C-64 Black Screen and Voltage (Bridge Rectifier) issue.

[DistantStar001]

I've been working with a C64 motherboard (Rev. 250407) that has developed some issues. At some point, it just stopped giving a picture. Not a black screen, just no video signal at all. At first, I thought it was the ICs responsible for the timing circuitry, so I replaced them with no change. Then I noticed a broken trace on the back (not sure if it was there before or after I replaced the ICs, but once patched, I get video again. Unfortunately, all I'm getting is a black screen. The VIC II has tested good in other machines, the SID has been removed (but not tested yet), the PLA is PLAnkton replacement and is working to the best of my knowledge, all other ICs are soldered into the board at the moment, so I haven't been able to individually test. Even so, I think they're fine. Obviously, I can't be sure, but the 5v DC rail that feeds into the ICs is good. The voltages that come off of the 9v AC rail are another story. The output on the Bridge Rectifier is only 5.72v (compared to 8.2v on one of my working C64s with the same board revision), which is what I'm also getting on the input of the 7805. The output, however, is only 3.2v. The 7812 doesn't seem to have a problem and is putting out 12v even. This tells me that the VIC II isn't getting the right voltages, which is preventing the computer from booting.

 

The PSU is from Ray Carlson and works perfectly with at least five other C64s. So I think I can safely eliminate it as a potential source of my problem. I have also swapped out the 7805, and large capacitors with no change. 

 

I'm leaning towards the rectifier as the culprit, as it's the next thing up the line and directly connected to the switch, but wanted to know if there were any other components that could be an issue? 

[Markeno]

Poor contact inside the power switch possibly, that causes the voltage to potentially drop at the switch.  Is the input voltage low on the Bridge Rectifier(remember that will be AC not DC)?  If the input is about what it is on your other C64 it won't be the switch at fault.  

5.72V is to low for the 7805 to be able to output 5V.  The 12V get a Voltage Doubler before it so it is possibly still getting enough voltage to work with.

If the Rectifier input is correct, but the output is not, then it may be one of the diodes has failed in the rectifier and it needs replaced.

Also maybe a bad filter capacitor draining off the power or not keeping up?