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.