Maybe you don't. You can only be certain by testing your machine with a known working one from a friend or whatever.
Even if your PSU is OK, if one was faulty and it sent overvoltage to the C64, your machine will now be screwed and wont work with any PSU.

Yea, I measured as an AC line on both supplies and they display between 4-7v. How can I have two fucked PSUs?

Uhg, just gonna buy a supply for Ray Carlsen....

Sounds like your PSU got fucked and took your C64 with it, a classic.
Did you measure the 9V line as an AC line?
Anyway, get a proper PSU and then see how much damage it did to the computer.

Ok, so, I tested out my power supply (supplies, as it turns out) as instructed. Here they are:

One of these came from a VIC-20 I purchased a while back. I'm honestly unsure of which is which. I mixed them up during testing a few months ago. Is there a serious difference between them? Anyway, both of them put off 5v on pin 4/5 fine. I could not get pins 6/7 to read 9v. It always hovered around 4-5v. Initially spiking at 7v. It also seemed like it was either pin 6 or 7 putting out power rather than BOTH. This happened on both power supplies. Do I have my multimeter on the wrong setting?

Could both supplies be malfunctioning? Which one should I be using with my c64?

In relation to one of my earlier question about chips, here's my pcb:

ASSY NO.250407 REV.A

I've heard that "fried" chips get a glossy sheen on the top of them rather than the matte finish they usually have. It seems like the two chips in the upper left have a different texture than the rest. Are they fried? I did notice some funny performance when loading up Turrican. Sometimes the whole system would slow down when certain effects were used on the title screen. Maybe it was just a bad crack of the game but it seemed almost as if strange characters were showing up instead of the proper effects.

Thanks again for your help!

without a load on the transformer the AC output will be closer to 12v.

Maybe I can ask a general c64 PSU question: what's considered acceptable for the AC voltage? Seems like I've seen ~12v, is that a problem?

Maybe I can ask a general c64 PSU question: what's considered acceptable for the AC voltage? Seems like I've seen ~12v, is that a problem?

You've been very didactic Kvee, that's kind of you. I did the same for my PSU, except I wasn't careful enough and I shortcut two pins, which burned the fuse... I suggest you use a plastic cylinder around the pins (from a ball pen maybe?), that could help to isolate the outer ring and other pins.

Wow, thank you so much. I'll give this a try tonight. You've been very helpful!

I'm gonna assume you have this model: https://www.amazon.com/Craftsman-11-fun … B00GUJK7GW

1. Open this: https://www.c64-wiki.com/index.php/Powe … _Connector.
2. Set the meter to "VDC"
3. Put the black cable in "COM" and the red one in the connector that has "V Ω CAP Temp Hz μA mA".
4. Plug the PSU in the wall outlet
5. Look at the picture in step 1, connect the black test lead to pin 2 (bottom, GND) and the red to pin 5 (you can also try 4, but there might not be anything depending on the PSU model). This should read a voltage somewhere around 5 V (some small difference is OK).
Be careful to only touch the specified pins and even more so to not short anything with the test leads (don't even touch the metal outer ring)! This can be tricky - fixing the PSU connector in place is recommended, also maybe rest your hands against something if they're not steady.
6. Remove the test leads and switch the multimeter to "VAC"
7. Once again put the black lead to pin 2 and measure pins 6 and then 7 with the red one, they should both be at about 9 V.

That should about do it.

(For a bit of a background: The meter (in VDC / VAC modes) measures the voltage difference across the COM and the right socket. COM means a common wire (that has the same voltage in a system) - it's usually used for ground and that means you'd expect 0 V on it. Black color is commonly used to mark that. Then the red lead is used to compare any voltage in the system against the reference/ground/common voltage.)

You know, I would love to but I have no idea how to do that. I do have a Craftsman 11 multimeter - maybe now is a good time to learn how to use it? Care to give me a tutorial?

Hopefully your power supply didn't crap your system. They are "suicide" power supplies. They didn't have any built in protection in them for when they go bad.

Could you try to check the psu voltage first? (I mean voltages)

No problem!
Cool, didn't know about this site. The note about voltages reminds me though - the rated voltage must be the same or bigger. Don't replace with smaller values, unless you're sure. They probably only warn you because they send some with bigger voltage rating (and that is OK).

If you by any chance run into a situation where the original caps are rated higher, it's better to double check with the schematic or ask someone to check for you. Generally, most of the C64's internals run on 5 V or 12 V, but the internal power circuit could require bigger values.

Thanks for the response! I'll definitely be looking in to everything you talked about.

And just in case you didn't know about this place:
http://store.retroleum.co.uk/c64-capacitors