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Question regarding C64 power brick voltages

br44

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Jan 19, 2017
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Hey folks!

I picked up a bread bin C64 recently, and I'm building a new power supply for it. I'm trying to find a range of voltages that are safe, sort of like a GO/NO GO gauge for my supply. All the sources of information I've found have been very conflicting (been researching for a few days). Some people are saying 4.7v-5.4v dc is safe, and 8-11v AC is safe. Others are saying 4.9-5.1 dc and 9v-10.5v AC is safe. Some people claim it should be dead nuts on 5v and 9v.

So what are the minimum and max safe voltages for the 5v DC rail and the 9v AC rail? Is it as critical to get the 9v rail close to 9v as it is to get the 5v rail close to 5v? How much headroom do I have here?

Thanks!
 

VERAULT

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It doesnt need to be dead on, thats ridiculous. IT should "Operate" at 4.4Vdc to 6.2vdc The 9V AC rail is what you need to worry about coming out of the wall wort. If its above 11 when the machine is on dont use it. If its putting out 12 or 13VAC when its not plugged in its suspect.

There is no minimum for the 9Vac line it running high and out of spec is the issue.. In a nutshell buy a new modern psu as the old wall worts are constantly frying systems.
 

br44

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In a nutshell buy a new modern psu as the old wall worts are constantly frying systems.
That's why I'm building a new supply. I never even bothered using the original C64 brick, I just immediately gutted it and salvaged the useful bits.
 

br44

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Ok good, your off to a good start.

Dont forget to reference Ray Carlsens documents to find information you might need.

I bought his new PSU for all my C66, C128 and Amiga needs. Its built like a tank.
So it seems that according to Ray Carlsen, the ram is the most sensitive chip to over voltage, with an absolute max of 5.5v. He mentions that original commodore power supplies sometimes seem okay when they're cold, but then end up producing too much voltage when they get hot, thus popping chips. Well now.... it just so happens that this C64 of mine has EVERY SINGLE LAST ram chip socketed and coated in flux. Which means the previous owner was in there replacing ram chips. HMMMMMMM, I say. I think that's exactly what happened to the previous owner. The dang supply popped all the chips, probably multiple times.

Reading onwards, Ray Carlsen makes no mention of the absolute max safe voltage for the 9v ac rail, but he does say that it should come "directly from a transformer". If that's the case, and he knows what he's talking about (I'm sure he does), he should know that transformer voltage levels vary wildly depending on what the mains line voltage is on any particular day. Which leads me to believe the 9v rail is not a major sensitive issue. I managed to salvage the original commodore 9v transformer from the original brick. It produces 10.2 volts unloaded. It was certainly fun nutcracking that thing out of the original epoxy! I have no reason to believe this transformer is cheap or likely to fail, after all a transformer is really HARD to mess up. They're pretty simple devices. So I'll reuse it.

Searching through my stash of junk, I found a 5v 2.6 amp switching power supply. Unloaded it produces 5.2 volts right on the nose. He states that Commodore came out with a supply later on which did 2.5 amps, and mine does 2.6 amps.... so PERFECT. It also produces less voltage than the absolute max.... so PERFECT. And best of all... it's free.

So I think I'll give it a day or two for other people to chime in. Unless someone comes up with a reason why my new circuit won't work, I'll build it at that time.

Also, I attached the document from Ray Carlsen that I am referencing in this post.
 

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VERAULT

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i have never tried to reuse a bad c64 wall wort but seeing its ib epoxy its bot worth the work. Someonw correct me if I am wrong but the issue with thr 9v rail going high is the capacitor in the wall wort fails.
 

jafir

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I thought it was the 5v that went high, killing everything. Something about the regulator shorting out.
 

br44

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I thought it was the 5v that went high, killing everything. Something about the regulator shorting out.
It is. The 7805 regulator commodore used was cheap, the rating was too low (which caused it to run hot) and it couldn't dissipate that heat due to the epoxy. This caused it to sometimes (not every time) fail shorted. Sometimes they fail open.

If someone was so inclined, they could theoretically fix the original supply by recapping the entire thing and replacing that 7805 with a better one. That is pretty impossible for the epoxied units though, you'd have to stick with the non epoxy ones. It was a pain for me to just salvage the 120v-9v transformer out of my epoxied brick from hell.

Honestly I'm not too upset with commodore. While the supply was a bad design, it was clearly built to a cost. In units of millions, every penny counts. They figured it would last 3-5 years, and that's exactly what it did. And in 1982, a penny was worth more.
 

jafir

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I’ve got a commodore one that isn’t epoxy-filled, so I’ll probably do that to that one. I’ve also got an aftermarket “Billion” brand one that is also repairable.
 

br44

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Jan 19, 2017
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Update!
So I assembled the power supply and tested all the voltages. Unloaded the 9v rail was reading 10.3-10.4 today. Loaded it reads 9.28v ac. Lovely!

On the 5v rail, unloaded it was reading 5.2 volts dc. Loaded it reads 5.02 volts dc. Also lovely.

My grand total in this power supply project is.... zero dollars. Yeah. I used only junk I had laying around, like an old 5 volt wall wart for who knows what long dead gadget. And I reused the original commodore 9v transformer, because it's a transformer and they don't really go bad. Probably the most expensive part of this project was the tube of Permatex Ultra Grey I used to glue the transformer into the original case. I could have used anything, but it's what I had laying around. Next time I might just use hot melt.

Now I just need to fix the actual C64 itself, but that's beyond the scope of this thread. Here's some pictures of the brick though. Can't even tell when it's all sealed up. Just gotta wait for the permatex to dry completely so I can scrape away the excess. I sort of just boogered a lot in there so I could use up the last of the tube.



IMG_20220410_2155461.jpg
IMG_20220410_2157051.jpg
 
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