Many years ago I had the sound go out in a Space Shuttle pinball I used to own.
Come to find out the problem was cold solder on the back of the power supply board that sent the voltage for the sound circuitry to the CPU and Speech board.

Worth a try, pull your power supply out and look at the back for cold solder at the connector pins.

I just looked at the schematic for Comet. There are several possibilities here. The 2002 audio amplifier might still be bad. The best way to troubleshoot this is with an oscilloscope looking at the input to the amplifier (pin 1). If you see audio there, but nothing coming out... there is your smoking gun. It is also possible that the DAC chip at U48 could be defective, U13, the 6821 PIA that sends data to the DAC, or possibly Q51 (2N3904), which appears to be a preamplifier transistor for the main audio amplifier. C32 is a coupling capacitor off of the preamp and if it was open, it would cause the problem you are describing as well. It's difficult to pinpoint without the board in front of me, but these are some places that I would look at first.

Jeff

Also, check to see if you have -12V at one end of R77 (the end not connected to pin 4 of U48 and the emitter of Q51). -12V should also be present at pin 3 of U48. This missing voltage could also cause the problem you are experiencing.

Jeff

-14.3V seems a bit excessive to me, but it's not over the top crazy and probably not causing the problem. An oscilloscope would be such a big help for this problem. Without one, your troubleshooting is limited to the technique you describe. Given this situation, I would probably start at the output and work my way backwards back to the PIA. Hopefully the audio amp is the culprit

The only way I know to test the chips is with an oscilloscope. It's my favorite troubleshooting tool by far and I use it a ton.

Please keep us posted with your progress. You will get it!

Jeff

http://pinside.com/pinball/forum/topic/new-space-shuttle-1984-williams-manual-done

More legible schematics for the CPU board.

Per the manual you need to check the -12v filter cap at C7 (100uf 25v radial) for excessive ripple (.75 vac or more.) Without an ESR meter or the ability to power the board on the bench you might as well just replace it.

You can also try swapping the two CPU chips (if they are the same) between U11 & U17. If the machine suddenly doesn't boot you have a bad CPU chip (though I doubt that is your problem.)

After that, I would try a new ROM at U49. If that doesn't solve the problem, without a scope or logic probe and test ROM, you're left to shotgunning the rest of the sound section - not the ideal way to fix it. I usually only advocate doing that if everything you are shotgunning is socketed or you are rehabbing a board.

viperrwk

OK, good job finding the jumper to eliminate the speech board as potential problem. You'll still want to remove the speech ribbon cable to keep something from that board pulling down the sound line.

The humming moving up and down with the volume knob pretty much eliminates the TDA2002 as the issue.

The rest of the sound section is just the processor (at U11), ram (at U12), eprom (at U49), PIA (at U13), 1408DAC (at U48) and an ls138 (at U50) used for addressing the speech roms. looks like U55 and U28 are use in addressing as well.

Now if the DAC was bad you would still get speech assuming that the speech board also doesn't have a problem. First thing is to swap the processors as someone already recommended. Watch out when switching the processors, if you swap a 6802 Motorola for a Hitachi or vice versa you will need to change a jumper. After that the most common problem I run into is bad RAM chips and then bad Eproms.

There is a Leon Test chip for testing system 9 and system 11 MPU based sound sections (might work on separate sound boards as well, not sure) that can help diagnose this.

-12V is unregulated so seeing almost -15 volts is pretty normal.

At this point if you believe there is trace damage, your troubleshooting efforts may be in vain until it is corrected.

If you pull the board to test it on the bench you can buy the test ROM from John Wart Jr.

As Kris indicated, you may not be getting speech if the speech board isn't working - could be bad sockets for the ROMs.

There are lots of board repair guys out there - Rob Anthony, Chris Hibler, Hans Haase, John Wart Jr, Clive @ Coin-Op Cauldron, even Kris above may look at your board as well. If you were local I'd just come over and lend a hand.

viperrwk

Not likely. The chip at U49 has the operating code for the sound section, so without it the processor can't execute the code to talk to the speech chips.

Good news for sure..... Hoping that is the fix on BigB's Comet also......

This thread is gold for my current space shuttle intermittent sound issue. Going to spend some time with my is cope this weekend chasing audio.

Comet sounds working. ... But missing the railroad and Dunk Dummies Heckling..Maybe a couple others.

Thoughts. Sound Chips?? Ribbon issues. Board..??

I installed a known working board... And all is fine..

THx. for any help.

Still learning. electronics , wiring.issues.

I had the same problem and your post nudged me to try a new 6810. Sound is back! Thanks.

Just wanted to send a quick thank-you to everyone on this thread; I also replaced 6810, and I now have sound and speech back, too! THANKS!