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C64 250407 No Sync even after replacing numerous parts?

C

Capt. 2110

Experienced Member
Joined
May 23, 2015
Messages
362
Location
Texas
So, I was using this machine a few months ago and, while listening to some SID music, the screen suddenly went black. I tried rebooting it, but there was no sync. I've replaced the following chips with known good ones to no change:
6510 CPU
6567 VIC-II
PLA (It has the 82S100 in it right now)
556 Reset Timer
7406 at U8
7805 5 volt voltage regulator

Nothing else I've seen can cause a no sync issue, other than the RF modulator and the 4 chips next to the VIC. Is there a way to test the RF modulator so I don't waste any time or risk the board replacing the 4 chips next to the VIC? Thanks!

(I also have a newer board with the 8701 that has the same no sync issue, I replaced the 8701, but it didn't change. I plan on swapping U27 next since it's a MOS 7712, but if I should look elsewhere, I'd appreciate advice. Thanks again!)
 
Did you do any other tests before randomly swapping chips?

You say "no sync". The C64 can output via RF, Composite and S-Video. Did you try them all to narrow-down the problem?
 
I tried seeing if the 1541 initializes on boot, and it looks like it does. I did temperature checks and the 6510 was hot to the touch within a few seconds, which is why I assumed it had failed. I checked the voltage regulator before swapping the CPU and it was low (around 4 volts), so I think the CPU had an internal short that broke something else (or vice versa). I swapped it mostly because I don't trust the 30+ year old regulators. Most fail open, but I've seen enough that failed closed to not trust them.

I've tried via composite and RF and neither worked. I don't have a cable (or TV) for s-video, but I'm guessing if composite doesn't work, s-video won't either. RF doesn't change on power up, and composite doesn't give the flash it usually does on power up.

Thanks!
 
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