https://www.youtube.com/attribution_link?a=jw9pC_ARHNCreSJ3&u=/watch%3Fv%3Dc8QSMcKkx-s%26feature%3Dem-upload_owner]YouTube[/URL]
ALSO, I've conjectured for a long time Teac built in the potential for the each I/O Module to have a balanced line output, and maybe a balanced line input too. Well, with a fairly good understanding now of how the ultra-high headroom line amp works, and taking another look at the buss amp PCB, all the vacant component sites, the vacant holes in the jack panel chassis, and being able to study the balanced output driver in the Control Module, it is for certain the M-__'s group and aux buss outputs could be balanced, and it actually wouldn't be that hard to accomplish! More on that later.
[url=https://www.youtube.com/attribution_link?a=jw9pC_ARHNCreSJ3&u=/watch%3Fv%3Dc8QSMcKkx-s%26feature%3Dem-upload_owner]YouTube[/URL]
ALSO, I've conjectured for a long time Teac built in the potential for the each I/O Module to have a balanced line output, and maybe a balanced line input too. Well, with a fairly good understanding now of how the ultra-high headroom line amp works, and taking another look at the buss amp PCB, all the vacant component sites, the vacant holes in the jack panel chassis, and being able to study the balanced output driver in the Control Module, it is for certain the M-__'s group and aux buss outputs could be balanced, and it actually wouldn't be that hard to accomplish! More on that later.
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bachelorb
Cowboy Chord Virtuoso
Glad to see you working on the beast again. I sure know what you mean about life/ burnout. I’ve forced myself in the room a couple of times here, but nothing stuck. ....but I’m playing the guitar more and more, so maybe my burnout is just seasonal ......
Alright..... I’m off to watch the video.... maybe I’ll learn sumpin.......
...... Alright..... I’m off to watch the video.... maybe I’ll learn sumpin.......
sweetbeats
Reel deep thoughts...
Yeah...I've been thinking more about it, and it actually would be easy to set it up like the M-50 or the M-500 consoles where the balance amp is independent...so any unbalanced -10dBV output can be patched into the input of balance amp and then that signal gets balanced and the level boosted, only on the M-50/M-500 consoles those independent balance amp sections are over in the master section of the jack panel (but ANY outputs can be patched to their inputs). The M-50 has only 2 balance amp channels but they are +/-35V powered by the same semi-discrete line amp that's in the M-__; ultra high headroom, and up to +8dBu nominal output level. The M-500 consoles' balance amp sections feature 8 channels, but driven by a more garden-variety +/-18V powered opamp-based line amp. Still higher headroom and +4dBu nominal output level, and a really really neat feature, but not the same animal as the M-50 or M-__ line amp. On the M-__ there is potential for 12 of these ultra high headroom balance amp channels, one on each I/O module. So that would make it so, with a small RCA -> RCA patch cable I could, for instance, have my choice of level bumping and balancing the pre-fade/EQ output, direct out or buss out on each module...like a mini patchbay on the back of each module...or of course with longer patch cables patching in ANY -10dBV unbalanced output.
Here is the jack panel of the I/O Module...the XLR output jack would mount just to the left of the MIC in jack, and I would use one of the several parallel RCA output jacks and designate that for the balance amp input:
Here's a YouTube video that goes into a little more detail:
YouTube
Here is the jack panel of the I/O Module...the XLR output jack would mount just to the left of the MIC in jack, and I would use one of the several parallel RCA output jacks and designate that for the balance amp input:
Here's a YouTube video that goes into a little more detail:
YouTube
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sweetbeats
Reel deep thoughts...
So this thread has been quiet, but that's only because I've had my head buried pretty deep in the project with a number of separate things going on at once, and its been a bit overwhelming to be posting about each thing. Read below to see why...
Oscillator
I'm still working on this. Its still not 100% and I've been getting some expert help on an email listserv I joined. I have some things to try in the works but can't try them until I finish getting an order put together from Mouser, and that's hanging on several other facets at the moment. So for the time being only two bands of the oscillator are working reliably all the time. Once things warm up *sometimes* some or all of the other bands work. It’s a noise issue. We usually try to minimize noise. The quadrature oscillator on the M-__ (same type as is used on the M-50 and M-500 series) relies on noise in the circuit to kick-start the oscillation and mine is either too quiet for some reason. There are ways to add noise, but the goal here is to see if I can get it working reliably without having to modify the circuit. Yes, there is always he question “well did it ever work? It is, after all, a prototype…”, but this a known circuit design (the quadrature type oscillator), and is nearly identical to the production mixers that followed the M-__, so I choose to believe there is just something wrong with mine irrespective of the design.
Control Room Output
While working on the oscillator issue the Control Room outputs have gone wonky. The switchable output level is not working right, there's a lot of DC coming off the outputs, the "dim" attenuation function is not working and the output quality is (intermittently) distorted or super quiet. I've tried a few coarse tactics to resolve the issue that haven't worked. I have the problem isolated to areas on two of the four boards in the master section, but its a (for me) complex circuit to troubleshoot because it includes 4000 series CMSO logic, FET analog switching arrays, and resistor arrays among the usual stuff. At this point in order to help others help me I'm in the midst of reverse-engineering the circuit and drawing the schematic...which is a challenge for me. And to make matters worse, my bench supply lost one side during testing, so I’m now having to try and figure that problem out before I can continue testing.
