Tascam M-___ Story...

[sweetbeats]

So to summarize at this point...

I'm continuing to:

• Clean things up (PCB's, jacks, dress panels, knob and switch caps)
• Refurb the frame (removing rust, painting components, reinforcing the attachment points of the side panels and front cross member)
• Map the channel PCB's interconnections

Where I really need to focus my attention at this point is on trying to get a handle on what the 35V rails power.

My spare PS-520 power supply is now in evm1024's possession...we worked a deal and fortunately I get to keep all my children... He's taking a look to see why the 6VDC rail is not functioning and why one of the 0VDC rails is not pulling down to 0 volts reliably. Then from there it is a matter of seeing if there is a reasonable way to create a set of 35 volt rails, but of course it helps to know something about how much current draw there is, so I'm going to try and gather some of that information.

Also, I stopped by my favorite local electronics shop...the kind of place that has a boneyard in the back with lots of wierd surplus stuff...wierd music playing the background, loads of obscure parts and tools, and really cool down-to-earth people that get excited when you bring something wierd in and put it on the counter...well, I brought in the mother PCB assembly. I was excited to get a "Wow!" I was hoping to find the connector that mates with the power supply connector. No such luck, but we determined that it is indeed a Molex connector, at least thats what the pins are inside. There was a temporary debate as to whether it is Molex or Amp, but it looks like Molex pins. I did find an identical connector set in a 6-pin configuration on a transformer in their boneyard though, so whatever it is it is most certainly a line geared toward power supply components...time to start digging. I did end up buying a set of 12 pins in case I decide to build an adapter to mate the Molex connector on the M-___ board with the Hirose ends on my M-520 power umbilical...that may be what I end up doing, as long as I can find a way to mount the Hirose connector to the M-___ frame without drilling any holes or otherwise permanently modifying it...I want to try and keep it as original as possible for posterity, or at least make changes easily revertable...though I guess I did paint it, but I think that's different. Rust spots and chipping paint make for a housing that is not clean for the sake of surrounding components, or the internals of the mixer itself.

blahblah....bl-bl-bl-bl-blah.

 

[Richard King]

This reminds me of something I saw at a satellite convention a few years ago. I spent some time in the Dish Network booth where they were showing off all their new toys. Sitting on one of the shelves was a new satellite receiver with a built in DVD player. This was when DVD players cost much more than they do today and such a product made some sense. However, it never made it to market and was never to be seen again. I suspect that your M____ board is similar to the combo satellite receiver. It certainly is a product that never made it to market, no question. What would be fun to determine is where did it come from and what was the original purpose? Was it a hand built product assembled in the US? Was it a prototype of a product that just never made it to market? If so, why did it not make it to market? Was it custom built for a specific customer?

I have sent an email to the last place that I could track down Bill Mohrhoff to. He was the top man at Tascam through the '70's and '80's. A Google reveals that he moved to E-Mu Systems in 1990. I suspect he is retired by now, but I sent an email to E-mu to try to find out where he is. Hopefully I will get an answer back.

 

[sweetbeats]

Meant to do this last week...

Put up a pic of the trace side of the Mother A and B PCB's...the sockets for the channel and master section cards are on the other side as well as jumpers to link the individual PGM and AUX buss traces to the appropriate channel card for the respective output jacks...

MONITOR and REMOTE A & B busses from the master section are on the Mother B PCB (on the left in the photo) and the other busses (PGM, STereo, MONItor) to their respective master out jacks or to the master module, as well as all the power supply rails are on the Mother A PCB (on the right in the picture).

 

[sweetbeats]

I have sent an email to the last place that I could track down Bill Mohrhoff to. He was the top man at Tascam through the '70's and '80's. A Google reveals that he moved to E-Mu Systems in 1990. I suspect he is retired by now, but I sent an email to E-mu to try to find out where he is. Hopefully I will get an answer back.

Very cool, Richard...thank you.

I have yet to hear back from Randy Alberts, the Author of TASCAM: 30 Years of Recording Evolution...I found a hotmail address online and emailed awhile ago. Probably is an invalid address, or he's just ignoring me.

Anybody have any other ideas on how I might reach him? I am trying to reach him based on the suggestion of Jimmy at Tascam since they have no company archives and Mr. Albert's work is the closest to an archival collection that Jimmy knows of; that Mr. Alberts might have some direction toward answers...

 

[Richard King]

I would check with Hal Leonard Publishing for contact information on Randy.

 

[evm1024]

SNIP!

