Tron noise/sound/ground problem

liquidx

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Hi,

I have a tron that's basically producing noise/thumps/clicks instead of sound. Running the channel test gives me these cracks and thumps on all 6 channels. All the usual things were done, switch AY3s, check voltages, new ribbon cables etc.

Anyway, after moving around with a logic probe I noticed that when I touched the ground pin on the LM3900 with it I could hear the sounds faintly, so I guessed I might have a grounding problem. And, indeed, when I jumper the ground pin (7) to the board ground I get all the proper sounds and music.

Looking at the schematics I see there's a separate ground for the analog/audio section which is connected via the amp board down to the 12V output of the isolation transformer. I checked this for continuity and it looks ok.

So, my hack works but it's obviously not the way things are supposed to be as I'm basically connecting 2 grounds that were designed to remain separate.

Not sure where to go from here apart from the capacitors but I must be missing something.

By the way, this problem exists both with the original power supply and a switcher adapter.

Thanks
 
ON MINE IT WAS DOING THE SAME THING.. i had a broken shield ground at the audio cables connection to the pcb.. also the terminals lost tensiona nd werent grabbinmg the pcbs pins properly. Check that cable... i almost guarantee theres an issue on one end or both.. also check for cold solder connections on those pins of the pcb and of the audio amp pcb
 
I thought about that but I have continuity from the analog ground pin of ICs on the board right down to the power supply, even if I wiggle things about I still get a nice constant tone from the DMM.
 
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