Old Power Supplies, revisited

From: Joe <rigdonj_at_cfl.rr.com>
Date: Sun Apr 28 00:00:42 2002

At 11:07 PM 4/27/02 -0400, you wrote:
>I am putting my SWTPc 6800 back together, after complete disassembly
>for cleaning. So, on to testing the power supply.
>It works! But here's the question, which I know has been discussed
>before with respect to S-100 machines.
>The power supply is a big transformer with one really big (91,000 ufd)
>capacitor, and the usual rectifier stuff, etc. The documents call for
>7-8 volts unregulated, along with + and - 12V.
>I am getting something like 9.3 V, and + and - 14.3 V. Are these
>voltages too high?

   Those readings sound about right for a PS with no load. You have to remember that this is a totally unregulated circuit and that the caps will charge to the PEAK voltage instead of the average (RMS) voltage since there's no load to draw their voltage down.

 Of course, on the SS-50 and SS-30 cards there are
>voltage regulators, but I don't want to cause any damage. This thing
>probably hasn't been powered up in 25 years.

  Assuming that the PS still puts out the same voltage under load, your regulators should handle 9.3 and 14.3 volts, at least for a time. The problem with excessive voltage in a situation like this is that the regulators will have to drop more voltage (9.3v -5v) vs (8v -5v) and to do that they convert it to heat. So they have more of a tendency to overheat. But my guess is that once you put a load on it you'll find that the PS voltages are nearer to normal and the regulators will handle it just fine.

>How can I check the capacitor function without an oscilloscope or other
>fancy equipment?

   Connect a dummy load and check the PS voltage with any decent meter. If the caps are shorted the voltage will be zero. If the caps have little or no capacity then the voltage will be low. How low depends on the current capacity of the PS, how much capacity that the caps have and how much of a load is applied so it's MOL guess work. You can use just about anything for a dummy load. Light bulbs would be a good choice but I've used old disk drives and all sorts of things that were disposable. I'd start with something that would draw about 1/10 of the rated current and work up to perhaps 1/2 of the rated current.

Should I worry about "reforming" the capacitor?

   If you're already powered it up then the capacitor has already been "reformed". If they were bad you'd have almost certainly known it by now (blown fuses, zero voltage output, etc).


>Thoughts appreciated.
Received on Sun Apr 28 2002 - 00:00:42 BST

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