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If you know anything about the Commodore 64 (and other Commodores, really) you know that Commodore really went the cheap route and produced some awful power supplies. The Commodore 128 PSU isn't quite as bad as the C64 version, but it's nowhere near perfect.

If you open up the PSU for a C64, you will see that the insides of the brick are completely filled with epoxy. There is no way to salvage the parts or repair them.

My assumption is that Commodore didn't want average folks repairing their own PSU when they went south (which happens a lot). And, Commodore would much prefer you buy a new PSU. But was that really the reason? Or, did Commodore think they were making them safer by doing that?

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    It stops humming from the transformer.
    – Jan Mattsson
    Commented Aug 7, 2017 at 16:34
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    It was handy when my C64 power supply failed and I kept it going by putting it in a bucket of icewater. When my mom found out, she made my dad buy me a replacement, saving me $50 in precious allowance money.
    – Ben Jackson
    Commented Aug 8, 2017 at 6:53
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    @Nelson Potting is one of the most effective ways to keep water out of electronics. As for how it fixed it, it likely had a fault causing overheating.
    – SomeoneSomewhereSupportsMonica
    Commented Aug 8, 2017 at 9:40
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    Over 30 years later I still have a scar on the second knuckle of the first finger of my right hand from trying to open a C64 power supply. That epoxy is sharp in places.
    – Tim Locke
    Commented Aug 8, 2017 at 15:08
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    Being a young computer dork with a C-64 and living in the middle of nowhere - when my C-64 power supply fried my Dad fixed it for me. He carefully cut away the epoxy, reverse engineered the circuit, built a new one, and put the whole mess in an old chassis left over from some earlier (read: 1960's) project. I still have that power supply, and it still works perfectly. It's not impossible to salvage or repair a potted C64 power supply, it's just really hard.
    – Geo...
    Commented May 24, 2019 at 16:24

2 Answers 2

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Epoxy offers two advantages -- it is an electrical insulator, and it conducts heat better than air. Transformers and inductors are generally potted with epoxy for this reason. [ref] Perhaps the cause of the high failure rate is that Commodore engineers decided they could use cheaper components to build the power supplies, depending on the epoxy properties to balance out any deficiencies.

In 2012 Bil Herd (Commodore engineer) speculated:

They were made for CBM by the boatload, they got warm and were not rated for things like the CBM cartridge [Bil probably meant CP/M cartridge], etc, though they would continue to make voltage they would just run hotter reducing their life. Potting them was probably to protect CBM as it's hard to start a fire from within a pound of epoxy. The potting would have made the hottest components slightly less hot and everything else too hot.

Remember that the VIC 1 had started at least one fire, I think something like 3. The case used to melt and sag over one of the heat sinks. Supposedly a programmer tried to design apiece of the power supply without really knowing heat and wattage calculations. So a good reaction would be a supply that never caught fire, was impervious to static and even water, and was a bitch to air ship.

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    That is an excellent answer. And, it makes total sense. I guess Commodore wasn't as stupid as I thought they were. Too bad they didn't design a better PSU before they potted it. :-)
    – cbmeeks
    Commented Aug 7, 2017 at 17:22
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    The explanation I remember hearing at the time was it was done for safety reasons, maybe even specifically to get certification, and that the usual failure mode was that expansion and contraction would eventually crack the voltage regulator or break one of the connections.
    – user722
    Commented Aug 7, 2017 at 17:43
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    Even without overloading those C64 PSUs died regularly. I used to fix them by piggy-backing a new 7805 on and replacing the removed case bottom with a generous heatsink - did at least two dozen of them.
    – Zac67
    Commented Aug 7, 2017 at 19:27
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    FYI, "CBM" is Commodore Business Machines.
    – RBarryYoung
    Commented Aug 8, 2017 at 14:10
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    @RBarryYoung Thanks, but I know. If you are responding specifically to my internal quote note where I referred to "CP/M", I believe in that one instance Bil meant "CP/M cartridge" and not "CBM cartridge". The Z80 processor and other components within that cartridge would have added a noticeable load on the power supply, which in context is exactly what is being discussed at that point. FYI, Bil Herd is the lead engineer for the C= 128, and the one who decided to add the Z80 to its motherboard. So it makes sense that he would specifically reference the C64's CP/M cartridge.
    – RichF
    Commented Aug 8, 2017 at 16:41
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Electricians some times (or used to) do similar techniques when configuring wiring -- once the wires were in place, they would fill the cavity with a non-conductive resin or epoxy, so that the chance of any movement or shift in the wiring would cause a short or a disconnect is greatly reduced. I've seen this in numerous situations myself, including air compressors, where there is quite a lot of vibration.

Personally I think it was a cheap way to make their power bricks last longer because people move those things around, they get kicked a lot (I had a C64 for a very long time), and probably had to hold up the weight of many-a-12-yr-old when they stepped on it, because well, they're 12 and they can.

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    I would also assume that the design was more to avoid mechanical than electrical damage - A power supply of that time had to consists of some pretty heavy components (transformator, large capacitors, large inductors) that were hard to mechanically fix with the solder joints on the PCB only.
    – tofro
    Commented Aug 8, 2017 at 10:25

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