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Williams system 6 sound board help please!

BallyWilliams suck

BallyWilliams suck

Registered
10 Years
Joined
Jul 21, 2011
Messages
465
Hello all...

I don't seem to be getting very far with this Gorgar sound board. After having a bad PIA, and a bad 6810, and replacing several out tolerance caps, I now have sound - although it's so faint I can hardly hear it. However, with my ear pressed to the speaker I can just about make out the test sounds. I assume most of the board is running....

I've jumpered the volume control, so I expected to get a loud hum or static from the speaker - but I only get a faint bit of noise. The Tda2002 appears to have the correct power - so I used a scope on the output (which I think is pin 4), but it's extremely low. Rubbing my finger on the pads where it's connected to does give me a bit of quiet static, so I think it's OK. I was expecting to see the signal coming into the TDA2002, but again, everything seems so low I can hardly see it on the scope screen.

I'm trying to look at a bad photocopy of the schematic, and think I should be getting inputs (at a readable level) on the DAC (IC 13,, pins 5-12) - but again, I'm getting very little. There must be a signal somewhere (!) as I can hear the sound extremely faintly, so it's running.... I appear to be getting inputs on the 6821 pia (26-33?) , but no output on pins 2-7 which feed the DAC. I've checked and replaced the PIA, but still the same. Could anyone suggest a way forward please?

Many thanks!
 
I always jumper the volume pot in the same way when I'm bench testing these. Just a straight link between the pins, which usually gives full volume. You can use a resistor between the pins to lower the volume, but I don't usually bother for bench testing.
 
Thank you - I did check out the sites, but most of them are explaining how to get the boards running - which this one seems to be. The sound output should be the easiest part, but I can't work out the theory of operation from the pulses out of the PIA, as it looks like they're being stopped by something externally. The data and control busses appear to all be running the correct signals, but I'm unsure on the exact signal path to follow the generated sound signals through to the amp.
 
There is a much better schematic on the first link , which I did miss - thank you!

It seems pretty simple that the outputs of the 6821 are on pins 2-7, which feed the DAC. Unfortunately, there doesn't seem to be any outputs, despite good inputs. The 6821 is fine, and I've replaced it twice to make sure! Something externally is almost muting the output of the PIA.
 
Having slept on this, I've realised it can't be that complicated. If I'm able to hear sounds (even very quietly) then the DAC must be working, and it must be receiving the correct signals via the PIA. It must be the amplifier stage causing the issues. I'm just unsure why those signals are extremely low. I need to check of there's a broken link in the volume control section - although I have already checked this, and it seemed OK....
 
I don't think that there is digital volume control here. So the PIA will output the digital data stream, the DAC converts it. Preamp is Q2, have a look there.
 
drhex said:
Nice schematics here: https://pinwiki.com/wiki/images/d/d9/18-soundlogic.pdf
Pin 4 of the TDA2002 is the output indeed. What do you have at Q2?
Click to expand...
Thank you drhex. I don't seem to have a readable signal coming out of the DAC towards Q2, but there must be one, as otherwise I'd not have the sounds at the speaker. It must just be extremely low. What I need to locate is where the level is altered. Although, it doesn't explain why I can't see readable outputs from the PIA into the DAC, or out of it, as it seems the volume control section is AFTER this point.... Aaaaaargh
 
There must be something otherwise you would not hear anything. There is enough signal on the collector of Q2 to drive a small speaker, you should be able to see that with your scope. Also, there must be signals on pin 2-9 of IC10 (PIA) which drives the DAC. DAC out is on pin 4 - BE VERY CAREFUL TO NOT SHORT TO PIN 3 (-12V) if you probe as that will blow up the DAC. Better check continuity to Q2 collector from pin 4 when off and probe to the former. Touch the solder points of the TDA with your finger, you should get hum - if that doesn't happen, the TDA is toast or its power supply is out.
 
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