MITSUBISHI M-A5200 Woes continue

Radiomandan

New Member
So long story short not to rehash all the old threads, friend hires me to repair his Mitsubishi M-A5200 as it was blowing the mains fuse. Replaced the STK-4048V Chips and unit ran a few days till the chips shorted out again. I found a used 5200 off Ebay and plugged it in, and I had the green light, but no output. Realized I has forgot to plug in the input to the amp. I plugged it in and powered it up and was greeted by a nice lightning storm as the chips failed yet again. But it happened on power up well before the AMp actually went into Green Mode.

I tested the speakers the best I could, both ohmed out identical, no shorts in any of the coils, and they have been playing beautifully hooked up to a SONY Stereo.

Anyway, I figure now the problem is in the Preamp that is the master control for this system. I measured 52v AC on the Neg of the output, and 27VAC on the positive. Something I never checked when looking the system over. I'm a Tube Guy, this solid State is difficult when you don't have a single diagram to even start troubleshooting.

So now I have 2 Blown 5200 Power amps sitting here with blown STK Chips. and a questionable preamp, that houses all the brains. the CD and Tape decks all slave off the preamp as well for control. so I cannot simply replace the preamp, with a newer unit and still have control over the CD and Tape, without manually pushing buttons on the system.

Customer is curious to know if a Mosfet or newer style amp could overcome the voltage issue of the preamp, I doubt it. I am thoroughly stumped at this point, and trying to find a decent full system is not easy, or cheap right now with the resurgence of vintage audio.

Like I said in earlier posts he needs something that can run at Vol 11 for long periods, without breaking the bank. OR I have to go back to square one, figure out the issue with the preamp now, and replace another set of STKs. he just really likes the system, and I have a hard time telling him at this point, lets sell the components and put it toward a nicer system, that can handle the abuse of running loud. I am still open to rebuild a new style amp inside the chassis of the old poweramp. but ID hate for it to blow up, till I can figure out what is going on with the pre amp. It also has an STK chip inside, and he did say that the surround sound was far quieter than he remembered, it was playing fine, and will play fine until you hook the power amp to it.

ALso everything plugs into the back of the preamp, and the preamp plugs into the wall. I did test voltage at all the plugs and all were 121V at the plug and 122.5 at the wall.

Im game to just sell everything, unless I can find a diagram.

Any ideas? or time to cut losses and start rebuilding? I Know these STKs are a ticking time bomb to begin with

Thanks,
Dan
 
Reviving a dead horse here but now I've got 2 of these power amps. Need to see if I can make one working unit out of 2. Ended up being a random short issue with the preamp circuit. Leave the preamp unhooked to it and it would power up just fine wait for the relays to click and plug in the Poweramp to the preamp and it would be fine. If you let the power amp power on before the preamp was ready pop went the fuse. After a few time it blew the chip in the second amp I found. It was hard to catch but I finally saw 44v spoke thru the left channel output to the Poweramp. It was very fast and happened totally randomly. But it's not worth digging into the pre amp. Going to try and find the issue at some point but for now hunting for similar to what's on my bench now for him. Pioneer sx850.
 
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