Bizarre issue with NTSC C64 - running too fast?!?

[ComputerCompulsive]

Hi there,
Between parts for my various other projects I decided to boot up one of my C64s and verify that a disk drive I'm about to sell is still working. Well the good news is the drive is working the bad news is that the C64 has developed a couple bizarre flaws.

1) The disk drive was constantly spinning and would not stop when connected to the c64
2) When I tried playing a cartridge game it played both the game and the sound WAY too fast! (see video)

C64 Glitching Video

Wondering if anyone has any idea's what I should look at for faults? Would this be a timing chip failure? (MOS8701) or is it a weird memory issue?

 

[3lectr1c]

and they said you couldn't overclock a c64...

 

[ClassicHasClass]

Check your CIAs and that you have a proper 60Hz signal on the AC power supply lines. That looks like the Timer A interrupt going crazy.

 

[ComputerCompulsive]

Check your CIAs and that you have a proper 60Hz signal on the AC power supply lines. That looks like the Timer A interrupt going crazy.

Well I've tried to swapped the CIAs (from one socket to the other) and it seems the same. How would I check the 60Hz signal please? I know the power supply I have is good as it's brand new and the multimeter shows the right voltage. It also works on another breadbin just fine.

 

[ClassicHasClass]

If you can't replicate the same issue on another breadbox with that power supply, then the 60Hz signal is probably OK. Do the symptoms show up with other software?

You could try replacing the 8701, sure. However, I would expect a failure there would just black-screen the entire system.

 

[ComputerCompulsive]

If you can't replicate the same issue on another breadbox with that power supply, then the 60Hz signal is probably OK. Do the symptoms show up with other software?

You could try replacing the 8701, sure. However, I would expect a failure there would just black-screen the entire system.

Same game cartridge worked fine in another c64 and couldn't load a floppy disk to test as the drive just kept spinning when attached to this machine. Same drive attached to the other c64 worked fine. Very peculiar.

 

[ClassicHasClass]

For the runs and runs part, my 64 service manual suggests checking the 7408 at U8 (and, of course, CIA #2 - always possible you have a double failure, though that wouldn't be my first guess). However, that doesn't explain the rest of what you're seeing, so I'm out of ideas.

 

[ComputerCompulsive]

For the runs and runs part, my 64 service manual suggests checking the 7408 at U8 (and, of course, CIA #2 - always possible you have a double failure, though that wouldn't be my first guess). However, that doesn't explain the rest of what you're seeing, so I'm out of ideas.

I'm thinking of getting a diagnostic cartridge and see if it points out a failed component. The old dead test cart I have seems to think everything is fine including the CIA. That said I only let it run a short while; maybe I should see if they start to drift if I leave it running for a while.

 

[ComputerCompulsive]

Diagnostic Cartridge came in and it doesn't find any faults I'm afraid :/
I did notice something while I was testing though. If I leave a serial cable hooked up to a 1541 drive and power on the c64 the drive spins forever. If I then unplug the drive and put in a cartridge I get the super fast error on the c64. If I unhook everything (cartridge, drive, power) let the system sit for a minute or two and then plug back in the cartridge and power the game boots up normal and is fully playable.

I tried swapping the CIA's from one to the other position and the behaviour is the same. It's not the drive or the cable specifically at fault either as I can swap to a different drive and cable and the same issue plus I can hook any of them up to another c64 and it works fine. It has to be something involving the connection to the disk drive. Thinking maybe its U8? Any other ideas please?

 

[ClassicHasClass]

That's all I got. You've stumped me on this one.

 

[ComputerCompulsive]

That's all I got. You've stumped me on this one.

Well it's not U8 as I've replaced U8 with a chip from a spares board and it does exactly the same thing.

So something new I just noticed is that the 1541 drive spins forever (without ever finishing) if it's hooked up to the c64 serial port even when the c64 is otherwise completely unplugged and turned off! Makes me wonder if the problem isn't with the serial port itself rather than a chip since the c64 board is completely without power when this happens.

 

[ComputerCompulsive]

It has been fixed!
Huge kudos to Baptiste Hobbé from AnalogThinker for doing the hard work and finding all the faults!

So here is what Baptiste told me he found. He said ultimately there were two separate faults at work causing two very different symptoms.

The First issue was the super speed. As it turns out the 6510 cpu socket solder joints had gone bad and the board was tweaked by inserting the serial connection a certain way the machine would run super fast! Re-flowing the solder joints on the socket addressed this problem.

Second issue was the bizarre disk drive behaviour. The drive would spinning continuously without stopping when the serial connection was connected to the c64. As it turns out there was a transistor on the c64 that was acting like a diode in reverse! Baptiste replaced the transistor and BAM! disk drive is back to normal!

Awesome work!

Edit:
For anyone with a similar problem it was transistor Q3 on a revision 250425 board that was swapped out.

 

[ClassicHasClass]

Great detective work. Glad another system is restored to function.