Stampede

[mark1505]

I have a stampede that the sound is distorted. It was working fine. I put a different coin tray with  speaker and same sound. I get a clear startup bong. where do I start?

[rokgpsman]

Since you've eliminated the speaker as the defect this is what I'd try next.

I believe most of the audio output circuitry is located on the I/O board, which is the upper board in the card cage. If you have another I/O board to substitute that would be a quick test, there aren't any game-specific parts on the I/O board so you don't have to worry about swapping roms around. Before you do that you need to verify that all the power supply voltages are correct, especially the -12vdc. With the machine turned on take a look at the red LED's on the front edge of the I/O board, they indicate the status of the main power supply voltages. All 5 LED's on the right-side of the I/O board should be lit, they indicate status of voltages for 5vi, 12v, -12v, 5v, 18v, see the attached photo for clarity. The audio amp and associated parts are on the I/O board near the large white connectors in the picture below. If all of your voltages are good then you could try replacing the audio amp or nearby filter caps. The -12v and +12v voltages are important for audio.

The sounds themself are stored in digital image on the sound roms that are located on the CPU board, which is the lower board in the card cage. The sound roms are located at xu17, xu18, xu30 and xu31 which are in the center of the CPU board. Some games don't use all 4 of these chips so don't get worried if an ic is missing. Maybe one of the ic's has creeped out of its socket or its contacts have formed some surface corrosion, you can try pressing down on each one to reseat it. If you do this be sure to rest the board on something sturdy like a thick phone book or a folded towel on a table or countertop, you don't want to flex or bend the board and damage any traces/circuit paths.

There is a sound test in the diagnostics, it tests all the various sounds your game can make. This is covered in the WMS 4XX manual for your machine, available on this website. But I don't think it will ID any bad components, it just makes the sounds for different events that happen.

Whenever you handle circuit boards you need to be aware of causing damage from static electricity from your body, but if your humidity is anything like ours this summer (high!) it isn't a big worry. You can temporarily remove static elec by momentarily touching an unpainted metal part of your machine, provided it is still plugged into your elec socket and properly grounded. Try to handle the boards by the edge when possible. Movements like walking around on carpet and very dry air (low humidity) can increase static electricity so be careful.