Vanguard wrong/missing sound fix .....and the ongoing static hiss issue

TheGond

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Hi guys

Recently acquired one of my all time favourite games from back in the day - I put so much lunch money into this every day for about 6 months ! Don't tell my mum though ;)

When I first got it I immediately realised some sounds were not correct.

#1 The player shot/firing was an extremely high pitch and annoying 'pew pew'.
#2 There was no explosion sound when the enemy ships or your ship exploded

I'm new to all this so after much googling I found someone with missing sounds suggesting replacing the LM3900N Op Amp at IC37 can fix some sounds. This is on the main board hidden below the small speech board.
I removed, socketed and replaced it . No difference.

After more googling I then did the same with the LM324N at IC46.
Success ! I have the correct ship firing noise and all the explosion noises back (y)

See pic below for the 2 ICs I replaced.

So, very happy to have the correct sounds HOWEVER with a working LM324N in place I now have the *loud* static hiss present - which I believe is a very common issue.

This wasn't present when I had the missing sounds and the faulty LM324N in situ. So I'm assuming it must be something to do with this part of the circuitry ?

After reading around I'm failing to find a definitive fix for this issue. I see some suggestions regarding adding caps or more grounding. I also see folk saying you won't fix it and just live with it ? This seems pretty odd - surely they didn't come out of the factory like this ?

Any new thoughts on this ? I appreciate there are a few older threads already on this but resurrecting them doesn't always gain traction.

Thanks in advance.

Vanguard.jpg
 
The 4066 could be introducing noise. Otherwise it could be a sound generator chip, like the SN76477. If you have another LM324 it also might be worth swapping to ensure the one you replaced is 100% good.
 
The 4066 could be introducing noise. Otherwise it could be a sound generator chip, like the SN76477. If you have another LM324 it also might be worth swapping to ensure the one you replaced is 100% good.

Thanks

Yes I tried another new LM324 and no change to the issue.
 
Unfortunately the schematics are drawn horribly.
Best thing to do is start pulling the .1u (104) caps that couple sounds into the amp to isolate which discrete circuit is causing the hiss.

Good chance it's a leaky electrolytic in one of the enables that's preventing one o the sounds from turning off.
 
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