Output Balance Amp
So I've mentioned it appears the Buss Amp boards in the I/O modules were designed to provide for balanced line level outputs if the boards were populated with the necessary components. As assembled only one side of the high headroom line amp is there providing for unbalanced output. The traces, lands and screening are all there for the other side of the amp to make it balanced...and the chassis was pre-punched for the XLR jack...the dress panel was not. But it can be done. I posted the project to the email listserv I mentioned above, and when folks looked at the schematic (since the circuit is identical to the balance amp in the M-50 mixing console), one fella correctly deemed the amp worthless for driving low impedance balanced inputs as are commonly found on vintage pro tape machines like my Ampex MM-1000 I'm planning on feeding with the Tascam M-__. The greater than +28dBu high headroom capability is there, but there will be unacceptable LF distortion levels when driving low impedance loads especially transformer coupled, and he thoughtfully explained why. “Crap” thought I…Then the same list member who was pointing this out offered to partner with me to modify the circuit so it could work well with low impedance loads. He said “It can be made to be more than adequate, even superior…” So I've got a couple spare balance amp boards from a parted-out M-50, and I'm going to strip the caps, resistors and output transistors off of it, replace the output transistors with Darlington parts, recap, replace the resistors with 1% metal film parts, and make some adjustments to the values of some of the resistors as directed. Then there's an initial test to fine-tune the bias resistors so the DC offset is as close to null as possible (doing this with trimmer resistors...put a heavy load on the amp, and measure the DC offset while adjusting the trimmers, then remove power and measure the final resistance value...replace trimmers with fixed value parts). This will negate the need for output coupling caps. Once that's done the amp will need to undergo distortion analysis testing which will result in additional tweaks. Then a final parts list can be made and the existing stock unbalanced line amps on the I/O modules can be gutted and replaced with the parts for the upgraded/modified balanced line amp. I'm still not sure how I want to handle access to the amp, whether it is an independent balance amp like on the M-50/M-500 series consoles, or whether it is permanently tied to the buss out, or some other output...and furthermore there will be decisions that need to be made with level switching and whether I want to still be able to use the high-headroom circuit unbalanced as well.
Balanced Line Input
I remarked in an earlier post that, in addition to the balanced line out capability, it appeared the I/O modules also were designed with balanced line input capability (additional unpopulated component sites on the buss amp board). Well, I verified each buss amp board contains traces and lands for TWO balanced inputs...it is a 4-resistor differential balance amp circuit. I thought there might be additional clues to what actual parts (resistor and cap values, etc.) were intended to be used by doing what I've been able to do elsewhere with this project, which is to get technical documents for other period Teac products that appear functionally similar, like the PE-40 EQ is similar to the EQ on the M-__...and the line out amp is the same as the M-50. The master module switching logic is similar to the M-50/M-500 series, etc. So what about the balanced line input? I found pictures of the main PCB in the original LA-40 (mkI) that made me go "hmmm", so I ordered the service manual from Teac. Sure enough, though not identical, there is a similar balanced line input circuit in the LA-40 as is found on the buss amp board in the M-__...similar enough that I think I can use the values from it along with initial suggestions from folks on that other list, draw up the circuit with those values for some additional analysis and recommendations. And, like with the balanced line out amp discussed above, the chassis is pre-punched for another XLR jack for a balanced input...the dress panel is not...but that gets me at least part way there. So between the balanced line input capability and the balanced line out, each I/O module *could* be setup like a mini 1-channel LA-40 (an LA-10 on each I/O module? Or all together an LA-120? hahah...), but there would be 12 of them. If I set them up as independent circuits, they could be patched to ANY unbalanced line level outputs from any I/O module or even the Control Module. I think this is probably what I’m going to do. It would simplify a number of issues that would have to be addressed if the balance amps were permanently tied to existing I/O, and would actually be consistent with how Teac incorporated this idea into production units. And the line outs would be ultra-high headroom capable of maintaining very low distortion spec down to 20Hz even when driving transformer-coupled 600ohm loads at high levels (up to +28dBu). This is too cool not to "go there". The back of each module would have 10 RCA jacks and 3 XLR jacks. I'd need to reconfigure the purpose of some of the RCA jacks if I was going to have the balanced I/O amps be independent and patchable, but that is possible since there are some parallel RCA jacks I wouldn't need. So I'm researching who could either make new dress panels or punch and re-screen the existing panels with new labeling. This is a bigger long-term project that would ensue after I've finished repairing the Control Module, and getting all the I/O modules back together and testing everything to deem the M-__ 100% and "done" prior to embarking on this “phase 2” stuff. I’m still left wondering why there are *two* differential input balance amps…My current theory, since each module has one MIC and two LINE inputs, maybe Teac was considering the possibility of a module configured as a mono input like the existing ones, and maybe another as a line level stereo input module. Dunno.
