[Heathkit] Puzzlement on HP-1144 A Power Supply

[Donald Spoon]

I happen to have two Heathkit HP-1144 Power Supplies... just by a quirk 
of fate.  The one that I had been using for the last 6-8 months started 
blowing the 20 Amp fuse in the + 13.8 Volt supply line whenever I 
pressed the "Tune" button on my SB-104.  It had done this once or twice 
before, but a fuse replacement "fixed" it and I continued using it.  
This time the condition seems permanent.

On a whim, I tried the other (older) Power Supply, which worked fine, 
and didn't blow fuses!  The SB-104 acts normal in every way with this 
older PS.  Upon closer inspection, I discovered the one blowing the 20 
Amp fuse was the HP-1144A model with the crowbar circuit, while the 
other one that worked was the older HP-1144 without the crowbar.  As 
luck would have it, the only documentation I have (including schematic) 
is for the older one...  so I am at a loss as to where to start to 
diagnose the problem.

I have given the failing PS a good visual inspection and have reset the 
voltage to 13.8 Volts as required.  I can vary the output voltage, and 
the SB-104 will work without problems in the receive mode,  Once I push 
the "Tune" button the fuse goes.  I think trigger for the crowbar is 
overly sensitive.

Where to start?  Has anyone run into this problem before and can give me 
some pointers?  Does anyone have a readable schematic of the "A" circuit 
that could be shared with me?

All advise welcome!

Cheers,
-Don Spoon-
K0APK