Can someone point me in the right direction on this please?

My Dr. Who is blowing the F501 and F502 fuses whenever I boot the machine. It has a blown trace on the back of the sound board and a few blown capacitors I repaired, but it is still blowing the fuses. Not sure if the problem is the sound board itself or something else.

Thanks in advance.

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Yes. Sorry. The diodes all tested good. First thing I checked.

I pulled the trace off. I was really hoping that was blowing the fuses, but nope.

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Where do I test the voltages on this board for the 25v and -25v?

All 4 diodes tested the same... good. What regulator?

Testing the amperage to the fuse shows .07 amps, which shouldn’t be blowing the fuses, right? Unless I’m doing the test wrong or missing something. All my test LEDs are correct.

Replaced the blown capacitors.
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Perhaps I’m testing the amperage wrong?

Overall the machine seems to work, except the sound board.

Ideas?

I noticed someone placed a wire in the center battery because the contact broke; terrible repair. Before I noticed it I replaced the batteries; as they were bad and I think I shorted out the middle battery (see photos) as it got hot and expanded (photos). So I repaired the “wire” bs and replaced the batteries. At some point I noticed the sound dying, but it was too late. My assumption is the middle battery short caused the blown sound board trace... After fixing box the battery issue and the sound board issues (capacitors and trace) the sound board is still blowing both F501 and F502 fuses upon startup. Ideas appreciated. I’m reluctant to just buy a new sound board if their are other issues, but all the LEDs and TP are correct. Ideas?

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There is a loud buzzing coming from the DMD.

Repair job.

I’m going to order a bunch of NVRAMS soon. Any favorite spots to get them?

Until then I want to fix my sound board.

Can’t wait to try this game.

Would I be correct to assume the issue lies on the audio board and not another board?

So I decided to try and unplug the main speaker and the fuses stopped blowing. When I replaced the speaker (was blown) and plugged back in the speaker wires the speaker makes a loud buzzing/humming sound and the speaker pulls in; which ostensibly blew the other speaker to begin with. Not sure what part of the board to tackle next... likely a bad amp?

Where I’m at now...

I read this... but now I’m starting to wonder if the hum from the display and speakers are related?

Hum is typically from failing capacitors on the sound board power supply. ... If not the smoothing capacitors are failing. This puts AC on the amplifier rails, and causes the hum.

Sending it for repair is the easy way. Why ask for help on a repair forum if I was someone who would just send the board off for repair? No thanks.

So anyways... if you’re someone, like me, who doesn’t mind fixing their own boards here’s the list of things that will likely need repaired on the board if it’s doing what mine is.

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Thanks for the link. I think I was looking at the wrong board; seems they tried a few soundboards in that era.

I’m hoping my electronics store has the capacitors I need. Planning on C20 C22 C23 C46 today and reporting back.

Very impressive that you wrote that. Someday I hope to be as good at you. I work repairing cable TV converters, etc. so this stuff is new to me, but not much different otherwise. If I can’t get it myself I’ll 100% send it to the expert.

So today I changed the C23, but my parts store only has a radial capacitor so I hope that’s ok? Anyways that didn’t do the trick. I have more capacitors to replace still, but I decided to check my speaker leads for voltage and they had about 8.8 volts going thru each. When I unplugged the ribbon cables I noticed the voltage went up higher too; around 15 volts or so, which I thought was strange, but whatever. I also checked for AC current on the two large capacitors and notice about .154 volts on the outer cap, while the other showed none.

Just hoping I’m not replacing all these caps for nothing. There’s got to be a better way to test capacitors other than pulling them out... no?

Here’s the radial cap I used to replace C23.

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Well I replaced C20 C22 C23 C46 C47 and have the exact same issue, so I guess I will replace the next series C26-C29, C37, C46, C47, and C48. Not sure where to go after that. Looks like the voltage regulators and then the amp chip???

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Personally I’m thinking the same but no idea where to start?

The machine was only originally blowing the F501 and F502 fuses when the blown speaker was installed. After removing it the fuses are good unless I let the new speaker blow. There’s something adding voltage to both the display and the sound board IMO; hence the hum from the sound board... ideas?

Which diodes specifically. I only checked the four main larger diodes.... bridge rectifier. Are there others I should be checking?

Ok. So I have a dead short on pins 6 and 7 which carry 20 volts AC. I’m starting to figure this out. Thanks for your patience. I just was able to print the schematic.

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So pin 6 and 7 should be ground on the board. Good to know. And everything seems in order otherwise. I’ll recheck my voltages and read more not that I have a good schematic and get back to you.

So I replaced the Amplifier chip and the board started working, but the sound eventually became more and more scratchy as the heat sink heated up. Ideas?

I pulled my c26 and removed the carbon under it and replaced it. I also checked all the blue 47mf capacitors (all tested good), so I replaced the board and turned it on. Made a funny noise initially then beeped like normal the my c26 capacitor exploded. Not sure why?

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