Repair methods (was Cromemco 3101/Beehive B150 score)

[Don Y]

Tony Duell wrote:
>>> I would love to see anyone understand an SMPSU without understanding
>>> analogue electronics...
>> That is, I think, why I have so much problem trying to fix them - my
>> understanding
>> of analog is inadequate.
> 
> The main problem I have with fixing SMPSUs is missing some seemingly 
> minor fault (like a diode that breaks down under load only) which causes 
> all the nice, expensive, new components that I've fitted to blow at 
> switch-on. A series light bulb helps (if only to reduce the shock to _me_ 
> when the thing tries to put a dead short across the mains), but won't 
> always save the expensive chopper transsistor.

We designed a 2KV, 2KW switcher many (MANY!) years ago.
Getting drive transistors was a *bitch* (expensive and
hard to come by!).

There was damn little you could do to "prevent" losing
the entire set in an ohnosecond.  Then, spend the better
half of the day disassembling, looking for possible
faults in the design, soldering in a new set of Q's
and doing the same thing over again the NEXT morning.

Advice to others trying this:  do it some MONTH when your
boss is on vacation!!  :-/