Tascam M-2600 Power Supply Issues

[rp3703]

I recently purchased a Tascam M-2600 off craigslist with the full disclosure that there were issues with the power supply. I have read quite a few posts on various forums about people who have had issues with the power supply on this board but never any answers. So I would like to post what I’ve done to solve mine to help the next unlucky person who comes into possession of one of these mixers.
When I first powered it on, only the solo light would come on on the board. No power indicator light no movement on the meters and no signal would pass through the board at all. It also had a faint squeal at the main outputs but I never checked what frequency it was. My first thought was bad caps in the power supply and I replaced them all. While replacing the caps I noticed that the voltage regulator(U3) had brocken off of it's PCB and someone had soldered it back poorly, so I soldered some leads to the chip and re soldered it to the PCB. No change, in fact, I think the solo light didn't even come on after I finished. At this point I figured my voltage regulator fix was either botched or the chip itself was fried, so I put a new one in. Now I got the whole board to light up, pass signal and the meters worked. Success! Unfortunately, the faint squeal was much louder now and the L-R fader controlled the amount of loudness. Since I had purchased new power transistors(Q2 & 4) when I purchased the voltage regulator, I threw those in. Louder still. In fact when I raised the fader it got so loud that the board completely shut down. No more lighted meters, no passing of signal at all. Only the power indicator light was working. After pretty much losing hope, I went back to it last night and took a look at the Diode bridge PCB. I noticed that one set of diodes had lifted the trace off the PCB and the diode chip was loose. After re-soldering it, everything came back to life. The best part was that the squeal was gone as well, that is, until I turn the power off. It lets out a quick fading squeal. At this point I feel that I need to either A. Live with it and be happy the damn thing works at all, or B. Seek the help of this professional forum to help figure out where next to look. Since there is such a low number of parts to this power supply, I figured my odds were pretty good using the guess and replace method but I'm starting to run out of guesses but eventually I could end up with a completely rebuilt power supply. If this helps you guys come up with an answer, here are voltages I’m reading from the power supply. I’ll admit that I have a cheap $20 Craftsman multi meter but I did just put new batteries in it. I get +/- 31.6V at the +/- 19V sockets and 68.5V at the 48V socket. The 5V sockets read correctly. I've attached a PDF of the power supply schematic.

 

[PRR]

> I get +/- 31.6V at the +/- 19V sockets

The 19V is probably very-very close to the MAXimum voltage of the audio circuits.

You keep putting a 50% OVER-voltage on them, you'll fry the whole mixer.

This thing is mighty complicated for "guess and replace method". I think you have used-up most of the more-likely "guesses". Now you need to disconnect the mixer, *think* about how a power supply works, and verify it step-by-step.

This can start with the F1 F2 F3 fuse ratings. I would suspect somebody has been putting in over-size fuses.

The squeal/mute is probably related to P.Mute. This holds some mute gizmo off until the power supply reaches happy levels. Since the supply is NOT happy, it should be muting. It seems the power supply is now SO un-happy that the P.Mute signal is doing a wrong thing, or the Mute function has been blown by 50% over-voltage rails.

 

[rp3703]

Thanks for the quick reply. Another bit of info I forgot to mention is that the power transistors I took out of the power supply were actually a 2SC5200 and 2SA1943 combo not the 2SC3281 and 2SA1302 combo listed on the schematic. I ordered new ones based on the schematic without looking to see what was actually in the unit. Do you think these could be causing the voltage increase? Since replacing them did not affect the squealing sound of the board I'm guessing that there was nothing wrong with the old ones and I was considering popping them back in and seeing if that corrects things.

 

[rp3703]

Ok, I just googled it and both power transistor combos are supposed to be the same, so that's not the issue. I should have admitted that I am not an EE. I don't know how to look at a schematic and tell what voltages I'm supposed to be reading at different points in a circuit. Since the whole mixer isn't worth much I was hoping to be able to fix it myself through trial and error which has worked for me in the past. Any suggestions of what to look for and where would be greatly appreciated. Thanks

 

[rp3703]

Update: Again, my intention for posting this was partly to provide info for the next victim of a PS-2600 failure and to get some help from someone who actually knows what they are doing. since the latter has not really happened I’ve continued my method of guessing and seeing what happens.
Since I was getting such hot rail voltages, my next guess for failing parts was the U1 voltage regulator chip. I replaced it but got no change in performance. Then going against my better judgement and since there were no more options left, I replaced Q1 and Q3. Results! My +/-31.6V rails were now back down to +/-19.5V. Again, I am not an electrical engineer but I would have thought a transistor would increase the voltage not decrease it. Oh well, it worked.
Now when I ordered the transistors for the power rail section, I also ordered Q8 for the power mute section and Q5 in the phantom power section. After popping those in, my 68.5V of phantom power was now down to 62.9. No real change there. To test the power mute, I plugged everything up and turned it on. Immediately I got the squeal but after about five seconds it went away. After that it makes a “woof" sound when I turn it on or off but no squeal. Since I’ve never heard this mixer running the way it should, I don’t know if the “woof” sound is the way the power mute is supposed to function or not. My guess is not since it seams like it would cause the woofer on a set of monitors to move quite a bit which seams opposite of what I would think a power mute is supposed to do but again, I thought I knew what a transistor did.
So with some progress finally happening, my next guess is to replace the voltage regulator chip in the phantom power section(since I’ve replaced everything else in that section) and Q9(the only transistor that has not been replaced in the whole power amp) and C36(the only capacitor that has not been replaced in the whole power amp) in the power mute section. If that does not fix all my issues, the only things left are diodes and resistors. Since I order these parts from China, I’ve really only spent about $50 on parts so far but I have had to wait a long time for the parts to arrive. I’ve been working on this unit for a few months now. I’ll try to update again a month from now when my next set of parts arrive.

 

[PRR]

> get some help from someone who actually knows what they are doing. since the latter has not really happened

I am sorry to have disappointed you.

 

[rp3703]

PRR-Please, no offense to you. I had no idea what the "P-mute" on the power amp schematic even stood for until you told me and I thank you for that. I guess I was just expecting a lot more input from this forum than I got. Yes, I realize that Im asking a lot with my own limited knowledge of the subject. I myself have not done much to help anyone else who may be searching for answers on how to fix their unit since all I've really done is replace everything on the entire PCB.

 

[thebaz]

Hi, I know this is a very late reply to this thread, but I thought I would express my gratitude for the schematic diagram I was able to download. It helped me repair my failed power supply. I was using the console when suddenly all sound went away. Turned out is was just a dry joint on the 24vac transformer adapter on the circuit board. The +19vdc and -19vdc both were zero. A quick solder and we were back in operation. Thanks again.