Apple II Plus power supply strangeness

[Mike van Bokhoven]

Tony Duell wrote:
> This PSU seems to have a crowbar circuit  (SCR Q5, zener CR20, etc), but 
> amazingly it seems to be connected to the 12V output. This is, IMHO, 
> ridiculous.
I  have a few ideas as to why it could be considered a bad thing, but 
I'd be interested to hear more details!

> When a PSU output sits at about 1V, I often suspect that a 
> crowbar is triggereing, which will shut the supply down, thus enabling 
> the crowbar to turn off, the cycle then repeats. So what you see is an 
> average of the 'too high' and 'shorted' output voltages.
>   
OK - I'll check the outputs with the oscilloscope when it's in that 
mode, and see if I can see a waveform that looks like that.

> If the output capacitors have high ESR (a likely problem due to old age), 
> there will be spikes on the output, sometimes around twice the normal 
>   
I've grabbed new replacements for all the capacitors, apart from the big 
high-voltage input-side ones. One of the output capacitors has shrunk 
its heat-shrink wrapping quite considerably, so I'm guessing that one 
has been running quite hot. Might as well replace them all though - the 
replacements only cost me NZ$10 (something like 3 pounds, maybe US$5, at 
a guess) so it's hardly breaking the bank.

Once I have the power supply behaving more sanely, I'll feel more 
confident in trying to track down any further logic board problems. 
Thanks for the suggestions, and also to Chris H for the patent info. I 
haven't had a chance to read it in depth yet, but I will.

Mike