EQ Section
I was originally not going to touch the EQ section in my list of I/O module changes/upgrades (such as replacing feedback resistors and changing out some of the opamps for critical amp stages), but I was having some fun the other night doing some experimenting and was A/B testing between a module with stock EQ, and one with some different opamps. There was a difference. Not earth-shattering mind you, but impossible to ignore. The bottom-end was more potent particularly on pedal tones…like there was more clarity and sustain for a given passage of music…the high-end had more detail to it, and overall the mid-range had more life and sass to it…overall when I’d switch back to the stock module after monitoring the modified module the stock module sounded lifeless and constrained. But when I did some DC offset testing I found some of the parts I’d normally like to use were maybe not happy where I was trying them out, so in order to get a better handle on what amp stages in the EQ section are important to the audio path, and what might work and what might not work in each of those stages as far as parts, the circuit needs to be reverse-engineered and the schematic drawn up. So that’s another project I’m working on in the periphery. This is easier than the Control Room buss circuit in the Control Module to reverse engineer since the PE-40 equalizer is a close cousin to the M-__ EQ, and I have the schematic for the PE-40, but its still a pretty complex circuit (for me)…one LPF, two HPF’s, and four EQ bands, swept hi and lo bands and two fully parametric mid bands…12 amps stages all together…anyway…
Power Supply
So there are two factors driving me to the point I believe I need to ditch the modified PS-520 power supply I’ve been using all these years: the +/-15V supply demand and the physical space needs of the +/-35V supply.
The issue with the +/-15V supply is that originally the PS-520 supply seemed perfectly adequate to power the M-__...the M-520 (which is what the PS-520 was designed to power) has a maximum theoretical load of about 1.4A. The M-__ in stock form has a maximum theoretical load of about 1.75A. Okay. That’s fine…not too far beyond the M-520. But now with opamp upgrades that 1.75A goes up to about 2.25A. I did some load testing on the transformer secondaries in the PS-520 that power the +/-15V rails, and, after studying the results, consulting with a friend and getting some education, in order to ensure the power supply has adequate recovery the main filter caps would need to be increased from the stock 4,700uF to about 33,000uF. That’s a lot. Its trying to squeeze too much out of a transformer that wasn’t designed to handle that kind of load. The power supply is so critical and we are talking about audio rails here…I’d like adequate horsepower driving the audio rails. The only way to solve that problem is more iron in the transformer, and we are talking about a transformer with multiple taps, some center-tapped and some not…a relatively complex transformer. So the easiest way to solve that problem is to look for a power supply with multiple rails that is designed to power something with a max theoretical load closer to what the potential load will be of the M-__ with the opamp upgrades. I want to try and keep it in the Tascam family just from an aesthetic standpoint, but anything I find on eBay (for instance) is at stupid prices. I looked at other options as well, like an Acopian supply or a collection of Power One modules. While I was grappling with this Miroslav rode in on a white horse and said “hey I’ll give you my spare M-3500 supply.”
Wow. See related thread here:
Maybe need a different power supply...
So I did some math and it looks like the max theoretical load of an M-3500 is greater than 2A, and the supply chassis is significantly larger and heavier than the PS-520…much of that increased mass is related to the transformer…more iron…so between the physical characteristics of the supply and the electrical characteristics of the load, I deemed the PS-3500 a near perfect solution, especially since Miroslav generously offered to just give it to me.
I will want to recap it, I need to look at the +/-15V supply for possible improvements in the noise floor with the regulator IC and the output transistors, and I’m also in the last day wondering about the possible wisdom of incorporating a soft-start circuit because the filter capacitance between the power supply and all the 100uF filter caps throughout the console is over 80,000uF. The other thing that will need to be done is dealing with the interface connector on the supply for the umbilical to the console. The PS-3500, like many other Teac/Tascam power supplies uses interconnects made by Hirose: Sumicon 1600 series parts. These are near impossible to find…long since discontinued. I’m confident that, with time and patience, I could find the parts. The PS-3500 uses a 28-pin connector, the 1628. I’d need a male cable mount connector and the associated hood. I did a quick search with no results. It wouldn’t be easy. Maybe 10 years ago I did a global search for 20-pin parts for my M-520 I had at the time. I finally tracked down some old stock stores at Canford UK. It wasn’t cheap with the international shipping, but I bought them out. They don’t happen to have any 28-pin parts. So, again, I’m sure with time and patience I could find what I need, but then I started thinking of other options, like DSUB connectors, and then it hit me: I’ve got quite a decent collection of vintage mil-spec Winchester and Cannon multipin circular and rectangular connectors thanks to a bunch of spare Ampex wiring harnesses. After a few minutes poking around in the shop I found parts that I’m confident will fit, and will do a *very* nice job providing interconnect long after I’m dead and gone. These connectors are extremely rugged and high quality…gold plated contacts, etc. So there’s that modification to do. And then there’s the +/-35V supply…
The PS-3500 chassis affords the space needed to add a separate power transformer for the +/-35V rails that are necessary to power the ultra-high-headroom line out amps discussed above. I bought a pre-fabricated precision regulated supply adjustable between +/-1.2V and +/-37V. Long story short, after much consultation I believe I’ve identified the right transformer, and have the other parts on the pre fab regulator assembly to drop in the separate supply rails into the PS-3500 chassis. There are many details to work out, like whether or not I pull the assembly apart and incorporate that into the main PCB in the PS-3500 chassis, or leave it on its own board, or something in between. I’d like to at least move the regulators on the pre fab assembly onto the large heatsink on the back of the PS-3500 chassis…but at least the PS-3500 chassis has the room inside for all this stuff. The PS-520 chassis was proving tight.
