[R-390] Detector tests followup

[n4buq at knology.net]

I tried some of Cecil's procedures last night.

I injected a 4V peak-to-peak 455kc 30% modulated signal at the grid/plate 
of the detector and the results on the dB meter were where they should be 
with a 0.5V RMS signal.  Too much voltage required to see the proper 
results.

I then disconnected the diode load jumper and injected the audio signal on 
pin 15 of the back panel.  The output wasn't up to spec, but then I 
realized there's a resistor in the path from pin 15 to the grid of V507 
causing a voltage drop from pin 15 to V507's pin 7.  Measuring the voltage 
at pin 7 of V507, I increased the signal level at pin 15 to see 0.15V RMS 
at pin 7.  The output level was on spec at that point.

Looks like there's still something wrong at the detector and assuming that 
eliminating the limiter as a problem by the above tests, there's not too 
much else at the detector to work on.  The choke has the right resistance 
so the 150pF cap to ground is my next suspect.  If replacing that doesn't 
help, does anyone else see anything else that might be a problem?

On another subject, I started looking at the capacitors in the various IF 
transformers.  As I mentioned before, these have a mixture of rectangular 
micas and what appear to be silver-micas.  In an effort to get better 
peaking in these cans, I'm considering replacing these capacitors.  The 
slugs are very near their limits and are very hard to turn at their peaks 
so I'd like to move the resonant point somewhere closer to mid-range of the 
threads.

I can read the rectangular micas, but I'm not familiar with the coding on 
the other caps.  They appear to have a large round purple(?) dot on one end 
with color code stripes along one side.  Can some tell me how to read 
these?  Provided I can stay away from the Litz wire connections (I don't 
want to resolder Litz wire if I can help it), it doesn't look like too bad 
of a job to replace these caps and if it will help the resonant point, I'm 
willing to do it.

Thanks all!

Barry - N4BUQ