I had that screeching problem on my Gottlieb Circus and it turned out to be the amplifier section of my sound board that went bad. Don't know that this is your problem but worth checking.

Does anyone have a spare 6 digit score display for a Black Hole? I had one drop out.

Nate

If desired...I made testroms for these boards. In order to use it, you will need to create a Jamma connector with some (dip)switches and pull up resistors to test the inputs. Send me a message if you would like to receive it.

Marco

My Mars sound/speech board started doing very a similar thing. When turned on there's a constant screech. Test button does nothing. Volume pot does affect the sound. Turning the cabinet off does play the correct sound.

Initially when I got it I recapped it and cleaned socketed chip legs. It was working good until the Tip640 on the sound/speech power supply took a crap on me and started blowing the power supply fuse. Replaced that and Im getting 12.3v and 31.5v at the sound board harness.

Anyone have any suggestions? I might have to send this one off for repair because Im scared I'll make it worse and these are so hard to come by!

You need to isolate exactly "WHERE" the screech is being generated.

Is is being made by the sound electronics OR merely the audio amplifier?

If you don't understand the difference, please send it out for repair.

I do understand that concept in theory but being able to actually put it into practice by tracking the problem down on my board is a different story. I would say it's probably not the amp though, because along with the screech I do hear the normal speaker hum, and it is properly playing the sound that occurs when the game is turned off. Regardless it will probably get sent out.

I probed the data lines coming off the 6502 and pin 31 and 32 are stuck high. Pin 22 is high and low but the pulse light isnt going off. Pin 37 is the same. Indication of a bad 6502? I somehow dont have one to swap it out with

I've realized the problem is probably either the clock, U25, U2, or the 6502, considering pin 37 of the CPU (the clock signal) is stuck hi/lo, not pulsing. Of course I've realized this after putting in my order for parts and not buying the crystal lol. Hopefully it's not the crystal and I can fix it.

Crystals rarely give trouble. It happens but very rare. I have (in desperation) replaced over 20 xtals in various items in the last 12 months and it wasn't the crystal one single time!

Don't be too quick to think the crystal is faulty - chances are that it isn't.

Careful with terminology as well. "Clock" means the entire clock generation circuit usually, not ONLY the crystal, which is but one component in the clock circuit.

Thank you, I did mean crystal. When I get my parts order in, I will start by swapping the 6502, then trying to piggyback U2, then trying to piggyback U25. Hopefully I'm on the right track

Attached schematics:

image_2021-06-07_215138 (resized).png