So anyway, lots of details to iron out, but thanks to Miro I’ve got the bones I need on the way. In the meantime the PS-520 will continue to be a good stop-gap supply until all those details can be worked out.
I’ll stop there for now.
Oscillator
I'm still working on this. Its still not 100% and I've been getting some expert help on an email listserv I joined. I have some things to try in the works but can't try them until I finish getting an order put together from Mouser, and that's hanging on several other facets at the moment. So for the time being only two bands of the oscillator are working reliably all the time. Once things warm up *sometimes* some or all of the other bands work. It’s a noise issue. We usually try to minimize noise. The quadrature oscillator on the M-__ (same type as is used on the M-50 and M-500 series) relies on noise in the circuit to kick-start the oscillation and mine is either too quiet for some reason. There are ways to add noise, but the goal here is to see if I can get it working reliably without having to modify the circuit. Yes, there is always he question “well did it ever work? It is, after all, a prototype…”, but this a known circuit design (the quadrature type oscillator), and is nearly identical to the production mixers that followed the M-__, so I choose to believe there is just something wrong with mine irrespective of the design.
Control Room Output
While working on the oscillator issue the Control Room outputs have gone wonky. The switchable output level is not working right, there's a lot of DC coming off the outputs, the "dim" attenuation function is not working and the output quality is (intermittently) distorted or super quiet. I've tried a few coarse tactics to resolve the issue that haven't worked. I have the problem isolated to areas on two of the four boards in the master section, but its a (for me) complex circuit to troubleshoot because it includes 4000 series CMSO logic, FET analog switching arrays, and resistor arrays among the usual stuff. At this point in order to help others help me I'm in the midst of reverse-engineering the circuit and drawing the schematic...which is a challenge for me. And to make matters worse, my bench supply lost one side during testing, so I’m now having to try and figure that problem out before I can continue testing.
Output Balance Amp
So I've mentioned it appears the Buss Amp boards in the I/O modules were designed to provide for balanced line level outputs if the boards were populated with the necessary components. As assembled only one side of the high headroom line amp is there providing for unbalanced output. The traces, lands and screening are all there for the other side of the amp to make it balanced...and the chassis was pre-punched for the XLR jack...the dress panel was not. But it can be done. I posted the project to the email listserv I mentioned above, and when folks looked at the schematic (since the circuit is identical to the balance amp in the M-50 mixing console), one fella correctly deemed the amp worthless for driving low impedance balanced inputs as are commonly found on vintage pro tape machines like my Ampex MM-1000 I'm planning on feeding with the Tascam M-__. The greater than +28dBu high headroom capability is there, but there will be unacceptable LF distortion levels when driving low impedance loads especially transformer coupled, and he thoughtfully explained why. “Crap” thought I…Then the same list member who was pointing this out offered to partner with me to modify the circuit so it could work well with low impedance loads. He said “It can be made to be more than adequate, even superior…” So I've got a couple spare balance amp boards from a parted-out M-50, and I'm going to strip the caps, resistors and output transistors off of it, replace the output transistors with Darlington parts, recap, replace the resistors with 1% metal film parts, and make some adjustments to the values of some of the resistors as directed. Then there's an initial test to fine-tune the bias resistors so the DC offset is as close to null as possible (doing this with trimmer resistors...put a heavy load on the amp, and measure the DC offset while adjusting the trimmers, then remove power and measure the final resistance value...replace trimmers with fixed value parts). This will negate the need for output coupling caps. Once that's done the amp will need to undergo distortion analysis testing which will result in additional tweaks. Then a final parts list can be made and the existing stock unbalanced line amps on the I/O modules can be gutted and replaced with the parts for the upgraded/modified balanced line amp. I'm still not sure how I want to handle access to the amp, whether it is an independent balance amp like on the M-50/M-500 series consoles, or whether it is permanently tied to the buss out, or some other output...and furthermore there will be decisions that need to be made with level switching and whether I want to still be able to use the high-headroom circuit unbalanced as well.
Balanced Line Input
I remarked in an earlier post that, in addition to the balanced line out capability, it appeared the I/O modules also were designed with balanced line input capability (additional unpopulated component sites on the buss amp board). Well, I verified each buss amp board contains traces and lands for TWO balanced inputs...it is a 4-resistor differential balance amp circuit. I thought there might be additional clues to what actual parts (resistor and cap values, etc.) were intended to be used by doing what I've been able to do elsewhere with this project, which is to get technical documents for other period Teac products that appear functionally similar, like the PE-40 EQ is similar to the EQ on the M-__...and the line out amp is the same as the M-50. The master module switching logic is similar to the M-50/M-500 series, etc. So what about the balanced line input? I found pictures of the main PCB in the original LA-40 (mkI) that made me go "hmmm", so I ordered the service manual from Teac. Sure enough, though not identical, there is a similar balanced line input circuit in the LA-40 as is found on the buss amp board in the M-__...similar enough that I think I can use the values from it along with initial suggestions from folks on that other list, draw up the circuit with those values for some additional analysis and recommendations. And, like with the balanced line out amp discussed above, the chassis is pre-punched for another XLR jack for a balanced input...the dress panel is not...but that gets me at least part way there. So between the balanced line input capability and the balanced line out, each I/O module *could* be setup like a mini 1-channel LA-40 (an LA-10 on each I/O module? Or all together an LA-120? hahah...), but there would be 12 of them. If I set them up as independent circuits, they could be patched to ANY unbalanced line level outputs from any I/O module or even the Control Module. I think this is probably what I’m going to do. It would simplify a number of issues that would have to be addressed if the balance amps were permanently tied to existing I/O, and would actually be consistent with how Teac incorporated this idea into production units. And the line outs would be ultra-high headroom capable of maintaining very low distortion spec down to 20Hz even when driving transformer-coupled 600ohm loads at high levels (up to +28dBu). This is too cool not to "go there". The back of each module would have 10 RCA jacks and 3 XLR jacks. I'd need to reconfigure the purpose of some of the RCA jacks if I was going to have the balanced I/O amps be independent and patchable, but that is possible since there are some parallel RCA jacks I wouldn't need. So I'm researching who could either make new dress panels or punch and re-screen the existing panels with new labeling. This is a bigger long-term project that would ensue after I've finished repairing the Control Module, and getting all the I/O modules back together and testing everything to deem the M-__ 100% and "done" prior to embarking on this “phase 2” stuff. I’m still left wondering why there are *two* differential input balance amps…My current theory, since each module has one MIC and two LINE inputs, maybe Teac was considering the possibility of a module configured as a mono input like the existing ones, and maybe another as a line level stereo input module. Dunno.