Where I really need to focus my attention at this point is on trying to get a handle on what the 35V rails power.

My spare PS-520 power supply is now in evm1024's possession...we worked a deal and fortunately I get to keep all my children... He's taking a look to see why the 6VDC rail is not functioning and why one of the 0VDC rails is not pulling down to 0 volts reliably. Then from there it is a matter of seeing if there is a reasonable way to create a set of 35 volt rails, but of course it helps to know something about how much current draw there is, so I'm going to try and gather some of that information.

SNIP

OK Here is the good news - all voltages are correct on your PS. The one gnd and +6 v read as they do because they are Isolated from the other ground and voltages.

The m500 series has a signal ground (c) and a display (d) ground that are joined together at only 1 place in the mixer. This is a common practice for mixed analog and digital systems. The display ground is floaty because it is not joined in the PS. Once you plug it into the mixer the 2 grounds are joined. The +6v is used to drive the indicator LED and the peak LED driver IC and is not part of the analog chain. So you would get a near 0 reading when measuring from +6 to analog (common) ground.

As for +-35 volts. This was a puzzle to me at first as that I could not think of anything that would use those voltages. Until I looked the M512 power supply. The M512 has a built in PS and shows unregulated +-35 volts sent to regulator daughter boards where needed. The daughter boards regulate to +-18v. THe M520 does the +-18 volt regulation in the PS.

That is also good news. I measured the unregulated DC supply voltages to the +-18 regulators and found that they supply +-27.5v. Not 35 bet then again more than 18....

So if you track the +-35 volts down and find some 7818 and 7918 voltage regulators you are golden as for as 35V goes. In that case we can just bypass the 18 volt regulators in the PS and use 27 volts which will work fine.

You may be further along than you know!

Regards, Ethan

 

[sweetbeats]

Ethan...yer a freaking wizard...

Wow...questions...

So if you track the +-35 volts down and find some 7818 and 7918 voltage regulators you are golden as for as 35V goes.

This is really fun and exciting...okay...haven't actually traced the 35V rail yet once it gets to the channel mother PCB, but my thoughts immediately went to a pair of regulator lookin' IC's on a simple heatsink on the PGM PCB...this would make sense since the 18V rail on the M-500 boards is (IIRC) for the PGM busses, and that function is spread out over each of the channels on the M-___.

These are what I'm talking about:

And here's a closeup...you can see that the part numbers are unfortunately not 7818 and 7918...they are C2824-Y and A1184-Y...I did some quick Googling and a brief search at Mouser...the 1184 reference at Mouser did bring up some regulator IC's...I didn't look deep as I was too impatient yet to get this post up, and next I'll be too anxious to start tracing the 35V rail...

HEY LOOK!!! Here they are again on the Balance Amp PCB's for the balanced stereo master output...

SO...

1. Is there some advantage to the 18V regulation being centralized on the M-500 consoles?
2. Is there an advantage to it being decentralized on the M-___ console?
3. Is it simply a logical engineering solution since the opamps fed by the 18V rails are not on a single PCB as in the M-500 consoles but are on separate boards?

This is cool...27.5V is OK right? You said that already but...I think you were the one that said that it is not uncommon for unregulated rails to vary even as much as 20% from spec and 27.5V is pretty close to that spec...

OK Here is the good news - all voltages are correct on your PS. The one gnd and +6 v read as they do because they are Isolated from the other ground and voltages.

Huh! Yeah...I think when I was testing it I had one lead clamped to the chassis ground lug, which is the analog ground right? And I do recall there being some minimal voltage reading...in the mV's...so is this the same with the 0V rail that was not pulling down properly? I wasn't measuring to the proper ground? I almost feel guilty...I got a discount from the eBay Seller when I complained that two of the rails weren't functioning properly...

Wow...this...is...um...cool. Wow.

So its possibly just a matter of making the cable???

Do you still advise trying to get an inventory of what each rail is pushing to make sure that the paths aren't going to draw too much current? I know...do the smoke and kill tests, and then power it up and see what happens right?

OH! But I do need to get more comfortable with what the supply rails that go up to the meter bridge comprised of...There are 5 lines, and I know 3 are +/-15VDC and 0V...the other two I betcha are 11VAC for the bulbs. I think it'll be a simple matter of seeing if those two traces on the meter amp PCB are isolated to the bulbs. They are the same bulbs as in the M-520, so that should settle it if those traces are isolated to the bulb mounts. If that all pans out that way then I'll turn on the PS-520 I have here and run jumpers to those lines from the AC rails on the PS-520 and see if the lamps glow and there are no sparks or smoke.

