Apple II Plus power supply strangeness

[Tony Duell]

> 
> 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!

There's nothing inherently bad about putting a crowbar circuit on a 12V 
supply, but in a microcomputer, the 5V rail is the 'important one' in 
that if it goes high it wipes out just about every chip in the machine 
whereas the 12V rail is used by the RAM and not a lot else. So it makes a 
lot of sense to sense the 5V rail for any protection circuits (like the 
crowbar). 

Having the crowbar short out the 12V rail (rather than the 5V one) is 
quite common in that the former can supply a lower current so it's easier 
to short out, and shorting any output will shut down a SMPSU. 

> 
> > 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.

In general a 'scope will tell you a lot more about why a PSU is failing 
than a simple (average) voltage check.

> 
> > 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.

Indeed. Dried up electrrlytics have a high ESR and will cause problems 
like this. I normally ger the 105 degree ones, they're not much more 
expneisve and may prove more reliable,. But it's up to you.


-tony