EQ Section
I was originally not going to touch the EQ section in my list of I/O module changes/upgrades (such as replacing feedback resistors and changing out some of the opamps for critical amp stages), but I was having some fun the other night doing some experimenting and was A/B testing between a module with stock EQ, and one with some different opamps. There was a difference. Not earth-shattering mind you, but impossible to ignore. The bottom-end was more potent particularly on pedal tones…like there was more clarity and sustain for a given passage of music…the high-end had more detail to it, and overall the mid-range had more life and sass to it…overall when I’d switch back to the stock module after monitoring the modified module the stock module sounded lifeless and constrained. But when I did some DC offset testing I found some of the parts I’d normally like to use were maybe not happy where I was trying them out, so in order to get a better handle on what amp stages in the EQ section are important to the audio path, and what might work and what might not work in each of those stages as far as parts, the circuit needs to be reverse-engineered and the schematic drawn up. So that’s another project I’m working on in the periphery. This is easier than the Control Room buss circuit in the Control Module to reverse engineer since the PE-40 equalizer is a close cousin to the M-__ EQ, and I have the schematic for the PE-40, but its still a pretty complex circuit (for me)…one LPF, two HPF’s, and four EQ bands, swept hi and lo bands and two fully parametric mid bands…12 amps stages all together…anyway…
Power Supply
So there are two factors driving me to the point I believe I need to ditch the modified PS-520 power supply I’ve been using all these years: the +/-15V supply demand and the physical space needs of the +/-35V supply.
The issue with the +/-15V supply is that originally the PS-520 supply seemed perfectly adequate to power the M-__...the M-520 (which is what the PS-520 was designed to power) has a maximum theoretical load of about 1.4A. The M-__ in stock form has a maximum theoretical load of about 1.75A. Okay. That’s fine…not too far beyond the M-520. But now with opamp upgrades that 1.75A goes up to about 2.25A. I did some load testing on the transformer secondaries in the PS-520 that power the +/-15V rails, and, after studying the results, consulting with a friend and getting some education, in order to ensure the power supply has adequate recovery the main filter caps would need to be increased from the stock 4,700uF to about 33,000uF. That’s a lot. Its trying to squeeze too much out of a transformer that wasn’t designed to handle that kind of load. The power supply is so critical and we are talking about audio rails here…I’d like adequate horsepower driving the audio rails. The only way to solve that problem is more iron in the transformer, and we are talking about a transformer with multiple taps, some center-tapped and some not…a relatively complex transformer. So the easiest way to solve that problem is to look for a power supply with multiple rails that is designed to power something with a max theoretical load closer to what the potential load will be of the M-__ with the opamp upgrades. I want to try and keep it in the Tascam family just from an aesthetic standpoint, but anything I find on eBay (for instance) is at stupid prices. I looked at other options as well, like an Acopian supply or a collection of Power One modules. While I was grappling with this Miroslav rode in on a white horse and said “hey I’ll give you my spare M-3500 supply.”
Wow. See related thread here:
Maybe need a different power supply...
So I did some math and it looks like the max theoretical load of an M-3500 is greater than 2A, and the supply chassis is significantly larger and heavier than the PS-520…much of that increased mass is related to the transformer…more iron…so between the physical characteristics of the supply and the electrical characteristics of the load, I deemed the PS-3500 a near perfect solution, especially since Miroslav generously offered to just give it to me.