Hm...the other issue gnawing in my mind is that the M-___ requires two 0V rails...How do I know which circuit in the console goes to which 0V rail on the PS-520??? MAYBE...if only ONE of the two rails is used in the meter bridge in the M-520, and only ONE of the two 0V rails goes up to the meter bridge in the M-___, then maybe its the same rail and that will tell which rail goes to the other circuit...maybe?

ohhhhhhhh boyohboyohboy!

 

[sweetbeats]

I'm a good way through mapping all the channel PCB interconnects...helpful in figuring out where the 35V rails go...

Well, the answer to the question "where do they go" is that they go exclusively to those presumed regulators mentioned in the last post.

Here is a crude first attempt at drawing a partial schematic that shows the 0V and +/-35V rails coming into the BUSS PCB and their path to the regulators:

 

[Muckelroy]

Holy **** dude, you're either a night owl or an early bird! Your last post was at about 6 AM Central Time!!

 

[sweetbeats]

Take your pick...option three would be "nuts"...I'm banking on that right now.

Pretty exciting stuff tho'...I've got a feeling this mixer hasn't been powered up in a long, long time, and it is appearing that it may very well be relatively simple to do, and that's...exciting.

 

[evm1024]

Now you are gettng somewhere

Just put a 2s infront of those part numbers. Japanese transistors use 2S as the prefix with the next letter telling you what type.

2SA1184 is a PNP audio power transistor that is the complement to the 2SC2824 NPN audio power transistor.

Finding opamps that can handle driving balanced 600 ohm lines is mildly hard today and perhaps was imposable then. I see on the balanced board that there is a pair of these transistors. I'm going to assume that these are the drivers for the balanced outs and perhaps for the non balanced outs.

The max balanced output on your 58 is listed as 28 dBm. If we use this number we see that 28 dBm is 19.5 volts into 600 ohms which gives us (19.5/600) 33 mA of current which is more than most if not all opamps couuld do in the time period. To find power we do 19.5 * 0.033 to get 0.637 Watts. 600mW is way too much for any opamp of the time. And we are not even talking about being musical.

So they are doing what we were looking for in a more direct way. We are looking at history here. 35 volt rails needed to drive some transistors needed to drive balanced outs - becomes - existing 35v rails go to 18 v regulators to power opamps (that can handle +-18v) to drive balanced outputs. (NB very few opamps can handle 18v rails and even fewer can handle more than 18v rails)

Ditto the 6v to drive the LED. 6v is a common incandescent lamp voltage.

As for the 2 grounds, you can join them where the power input cable ties the analog ground to the chassis.

--Ethan

PS and actually I expect that you could drive the 35 volt rails with 18 volts for testing....

 

[sweetbeats]

Well...well...huh!

2SA1184 is a PNP audio power transistor that is the complement to the 2SC2824 NPN audio power transistor.

Very, very cool. Okay.

Finding opamps that can handle driving balanced 600 ohm lines is mildly hard today and perhaps was imposable then. I see on the balanced board that there is a pair of these transistors. I'm going to assume that these are the drivers for the balanced outs and perhaps for the non balanced outs.

I'll have a look at that, and I am pretty certain you are correct...the unbalanced outs are switcheable from -10 to +4, balanced from +4 to +8.

The max balanced output on your 58 is listed as 28 dBm. If we use this number we see that 28 dBm is 19.5 volts into 600 ohms which gives us (19.5/600) 33 mA of current which is more than most if not all opamps couuld do in the time period. To find power we do 19.5 * 0.033 to get 0.637 Watts. 600mW is way too much for any opamp of the time. And we are not even talking about being musical.

So they are doing what we were looking for in a more direct way. We are looking at history here. 35 volt rails needed to drive some transistors needed to drive balanced outs - becomes - existing 35v rails go to 18 v regulators to power opamps (that can handle +-18v) to drive balanced outputs. (NB very few opamps can handle 18v rails and even fewer can handle more than 18v rails)

So is all this to to say, in part, that the PS-520 should be able to handle the current draw on those 35V rails?

As for the 2 grounds, you can join them where the power input cable ties the analog ground to the chassis.

I kind of understand this...

