Author Topic: Commodore 64 murder mystery - The power supply  (Read 1964 times)

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Offline jcrubin

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Commodore 64 murder mystery - The power supply
« on: October 30, 2017, 12:28:05 pm »
My commodore 64 power supply video with a halloween twist. Enjoy!


Offline Kleinstein

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Re: Commodore 64 murder mystery - The power supply
« Reply #1 on: October 30, 2017, 01:58:52 pm »
Using a center taped transformer and 2 diodes is nothing unusual - it saved 2 diodes at the cost of a little more copper in the transformer. So it was common in the old days, especially at 60 Hz.

Having some ripple at the filter cap and with load is perfectly normal. At that power level there is no requirement for PFC, not today and definitely not at the time when the C64 were build. The regulators are there to take care of it.

The main bad thing on the supplies is that they run rather hot. Chances are good the 5 V regulator has thermal protection and short circuit limiting. A fuse for the AC part might be a good idea though.  Having the external fuse for the AC part could be a bad sign - there might be no thermal fuse in the transformer than. Usually one should have at least one of these.

Offline Bruce Abbott

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Re: Commodore 64 murder mystery - The power supply
« Reply #2 on: October 30, 2017, 04:05:16 pm »
The most common C64 power supply fault I found was dry joints on the 5V regulator pins, caused by the thermal expansion and contraction that occurs with power cycling. This problem only affected potted power supplies because the regulator body was held in place by the epoxy so it couldn't move relative to the PCB, forcing the leads to break the solder joints as they expanded and contracted. If the ground lead broke free the regulator would put out ~8V, which would kill most of the chips in the C64.

I would not use a potted C64 power supply without first chipping the epoxy off the PCB and resoldering the joints on the regulator.

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Offline GlennSprigg

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Re: Commodore 64 murder mystery - The power supply
« Reply #3 on: December 07, 2017, 12:09:24 pm »
Excellent points Bruce !!!
Too often we plug in 'old' stuff to see if it runs!, without checking the 'Power-Supply' properly FIRST.
It's often believed that if the P.S. fails, then it will simply "Go Low", but that is not the case.
The voltages MAY also appear OK, UNLOADED, but upon any real load, they collapse.....
Certain OTHER power-supplies actually NEED a 'somewhat real' load, to even work in the first place.
The WORST, is when circuits/components fail HIGH !, (some regulators & switch-modes).
If the 'load' is 'TTL' based, then such over-supply voltages will cook your pretty PCB bits...   :(

Offline Halcyon

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Re: Commodore 64 murder mystery - The power supply
« Reply #4 on: December 23, 2017, 11:02:04 pm »
I've been guilty of that myself, including with a C64. Luckily nothing blew up, but I've stopped using the PSU that came with it.

I'm all for keeping things as original as possible but sometimes, you just need to replace components in order to safeguard the entire machine.

Those old AT computers (with the latching on/off mains power switch) are another example. I always replace those AT power supplies with a modern ATX power supply and use an ATX to AT converter cable to adapt the new PSU to an old cluncking power switch. It operates exactly the same way, but instead of switching mains voltage, you're just shorting the ATX soft power pins. You can't tell the difference unless you actually pulled the cover off the machine.

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