[R-390] Detector tests followup

[Cecil Acuff]

Did you record the signal level required at terminal 15 to get the .15v at 
pin 7 of V507?  Just curious whether it took just a bit more level or 
something closer to 4v p-p.

Simple test for C530....cut one end of it loose but where you can solder it 
back.  See if that brings the signal level up.  C530 and L502 are used to 
remove the 455kc signal from the audio...

It could also be a bad T503 secondary or C562.  Pull the cover off T503 and 
inject your AUDIO signal into pin 1 of  T503 with the diode load jumper back 
in place and see what you get.

I would also do a continuity test of the cable from terminal 14 back to pin 
1 of T503 and center to ground etc....

One thing that don't add up is that you were originally injecting .1V at 
Pins 6,7 of V506B and expecting to see the equivalent of .15v of audio at 
Pin 7 of V507 where you just had to crank the level going in at the diode 
load point to achieve .15 volts of signal at Pin 7 of V507.  Those number 
don't add up.  Where did you get the .1V of 455kc at the plate of V506B 
from...out of the manual somewhere?  My manual is showing a level of .3 to 
.4 V at pin 1  (grid) of the 4th IF amp V504.  I don't see anything showing 
the expected output on of that IF amp is but I would expect it to be fairly 
high to drive a decent signal through T503.  I haven't found any expected 
level at the input of the detector so far....but something approaching 1 
volt p-p might not be unreasonable.....100mv sounds low but I haven't 
measured a working radio that I remember.  The signal substitution process 
in my manual speaks only in general terms as to the levels to inject in this 
area...things like sufficient signal level to achieve an output etc....

Cecil....





----- Original Message ----- 
From: <n4buq at knology.net>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, September 19, 2006 8:50 AM
Subject: [R-390] Detector tests followup


>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
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