Recapping C64: What did I Break?

[Vinnie D.]

My breadbox C64 had suffered a failed PSU in the past forcing me to replace the 3 big capacitors and the fuse, and get a new PSU (this one has proper overvoltage protection). Well when I had it back up and running, something appeared to be wrong with Vsync. A line if distortion continually rolled down over the screen. It was an annoyance, but trying both my PLAnkton and the stock PLA had the same result. I decided to finish the recapping job, and this is where my woes began.

 

During the process I accidentally broke a pin off my JIFFYDOS. I put the stock ROM back in, and when I was finished the system booted up fine, and the annoying Vsync issue was gone. I thought to myself I could fix the JiffyDOS by soldering a couple of new pins to it. For extra safety I put it on a riser, so that these now hodgepodged pins weren't going straight into my motherboard's socket. This got me a black screen for my troubles. "Fine, I'll just use the stock ROM and get another JiffyDOS later" I thought. Sadly when I put the stock ROM back in the system won't start, and gives me only a black screen. I do notice that the light comes on, and accessories power up though. I also thought that if the problem is only the ROM then a cartridge might still work. That gave me varying results. Radar Rat Race gave me a white screen with border, but garbage inside. Others gave me a black screen.

 

I also noticed the rectifier at CR4, as well as the 2 voltage regulators warm up even when the system is switched off. The CR4 in particular gets quite hot. I'm starting to suspect an unrelated voltage issue arose at the same time. Perhaps leftover of the previous problem with the bad power supply.

 

Any suggestions? I'm out of ideas, other than start replacing components. I've checked and double checked the caps to make sure they're all the right ones, and in the right way.

[Vinnie D.]

Update:

 

I noticed that CR4 is now stone cold, and it's not even getting off the black screen with a cartridge, so clearly the situation has degenerated further.

 

Went in with a multimeter and started checking voltages. First I noticed that C102 was reading zero volts when it should read 5. Removed and checked the cap and it tested good. Tested the positive end against ground and got nothing. Ok so no voltage there. I found a pretty good explanation of the voltage rectifier circuit, and C102 is one of the last stops for smoothing out voltage, so I opted to go upstream. Next stop was the voltage regulator at VR1. It's putting out around half a volt, not even close to what it should read. I go upstream to the big capacitor at C19, and I'm again reading half a volt when it should read between 9 and 12. Next stop is the bridge rectifier at CR4. I measure AC across positive and negative and get 0.1v. Getting near the beginning of the line here. I test resistance on the fuse and it's a safe 3ohm. Fuse is good. Just to be extra sure I went up to the switch, then the plug, and finally the PSU its self (a Commodore 4ever Atom), and all tested good.

So it seems the culprit is the bridge rectifier. This is probably the source of a lot of my woes I've been having with dying power supplies (gone through 3 of them), and blown fuses and caps. It may be a caveman simple piece containing just 3 diodes, but when it fails it can produce lots of weird symptoms. Additionally there seems to be very little information out there about the bridge rectifier in regards to the C64. It's a Z2VR which I had some difficulty in finding.

 

I don't think they make it anymore, but I did find an equivalent that I hope will take its place. Either it was in the process of going out already, or the strain of fresh capacitors finished it off. At this point I think attempts to fix the JiffyDOS and switching back and forth between it and the factory ROM were purely coincidental. So now I've ordered a replacement, and hopefully this will bring the C64 back up and running, or at least to a state where I can see some progress.

[Osgeld]

a full wave bridge rectumfryer is 4 diodes, not trying to be an ass, I just have to

 

and yea any one that can handle the load should do the same basic job BUT it may not be the end of your troubles if something narsty is going on with the power supplies

 

putting in a nice new part that is not weakened might pop up problems, but they should be upstream instead of down (though the symptoms will be downstream confusing you )

 

 

good luck

[Vinnie D.]

Update:

I replaced the bridge rectifier. The old one did not want to come quietly (spiteful old bastard), but I managed to get it out of there. There is improvement, but it's still not booting. I've got a good 16v DC coming out of the bridge rectifier now, which according to the C64 service manual is the proper voltage. I'm getting higher voltages after that, but they still decline to unusable amounts before reaching VR1. So the good news is that the bridge rectifier and everything upstream of it are working now, but still having issues downstream. What's more annoying is that I notice the decline begin at C19, which is a brand new capacitor. I think either I've nicked a trace somewhere, or the old bridge rectifier decided to eat my new caps as well on its way out.

 

I still can't be sure if any ICs are damaged, because I am yet to get usable voltage that far down the board. Progress is being made, but I don't know when or if I can get this thing working again, or how much of the original machine will be left by then.

[Vinnie D.]

Correction. That should be 9vDC coming out of CR4, but no fear there, it's only putting out 1.6. Looks like this isn't the right bridge rectifier to replace it. I'll just get some diodes and build my own. In the meantime I've got another motherboard on the way, so I have a replacement if I can't fix it, and something to compare the chips against. If this gets too much more frustrating I may just send it off to get repaired, and end up with 2 C64s. At this point it's not even computer science anymore, it's electrical engineering.