Dead PSU - test-drb@ccmp.vtda.org

4 Jul 1998

I counted with a stopwatch. It beat very fast, so it might be even 
higher than 120Hz, but it certainly is much higher than 60. I don't know 
if they'll actually let me do this repair anyway.

...
 I think we need to determine if it really is a 120Hz
ripple, or  something 
...
 near it. If it really is 2* mains frequency, then start
looking around 
the mains smoothing capacitors first.

It's a very simple PSU, there's a bunch of capacitors, a single small 
chip, and a couple of every other component and that's it. This does not 
have the power-on from the keyboard thing. It uses a regular button 
which cuts mains off from the rest of the PSU. Two questions: what is 
the definiton of a Switch Mode PSU, and what is a Crowbar?
...

 Don't worry about the complex circuit with thround resistors and 
 little capacitors, it's working becasuse of that fluttering fan and 
 it did power up when you commanded it to turn on by the keyboard. 
Agreed. This doesn't sound like a control circuit problem (although,
having seen some strange SMPSUs in my time I'd not bet my life on it!.  A 
...
 few, a very few SMPSUs have a separate linear supply
for the control 
circuit that can inject mains frequency ripple).

 Jason D. 
-tony

______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com