Re: Vulcan IIgs Power Supply Repair

[Michael J. Mahon <mjmahon@aol.com>]

Polymorph <mike.a.stephens@gmail.com> wrote:
> On 7/07/2012 2:21 PM, Michael Black wrote:
>> These filter capacitors operate at a much higher frequency, which means
>> if they go bad, they will do a much worse job than if 60Hz was
>> involved.  The higher frequency ripple that they have to deal with every
>> day may also help to make them go bad.
>> 
>> Those filter capacitors after the transformer and diodes are doing much
>> harder work than the high voltage capacitors on the AC side.
> 
> Thanks Michael. I guess I'll continue and try replacing all of the
> electrolytics after the transformer. I should note though, that none of
> the capacitors (even the ones I replaced) show any signs of leaking or
> bulging - but I'm guessing they're still the most likely cause of the
> failure. The caps after the transformer are all low voltage (I think) and
> easy to come by for a few dollars.
> 
> Thanks for your advice. :-)
> 
> I'll get the parts tomorrow and post back my results.

I'd check all four outputs for shorts. 

The repeated shutdown/restart cycle can result from too much load as well
as too little. 

And the line side capacitors you replaced do every bit as much work as the
secondary filter capacitors. They have to rapidly charge to the mains peak
voltage, then sustain the supply until the next half cycle.

No supply ever cycled because of an open filter capacitor on an output, but
it certainly would if an output were shorted--possibly by a filter
capacitor, but given the history of your repair, I'd look for a short. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon