In article ,
John H. Gindlesberger wrote:
I'm working on a Hammarlund SP-600 and need some advice. It receives on all
bands, but seems like it overloads after it has warmed up. After it's been
on about 15 minutes, the meter pegs and the RF gain control does not seem to
have any effect on signal strength. The signals become extremely loud and
rather distorted. All black beauties have been replaced as well as most of
the power supply caps.

Ideas about where to begin looking would be appreciated.

Thanks,
John

John,
I had a similar problem with my SP-600. I had replaced all the Black
Beauties, but the AVC line would not develop much voltage when the
receiver was tuned to a strong signal. The problem turned out to be
C120, a 1300 pF mica cap _inside_ the housing of T4 (the IF transfomer
betwen the 1st and 2nd IF amps). It's part of the tuned circuit
of the transformer, along with C119 (300 pF). I couldn't find a 1300
pF cap in the junk box (surprise!), so I made one up using a couple of
silver micas. I replaced C119 too, just because. Then I re-peaked
the transformer. It's worked fine ever since.

There's a similar situation inside T3 (the preceding IF transformer).

As somebody here remarked the other day, mica caps _can_ fail...

You might also check to make sure you've got the proper bias voltage.
The voltage at the wiper of R93, the RF gain control, should be about
50V when the pot is fully CCW.

73,
Ted Zateslo, W1XO