My plan at this point though is, since there are clearly two distinct 0V rails on the Mother PCB A, and those stay isolated once you get into daughter boards, to look at what kind of work those two separate rails do in the channel modules, the master module and in the meter bridge and look for correlations in the M-520 schematics...I think it will be pretty easy to find the correlations...then connect them up. As you know, the power umbilical on the M-520 carries to 0V rails and a GND as well...The M-___ power connector uses 2 of its 12 pins for the two 0V rails, and there is a chassis ground lug that goes to on of the 12 pins as well...so I'm going to follow how those come over to the M-520 and replicate the connection ideology on the M-___...sound pretty safe?

Another thought...regarding the cable to be used...I talked with you about this the other day, Ethan...I was concerned that the 22AWG 20-conductor cable I got to build the M-520 umbilical wasn't going to work since the supply rail wiring inside the M-___ is all 18AWG. I got the sense that it wouldn't likely pose as a problem, but since the M-520 umbilical has 20 conductors, and the M-___ needs only 12, and nearly all of the rails are duplicated across the umbilical on the M-520 (i.e. there are two conductors used each for all the rails and GND except for the 48V for phantom), my plan is to also use two conductors each. See any fault in that plan?

PS and actually I expect that you could drive the 35 volt rails with 18 volts for testing....

So...you...are...saying.........I could power things up with the PS-520 that is sitting in the rack in my studio...oh my...

 

[evm1024]

Yes - that is what I was saying. You could power it up and see what smokes! Gotta do it sooner or later.....

Somewhere in your mystry-mixer the signal and switching grounds are ties together. This is needed to keep the noise down. One ground tie good, 2 ground ties bad (ground loops et al).


The PS520 is a big supply! It can handle it. The M520 is rated at about 100 watts consumption. the PS520 has more than 200 watts available at the rated fuse currents. Looking at the size of the transformer I would expect that you could pull 400 watt peaks out of the PS520 for short times.

--Ethan

 

[sweetbeats]

Yes - that is what I was saying. You could power it up and see what smokes! Gotta do it sooner or later.....

Boo yah...ahead! (he yells with reckless abandon...)

Somewhere in your mystry-mixer the signal and switching grounds are ties together. This is needed to keep the noise down. One ground tie good, 2 ground ties bad (ground loops et al).

Got it...but isn't it safe to assume that that is dealt with already in the existing circuitry and my job is to bring the rails to the connector?

The PS520 is a big supply! It can handle it. The M520 is rated at about 100 watts consumption. the PS520 has more than 200 watts available at the rated fuse currents. Looking at the size of the transformer I would expect that you could pull 400 watt peaks out of the PS520 for short times.

Cool, cool, cool. We like that.

I think I found the power supply connector... Now to see if I can find somebody that sells one...I'll get out the dial calipers tonight to compare dimensions, but it is keyed the same and has the same beads on the outside of the shell...

 

[evm1024]

Is is safe to assume that and then to test it. Put your ohm meter between the grounds on the power connector. Should be 0 ohms to each other.

 

[sweetbeats]

Okay...gotcha...but that will have to wait until I've got somthing plugged into the mother board..OR, I could just put my leads on the terminals for the 0V rails on the channel module or master module??

 

[evm1024]

Okay...gotcha...but that will have to wait until I've got somthing plugged into the mother board..OR, I could just put my leads on the terminals for the 0V rails on the channel module or master module??

That would be living dangerously

 

[sweetbeats]

Huh?

Maybe I didn't make myself clear, or I am about to learn something new.

What I meant is that I assumed the 0V rails tie together within the circuitry on daughter boards, so in order for me to confirm that they tie together, something (of those daughter boards) would need to be plugged into the Mother PCB...So I was saying that I'd plug a channel module in and then put my leads on the 0V pins on the power supply connector on the console...no connection to 110...then I was suggesting that, since its all torn apart right now but I'd like the answer sooner rather than later, it might be just as easy, or easier, to touch the DMM leads to the 0V terminals on a channel module sitting on the table. No power connected, just see if there is continuity between the rails.

Does that make sense, and am I missing something?

 

[Muckelroy]

This thread keeps getting better and better!

You'd better post a YouTube video of the first power-up of this machine!

 

[evm1024]

I was the confused one...

I misunderstood and envisioned you putting clips from your PS to the boards and powering them on. I just envisioned a clip coming off and shorting things out. Not that I haven't done that myself.

Just measuring resistance connector finger to connector finger should present little risk. I would expect the ground common connectionn point to ba part of the motherboard or (more likely) in the wiring harness where it also grounds the chassis.

Must have been one of those days.

--Ethan