Johnson Power Supply schematic
 

I roughly drew up the circuit.
I reckon what had happened was that the variable resistance in the base of
the regulator transistor had gone open. This resulted in no regulation -
rise in voltage which triggered the crowbar cuitcuit and blew the fuse.
Several fuses later I reckoned it was time to look things over.
If interested my schematic is he
http://members.ozemail.com.au/~pebar...os/johnson.pdf

Johnson Power Supply schematic
 

"Peter" wrote in message
...
I roughly drew up the circuit.
I reckon what had happened was that the variable resistance in the base of the
regulator transistor had gone open. This resulted in no regulation - rise in
voltage which triggered the crowbar cuitcuit and blew the fuse.
Several fuses later I reckoned it was time to look things over.
If interested my schematic is he
http://members.ozemail.com.au/~pebar...os/johnson.pdf




Thanks for the follow-up Peter. Remember to keep the thread context by
including pertinent text in your replies!!!! Like I have done here.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Never take a laxative and a sleeping pill at the same time!!

Johnson Power Supply schematic
 

Howdy Peter......Nicely drawn schematic of the regulator circuitry less
the heavy current feed to the 2 pass transistors and the rectifiers back
etc. Your theory sounds good....is that what you found in actual
testing? I'd troubleshoot something like that by disconnecting the
load, disconnecting the crowbar (if constantly tripping), measuring the
reg output voltage - if high, move to the base connections of the pass
transistors....at this point using a scope to look for glitching while
giving that adjust pot a swipe - if glitching is seen (high - near 24
vdc - spikes seen move progressively towards the pot. Get it stabilized
by repair then reconnect the crowbar.....or if nothing
found.....troubleshoot the crowbar ;)

Note that if the output AND the pass transistor base drive are all high,
disconnect and check those pass transistors for shorting. You probably
do something similar.

Gord