[Drake] L-7 vs. L7-PS failure

[Torrey L. Mitchell]

Good evening all.

As I was tuning up my L-7 amp this evening there was a very loud bang =
and a visible flash behind the plate voltage meter on the left.  Foolish =
or not I reset the power supply circuit breakers and tried again with =
low RF drive.  Same story again.  I took the top of the L-7 and could =
see no evidence of arcing or burned components.  Final tubes and =
attached resistors look normal.  In short, everything looks normal =
topside.  I have not removed the bottom panels to look.

The L7-PS power supply reveals charring and a burn-through of the R12 =
resistor (2 watt 0.82 ohm IRC type BWH).  The charring put a big black =
mark on the adjacent large tubular capacitor, but the capacitor does not =
look damaged and all the large resistors across these capacitors appear =
intact.

The R12 resistor appears to function as a protective fuse in the high =
voltage output line.  Am I thinking correctly?  If so, where could such =
a impressive short circuit have occurred in the linear amplifier without =
any evidence of damage topside?  I may have to send both units out for =
repair, but I certainly would like to avoid it...given all the weight, 3 =
boxes, etc.

Are the R12 resistors hard to find?  Hope not!  Specific recommendations =
for L-7 repair?

Any thoughts or help will be most appreciated.

Thanks,
Torrey N9AUJ
cell 515-778-2354



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