I will want to recap it, I need to look at the +/-15V supply for possible improvements in the noise floor with the regulator IC and the output transistors, and I’m also in the last day wondering about the possible wisdom of incorporating a soft-start circuit because the filter capacitance between the power supply and all the 100uF filter caps throughout the console is over 80,000uF. The other thing that will need to be done is dealing with the interface connector on the supply for the umbilical to the console. The PS-3500, like many other Teac/Tascam power supplies uses interconnects made by Hirose: Sumicon 1600 series parts. These are near impossible to find…long since discontinued. I’m confident that, with time and patience, I could find the parts. The PS-3500 uses a 28-pin connector, the 1628. I’d need a male cable mount connector and the associated hood. I did a quick search with no results. It wouldn’t be easy. Maybe 10 years ago I did a global search for 20-pin parts for my M-520 I had at the time. I finally tracked down some old stock stores at Canford UK. It wasn’t cheap with the international shipping, but I bought them out. They don’t happen to have any 28-pin parts. So, again, I’m sure with time and patience I could find what I need, but then I started thinking of other options, like DSUB connectors, and then it hit me: I’ve got quite a decent collection of vintage mil-spec Winchester and Cannon multipin circular and rectangular connectors thanks to a bunch of spare Ampex wiring harnesses. After a few minutes poking around in the shop I found parts that I’m confident will fit, and will do a *very* nice job providing interconnect long after I’m dead and gone. These connectors are extremely rugged and high quality…gold plated contacts, etc. So there’s that modification to do. And then there’s the +/-35V supply…The PS-3500 chassis affords the space needed to add a separate power transformer for the +/-35V rails that are necessary to power the ultra-high-headroom line out amps discussed above. I bought a pre-fabricated precision regulated supply adjustable between +/-1.2V and +/-37V. Long story short, after much consultation I believe I’ve identified the right transformer, and have the other parts on the pre fab regulator assembly to drop in the separate supply rails into the PS-3500 chassis. There are many details to work out, like whether or not I pull the assembly apart and incorporate that into the main PCB in the PS-3500 chassis, or leave it on its own board, or something in between. I’d like to at least move the regulators on the pre fab assembly onto the large heatsink on the back of the PS-3500 chassis…but at least the PS-3500 chassis has the room inside for all this stuff. The PS-520 chassis was proving tight.
So anyway, lots of details to iron out, but thanks to Miro I’ve got the bones I need on the way. In the meantime the PS-520 will continue to be a good stop-gap supply until all those details can be worked out.
I’ll stop there for now.
sweetbeats
Reel deep thoughts...
That's a good question. "Oscillator" is another name for "tone generator"...its a thing that generates sinus tones via frequency and amplitude controlled oscillation, so its just the thingy that produces test tones for tape machine alignment or other setup tasks. Your Tascam M-106, M-108 or M-312 mixers don't have an onboard oscillator. Your smart phone does if you download any number of apps.
sweetbeats
Reel deep thoughts...
It depends on the mixing console what kinds of internal routing features there are, but normally an onboard oscillator can, at minimum, be dumped to the mix busses (the "slate") since that's normally what is hooked up to the tape machine inputs. There is also often an output jack for the oscillator so you can patch that to...well...anything. So on the M-__ I can use it internally to calibrate the meters, and if necessary use it to trace faults. There are 5 frequencies on the M-__ oscillator (5 "bands"): 64Hz, 400Hz, 1kHz, 6.4kHz, and 12kHz.
sweetbeats
Reel deep thoughts...
So the donated PS-3500 power supply from Miroslav arrived yesterday. Huge thanks to Miro again for doing that...I feel very fortunate. You took a lot of time to dig it out, make copies of documents, carefully pack and send it...and it wasn’t free to send, and something like that has value and you gave that value up to contribute to the M-__ project. Thanks again.
I had a chance to have a little meet and greet with PS-3500 last eve...it’s going to work out well. There’s plenty of room inside the chassis for the second power transformer I’ll need to add for the +/-35V rails, I think my Winchester multi-pin connector idea to use in lieu of the factory Hirose connector for the console umbilical interconnect is a very workable idea that will resolve the issue of the unobtanium cable-mount connector, and provide a better more robust interconnect. And last but far from least I load-tested the transformer taps that power the +/-15V rails, and they can handle significantly more load than the equivalent taps on the PS-520 transformer.
Here it is...it’s in good shape, and I’ve also got some ideas with cosmetics rolling around in my brain:
Now, regarding the umbilical interconnect issue, my first task was to hunt down the connectors I’d need to replace the panel-mount connector and cable-mount connector with hood. I found a panel-mount connector on a spare sync relay module off of an Ampex MM-1000. It took like an hour to extract it. MAN they don’t make stuff like that anymore...here’s the module and I’m pointing to the connector:
And this module had been left for dead decades ago. I got it with the pile of spares I got with my MM-1000, and this thing was stored in an outdoor shed with a leaky roof, but the attention to detail in the wire looming, and the quality components...I think stuff like this looks pretty:
But anyway I got it removed and because it’s such good quality stuff it appeared it was going to clean up well in spite of the long term improper storage and bad environment. Next step was to pull down my large box of spare MM-1000 wiring harness sections. I think I’ve got almost two complete spare wiring harnesses for the MM-1000, some of it NOS never installed. I opened the box up. How did I accumulate this stuff... It’s hard to tell the scale from the pic, but that's like 60-70lbs of wire, connectors and spare chassis panels...the spread out pile is about 6 feet wide:
Anyway...geez I’ve got more of those 26-pin Winchester connectors than I thought. Digging through all the wire I just kept finding them...
And there’s at least one more in there, and that’s in addition to the ones on the MM-1000 already.
Then it dawned on the me the actual block that houses the pins or sockets is the same block used for panel or cable mounting. So I’m probably not going to use the one I spent an hour extracting from the sync relay module. I’ll use some new ones from the pile. I need to track down a pin/socket extractor tool and purchase some new pins and sockets...working on identifying the right parts.
