Re: Power supplies

[Rubywand <rubywand@swbell.net>]

JCA31@MAINE.MAINE.EDU writes ...
> 
> I have one of my dead power supplies back from the dead now. The fix
> was to replace the 10V, 220uF electrolytic cap with a new one. (I used
> a 35V piece). Assuming that the board markings are useful, this was
> C7, on an Astec board with the datecode "T8312" on it. I'm curious if
> anyone else that has replaced that cap has noticed any browning of the
> pc board where the leads enter from that 27ohm 2W 'R4' beast. ....


     Very interesting! A bad C7 would screw up the regulator's feedback
voltage. Replacing C7 sounds like the fix suggested by Owen (O Aaland
oaaland@aol.com):

>>
If the power supply you are working on is an ASTEC brand the most common 
problem is the 10V 220 uf capacitor located about 1 inch to the rear and
left of the transformer. I find this to be the cause in about 80 pecent
of the ones I repair. Use a 25V 220uf capacitor as a replacement and
chances are real good that it wikk not fail again.
<<

     Your observation of the heat spot on the PCB seems to indicate that
heat from the resistor is impacting the nearby capacitor. If there is
some way to eliminate this hot spot-- e.g. using a 10W resistor, perhaps
with a heatsink attached, or mounting the resistor on the power supply's
metal case-- many II+ and IIe power supply crashes might be prevented.

     Thanks (to you and Owen) for the valuable information. I'll be sure
to include it in the new Power Supply FAQs.

     So; is replacing C7 going to work with the other bad supplies? Be
sure to post an update. 


Rubywand