Commodore 64C no video problem

[7800fan]

I have a 64C (the later white model that looks like mini 128 that has power LED on but no video. Same power supply and video cable works fine when I used an older breadbin C64 so it is not power supply and the video cable is connected correctly to the monitor.

 

Suggestions?

[+sm3]

Hi, a blank screen can be a result of many different failures unfortunately.

 

Take a look at some of these symptoms and what might be their cause: http://personalpages.tds.net/~rcarlsen/cbm/c64/c64c-ic.txt

 

Failed or failing RAM is one of them. Check the ICs to see if any are hot.

 

Maybe download a repair manual or two from this site: http://www.bombjack.org/commodore/books.htm

 

A Dead Test cartridge is useful in situations like this. If you don't have one, an Ultimax type cartridge can be used to see if the issue is bad RAM or ROM. A failed CPU can result in no video as well.

 

I answered a simliar quesiton on the lemon64.com site today. Not sure if you are the same poster.

 

Good luck!

 

Update: Here is a list of Ultimax cartridges in case you have one:

 

Avenger, 1983
Billiards,1983
Bowling,1983
Clowns, 1982
Gorf (MAX), 1983
Jupiter Lander, 1983
Kickman, 1982
Lemans, 1983
Mole Attack, 1982
Money Wars, 1982
Omega Race (MAX), 1982
Pinball Spectacular, 1982
Radar Rat Race, 1982
Road Race/Night Driver, 1982
Sea Wolf, 1982
Slalom/Ski,1983
Speed / Bingo Math, 1982
Super Alien, 1982
Wizard of Wor (MAX), 1983

[7800fan]

No cartridge on hand other than that old parallel printer adapter and disk booster (speeds up disk load) Neither did anything. I checked everything on the board, nothing felt warm at all. I checked for 5v and 9vac on some chips according to schematic and all are present so it seems the power is fine.

 

I did swap the only 3 socketed chips (CPU, SID, and VIC) and nothing changed. That still leaves a dozen chips that aren't socketed to check.

 

Just checking but the CPU in my C64C has different number than the ones in breadbin, are those still same CPU with same pins and spec? If it's not the same, then I would need a known good 8500 to try and rule out bad CPU.

[+sm3]

There was a difference in the 64C models. Do you have the skinny board 64C or does it look like the size of the regular 64 but in the white case of the 64C?

 

Also, here is the service manual for the 64/64C:

 

http://www.bombjack.org/commodore/commodore/C64-C64C_Service_Manual_314001-03_(1992_Mar).pdf

 

You'll find some additional troubleshooting information:

 

Per the manual. For blank screen on power up:

 

Check External Power Supply
U4 (KERNAL ROM), U17 (PLA)
U7 (6510 MPU), U3 (ROM)
U8 (7406 IC), U19 (VIC II)
U9-U12 (4164 RAM)
U21-U24 (4164 RAM)
VR2, CR4, VR1

 

You have to be careful with putting ICs from a regular 64 into a 64C. Depending on the board revision, you can damage the ICs. For instance, the SID 6581 used 12v where the SID 8580 is 9v. Commodore changed to HMOS with 85XX chips which uses less voltage.

[7800fan]

Skinny board, rev A with separate color RAM next to the MMU (which has PLA and other chips combined in one from previous C64's) From what I can tell, MMU almost never fails (unlike the stand alone PLA which seems to burn out a lot) so that still leaves the rest of the chips.

 

Think I'll put this on backburner since I don't have exact CPU to swap with and the rest of the chips are soldered on.