Anyway, it was time to see if my idea would actually work...if the hood of the 26-pin connector would fit in the recess in the back of the PS-3500 chassis...and it looks like it will fit perfectly!
Clearly the dog is not as excited as me.
Then came the most important part...the reason I pursued an alternate power supply...to find something with higher load capability than the PS-520 supply I’ve been using because, with all the opamp upgrades I’m going to be asking too much of the PS-520. Were my hunches about the PS-3500 correct? Does it have higher load capability? In a word, yes!
The way to do this test is to connect a resistor (the “load”) across the transformer outputs, apply power, measure the voltage, power the system off and measure the resistance. Knowing the voltage and the resistance I can solve for current. A lot of you know this already, but voltage divided by the resistance equals the current “Ohm’s Law”: V/R=I. Looking at the formula you can see that, for a given voltage, as the resistance drops, the current flow increases, or for a given resistance as the voltage increases the current flow increases. As you increase the load (decrease the resistance), there is increased voltage drop...it’s a moving target. So you have to take a few measurements to determine the capability of the transformer. I have a big fat hot dog sized wire-wound resistor with an adjustable band on it so I can vary the resistance from 100ohms to short circuit. Now, because the voltage amplitude is going to drop (because the transformer is having to work harder) as the resistance is reduced (current increases), with a regulated supply there is a lower threshold to how low you can go. The regulator(s) need(s) some room to work with. A 15V regulator is going to need some amount of voltage above what the regulated voltage is in order to work. So with the load testing you either stop when your transformer output reaches the lower voltage threshold allowable for the regulator to work, or you stop when you reached the current capability you’ve determined you need. In my case I’d like the power supply to be able to handle at least 3A, 4A would be nice. And voltage-wise, well, looking at the datasheet for the Mitsubishi M5230 dual regulator used in the PS-3500, it says the regulator needs at least 2.5V more at input than the voltage to which it’s set to regulate. So for 15V we need to have at least 17.5V at the input. That’s bare minimum. Better to have more like 25% more, so maybe more like 19V minimum. The AC voltage at the transformer is not the same as the DC voltage at the regulator. We have to convert...we divide the raw AC at the transformer by 0.707, and then we have to divide that in half because it’s a dual supply. The result of the conversion is the DC voltage assumed to be present at the filter cap just after the rectifier. So my first test I set the resistance high at just shy of 90ohms. At that resistance the raw AC measured 38.3V. Solving for “I” that’s about 0.43A. Nowhere near my target of 3A or better, but that’s over 27V at the filter...lots of room before we hit the 19V minimum. After a couple more steps I was at 9.6ohms. That’s 3.83A...and the voltage? The raw AC was 36.75V. That’s right...I increased the load (reduced the resistance) dramatically, and there was only a minimal voltage drop. The transformer was handling a near 4A load without really breaking a sweat. 36.75V raw AC translates to about 26V at the filter. I think the PS-3500 has nearly twice the capacity of the PS-520 as far as the +/-15V rails go.
Here’s a scene at the electronics lab...dog has given up:
So it’ll work! Now to put together a parts order to recap the supply, determine if the +/-15V supply output transistors could benefit from an upgrade, reduce the +8V supply to +6V, work out mounting the Winchester connector, install the +/-35V supply transformer and regulator assembly, modify the ground scheme, and execute some cosmetic mods.
I had a chance to have a little meet and greet with PS-3500 last eve...it’s going to work out well. There’s plenty of room inside the chassis for the second power transformer I’ll need to add for the +/-35V rails, I think my Winchester multi-pin connector idea to use in lieu of the factory Hirose connector for the console umbilical interconnect is a very workable idea that will resolve the issue of the unobtanium cable-mount connector, and provide a better more robust interconnect. And last but far from least I load-tested the transformer taps that power the +/-15V rails, and they can handle significantly more load than the equivalent taps on the PS-520 transformer.
Here it is...it’s in good shape, and I’ve also got some ideas with cosmetics rolling around in my brain:
Now, regarding the umbilical interconnect issue, my first task was to hunt down the connectors I’d need to replace the panel-mount connector and cable-mount connector with hood. I found a panel-mount connector on a spare sync relay module off of an Ampex MM-1000. It took like an hour to extract it. MAN they don’t make stuff like that anymore...here’s the module and I’m pointing to the connector:
And this module had been left for dead decades ago. I got it with the pile of spares I got with my MM-1000, and this thing was stored in an outdoor shed with a leaky roof, but the attention to detail in the wire looming, and the quality components...I think stuff like this looks pretty:
But anyway I got it removed and because it’s such good quality stuff it appeared it was going to clean up well in spite of the long term improper storage and bad environment. Next step was to pull down my large box of spare MM-1000 wiring harness sections. I think I’ve got almost two complete spare wiring harnesses for the MM-1000, some of it NOS never installed. I opened the box up. How did I accumulate this stuff... It’s hard to tell the scale from the pic, but that's like 60-70lbs of wire, connectors and spare chassis panels...the spread out pile is about 6 feet wide:
Anyway...geez I’ve got more of those 26-pin Winchester connectors than I thought. Digging through all the wire I just kept finding them...
And there’s at least one more in there, and that’s in addition to the ones on the MM-1000 already.
