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Trying to fix MK2 Sound Board

DCrosby

DCrosby

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I was able to get my hands on (2) MK2 Sound boards, with the deal that if I could fix both I could keep one... well... the first one had 3/6 roms missing, and leaking caps.
So I purchased and replaced all electrolytic caps, and tried to plug it in. And it didn't work. So I started swapping Gal chips, still nothing, one of the dire warning signs was that I never got the red LED to turn on.
Then I replaced the Analog Devices ADSP-2105 KP-40 (The square socketed chip in the middle). I plugged it in, and Bing diagnostic tone, red led, and we have sound.

Now what concerns me is that the sound amp with the (large-ish) heat sync gets warm, really warm, uncomfortable to the touch warm, so I wonder if I fixed the mayhem that was caused by the amp overheating but didn't find the root cause. So what I'm concerned about is that if I play long enough, or let the machine run long enough I'll burn out another ADSP-2105 or something else. So to my questions:

1) How hot should the heat sync get? (roughly)
2) If there's something wrong where the amp overheats (Shorts to check) where do I start to check to see if something is wrong?
3) Are there some usual suspects to look at if the heat sync overheats? What chips typically cause this, and what am I looking for if the board functions normally (I haven't run the board for more than 5-10 min)

I hope this helps someone looking at what could be causing the LED not to light up.

Also the other sound board, is somewhat of a basket case, also bad looking caps, and missing amp, and other components, we'll see how I fair there, and I'll post some things I found / learned when I get to that board it's currently drying from when I gave it a good cleaning.

Thanks!
 
So I was able to fix the other board as well, new caps, and I think someone seated the ADSP-2105 KP-40 wrong in the socket, as the one I pulled out of the original, thinking it was dead, I put into the second board making sure the arrow aligned with the little plastic piece on the chip, and it fired right up and ran, but the amp was hot again, given I didn't have a heat sync for the second board, but it got very hot as well... so I wonder if that's just the way those go... and I appreciate the advice @ItsBobDudes but the old amp on board 1 is still working, and the new amp on board 2 is also working and both get hot enough where you can't touch them for more than 1-2 sec, without getting burned...
 
Also I'm seeing some odd behavior on the MK2 (Main) Board, the video sometimes flickers in intensity but continues, and then when I either touch (move or flex) or even if the board is cold the board watchdog's at the beginning when it's going through the roms, or when it's starting up for attract mode. Once the board is warm, and is running it seems to no longer do it, or is just much rare-er...
I thought for sure it would be bad caps/broken solder points etc... that just warmed up and closed. But A there are few coaxial caps, some tantalum but no electrolytic ones that I can see, and I've gone over the solder points with a magnifying glass, I may try a microscope...

So I'm a bit at a loss, maybe I'll put a logic probe on the watchdog, to see if that's going low, or if someone has a "I've seen this behavior before, and it's those dammed Xor gates" or whatever... then I can see what's going on there, but otherwise I'm just musing at the schematics, trying to see what would be worthwhile testing or replacing.
 
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