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 here:
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!!
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
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