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Forums > CSDb Discussions > C64 gradually losing sound output, but RF still works?
2020-10-03 12:34
Shadow

Registered: Apr 2002
Posts: 344
C64 gradually losing sound output, but RF still works?

My "daily driver" C64, a nice VSP safe C64C started acting up a couple of years ago. Sometimes sound didn't work at all when I powered it on. Powering it on and off sometimes solved the problem. Other times, simply leaving it running for a while made the sound come back. It got worse and worse, and finally I couldn't get any sound at all.
I've read about SID chips dying left and right, so my fears were that this was what had happened.
However, in my very limited electronics understanding, the gradual degradation seemed more like some like a capacitor or other component slowly getting worse and worse.

Today, I dug the machine out again and hooked it up with my S-video+audio cable that I always use - still no sound.
But then I tried hooking the RF up to an old TV, and sound worked immediately!
So SID chip seems to be alive! Could there be any other (capacitor or otherwise) hardware explanation to why sound output via A/V port doesn't work but RF does?
 
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2020-10-03 12:55
Shadow

Registered: Apr 2002
Posts: 344
Cable and monitor works fine, I get sound when I plug in my other C64 in the same cable, so it must be something internal to that C64.
2020-10-03 13:24
tlr

Registered: Sep 2003
Posts: 1402
Is the picture ok? Could be the grounding in the connector perhaps?

Otherwise I agree with TWW, it seems the audio out the connector is what goes into the RF modulator.

Found this schematic, not sure if it's the right rev: https://myoldcomputer.nl/wp-content/uploads/2015/11/252312.jpg
2020-10-03 13:32
tlr

Registered: Sep 2003
Posts: 1402
It could possibly be that C77 in that schematic is dried out, resulting in a higher resistance in the signal path, then the loading of the S-video+audio input might decrease the signal.

Easy to test. What happens with the RF signal if you simultaneously plug in the AV cable connected to the TV?

EDIT: seems a bit unlikely though as the RF modulator has a quite low input impedance already (pdf p37): C64_Service_Manual_314001-02_(1985_Feb)
Worth a shot anyway.
2020-10-04 00:45
Shadow

Registered: Apr 2002
Posts: 344
Quote:
What happens with the RF signal if you simultaneously plug in the AV cable connected to the TV?


Good call, if I have the AV cable connected, no sound on RF either. If (after powering down of course) I remove the AV cable, sound is back on RF.

The possible conclusion that could be drawn by this was that C77 might be broken?
2020-10-04 08:52
hedning

Registered: Mar 2009
Posts: 2949
Recap the whole machine. Get a new psu or a saver while at it.
2020-10-04 10:29
tlr

Registered: Sep 2003
Posts: 1402
Quote: Quote:
What happens with the RF signal if you simultaneously plug in the AV cable connected to the TV?


Good call, if I have the AV cable connected, no sound on RF either. If (after powering down of course) I remove the AV cable, sound is back on RF.

The possible conclusion that could be drawn by this was that C77 might be broken?


Possibly, yes. Might be that it's called something else than C77 on your board if it's a slightly different revision though.
2020-10-04 10:31
tlr

Registered: Sep 2003
Posts: 1402
Quote: Recap the whole machine. Get a new psu or a saver while at it.

Yes, probably a good idea, at least the power section ones.

I do find that in the bread bins I have those are pretty good quality brands but if they fail they might introduce over voltage on the +5 and +9/12V rails, potentially killing chips.
2020-10-04 12:48
Shadow

Registered: Apr 2002
Posts: 344
Yeah, I'll try to get the machine recapped at a reputable hardware wizard sometime soon!

Checking into the whole save/PSU thing, savers seems quite expensive, so maybe it's better to just pay a little extra and get a whole new PSU instead?

I did some searching and found a few different PSU replacement options:

https://www.c64psu.com/c64psu/43-1367-commodore-64-c64-psu-powe..
https://www.keelog.com/power-supply/#c64

Maybe there are more. Recommendations? Experiences?
2020-10-04 13:47
Frantic

Registered: Mar 2003
Posts: 1460
I have bought a couple of PSU's from Ray Carlsen. They work great:

https://www.carlsenelectronics.net
http://personalpages.tds.net/%7Ercarlsen/custom%20ps.html

Can't say whether other brands/varieties work well or not.
2020-10-04 13:48
TWW

Registered: Jul 2009
Posts: 478
Some things you could try:

- measure for shorts in the AV plug itself.
- measure for continuity between the AV plug output pin and the branch point for the audio out signal to ensure the trace is intact. If it's broken solder in a wire to replace the tracing.
- Try to re-solder the A/V plug to the board as sometimes the solder points on the board can come loose (due to mechanical wear of plugging/unplugging the cable or exerted force to the cable itself while plugged).
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