Gorf audio distorts picture quality

westal_sage

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My Gorf standup has issues with the audio messing with the monitor's picture quality. The sounds of the game cause a very noticeable distorting of the picture. It's really apparent on the blue-background screens. It's kinda like the picture jitters with wavy lines in time with the audio - sort of like when you're adjusting rabbit ears on an old tube. If I turn the audio all the way down, the picture stays clean, and the distortion gets worse as a I raise the audio volume.

Anyone know what would be a likely cause of this / what parts I might try replacing or rebuilding? Thanks!!
 
Two things, firstly (and most likely) if you have lost one of the two smoothing caps on the 12V line you can get the amplifier chip oscillating madly, it will get extremely hot and will eventually cook itself to death, but it can sound like it is working perfectly at the same time, this can cause interferance on the screen. Simplest way to test is to disconnect the 12V wire from the PSU and see if it this cures is. If it does then the problem is 100% contained in the amplifier section. Amplifier chips usually need a large electrolytic in the 1000uF or more on the 12V line to ground, and a smaller polyester cap of a few nF in parallel with it. Often the electro gets knocked and damaged, or it has simply dried up and needs replacing.

The second one, which is less likely is that one of the sound generator chips has gone wrong. I met a Yamaha sound chip once that was getting roasting hot and was causing the gfx to flicker badly. The audio was also ok while it was doing this, replacing the YM chip cured the problem. In this case removing the 12V line would not cure the problem as the fault was in the 5V section of the board.was getting roasting hot and was causing the gf
 
Thanks Womble! OK so I think part of the problem here might be that the game has been fitted with a switching power supply - completely bypassing the original power supply.

Where are the 2 caps you mention located? I'd like to just go ahead and replace them, but if they're on the original PS, clearly that's moot since the original PS is out of the loop. But if they're on the (audio board?), I would go ahead and replace them.

I did disconnect the 12v from the swiching PS, but then nothing will even boot up so I can't see if that cures it. I'm guessing I misunderstood that - maybe you're saying to disconnect the 12v from just the audio board - is that right?

What is the audio board? Is it the little board with the pots on it? Well, really I think I just need to ask - what 12v can I disconnect from where to do the simple test you mention? Or is that nonsense now because of the switching PS conversion? Thanks again!!
 
I think I found the 2 caps that Womble mentioned. The big one is on the original PS board, and the small one is on the little audio board with the pots and test switch.

Schematics show the big cap to be wired across pins 5 & 7 of the connector to the small amp board (red +). So I just tapped into those wires and jumped over to the big cap on the orig PS. I also replaced the small cap on the audio board. Oh and I found a new 15K mf cap in my parts bag so I went ahead and wired it up instead of the old one on the orig PS.

This did render a noticeable decrease in the gfx flickering / distortion. It's still not perfect though, but I'd say acceptable. If anyone reads this and has some further suggestions, I'd really appreciate it. Thanks again, Womble!
 
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