Then it dawned on the me the actual block that houses the pins or sockets is the same block used for panel or cable mounting. So I’m probably not going to use the one I spent an hour extracting from the sync relay module. I’ll use some new ones from the pile. I need to track down a pin/socket extractor tool and purchase some new pins and sockets...working on identifying the right parts.
Anyway, it was time to see if my idea would actually work...if the hood of the 26-pin connector would fit in the recess in the back of the PS-3500 chassis...and it looks like it will fit perfectly!
Clearly the dog is not as excited as me.
Then came the most important part...the reason I pursued an alternate power supply...to find something with higher load capability than the PS-520 supply I’ve been using because, with all the opamp upgrades I’m going to be asking too much of the PS-520. Were my hunches about the PS-3500 correct? Does it have higher load capability? In a word, yes!
The way to do this test is to connect a resistor (the “load”) across the transformer outputs, apply power, measure the voltage, power the system off and measure the resistance. Knowing the voltage and the resistance I can solve for current. A lot of you know this already, but voltage divided by the resistance equals the current “Ohm’s Law”: V/R=I. Looking at the formula you can see that, for a given voltage, as the resistance drops, the current flow increases, or for a given resistance as the voltage increases the current flow increases. As you increase the load (decrease the resistance), there is increased voltage drop...it’s a moving target. So you have to take a few measurements to determine the capability of the transformer. I have a big fat hot dog sized wire-wound resistor with an adjustable band on it so I can vary the resistance from 100ohms to short circuit. Now, because the voltage amplitude is going to drop (because the transformer is having to work harder) as the resistance is reduced (current increases), with a regulated supply there is a lower threshold to how low you can go. The regulator(s) need(s) some room to work with. A 15V regulator is going to need some amount of voltage above what the regulated voltage is in order to work. So with the load testing you either stop when your transformer output reaches the lower voltage threshold allowable for the regulator to work, or you stop when you reached the current capability you’ve determined you need. In my case I’d like the power supply to be able to handle at least 3A, 4A would be nice. And voltage-wise, well, looking at the datasheet for the Mitsubishi M5230 dual regulator used in the PS-3500, it says the regulator needs at least 2.5V more at input than the voltage to which it’s set to regulate. So for 15V we need to have at least 17.5V at the input. That’s bare minimum. Better to have more like 25% more, so maybe more like 19V minimum. The AC voltage at the transformer is not the same as the DC voltage at the regulator. We have to convert...we divide the raw AC at the transformer by 0.707, and then we have to divide that in half because it’s a dual supply. The result of the conversion is the DC voltage assumed to be present at the filter cap just after the rectifier. So my first test I set the resistance high at just shy of 90ohms. At that resistance the raw AC measured 38.3V. Solving for “I” that’s about 0.43A. Nowhere near my target of 3A or better, but that’s over 27V at the filter...lots of room before we hit the 19V minimum. After a couple more steps I was at 9.6ohms. That’s 3.83A...and the voltage? The raw AC was 36.75V. That’s right...I increased the load (reduced the resistance) dramatically, and there was only a minimal voltage drop. The transformer was handling a near 4A load without really breaking a sweat. 36.75V raw AC translates to about 26V at the filter. I think the PS-3500 has nearly twice the capacity of the PS-520 as far as the +/-15V rails go.
Here’s a scene at the electronics lab...dog has given up:
So it’ll work! Now to put together a parts order to recap the supply, determine if the +/-15V supply output transistors could benefit from an upgrade, reduce the +8V supply to +6V, work out mounting the Winchester connector, install the +/-35V supply transformer and regulator assembly, modify the ground scheme, and execute some cosmetic mods.
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Sweetbeats, somebody, sometime sort of promised some PSU schematics? I would also like to mention (again?) Douglas Self's book, Small Signal Amplifier Design. He goes into good detail about mixer PSUs (up to some 5A delivery) especially wrt noise and protection. One way to get RIPPLE down is a 2 stage filter after the rectifier so..Cap-R-cap where R is about 2.2 Ohms or whatever does not drop the regulator differential too low. Also some noise reducing tricks around the regulators.
Protection: 80,000 mfd of downstream capacitance could certainly put the regulation series devices in danger! Solution is a hefty diode rev' biased across the device. 'Mutual shutdown: Many ICs, the NE series for sure do not like having just ONE supply rail and will die. Self gives a simple diode circuit for protection but it probably could not be incorporated in those PSUs (can't tell. No map!) One bolt on solution is a relay that will only pull in for all the rail voltage. (can be bypassed with bleed Rs to cut the inrush current.)
Do not (grannies and eggs here!) be tempted to up the res' caps. That would bugger the 'power factor' and give the mains traff a harder time. (The R then cap avoids this problem)
Dave.
Protection: 80,000 mfd of downstream capacitance could certainly put the regulation series devices in danger! Solution is a hefty diode rev' biased across the device. 'Mutual shutdown: Many ICs, the NE series for sure do not like having just ONE supply rail and will die. Self gives a simple diode circuit for protection but it probably could not be incorporated in those PSUs (can't tell. No map!) One bolt on solution is a relay that will only pull in for all the rail voltage. (can be bypassed with bleed Rs to cut the inrush current.)
Do not (grannies and eggs here!) be tempted to up the res' caps. That would bugger the 'power factor' and give the mains traff a harder time. (The R then cap avoids this problem)
Dave.