[R-390] Detector tests followup

[Barry]

----- Original Message -----
From: "Cecil Acuff" <chacuff at cableone.net>
To: <n4buq at knology.net>; <r-390 at mailman.qth.net>
Sent: Tuesday, September 19, 2006 12:56 PM
Subject: Re: [R-390] Detector tests followup


> 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....

I did another test tonight.  I coupled the audio generator to terminal 15
and measured the voltage it required to get 0.15V at pin 7 of V506.  It was
approximately 0.33V.  I assume that would be about right given the voltage
drop across the grid resistor going to the limiter circuit.

Next, I shorted the diode-load terminals and watched the voltage on pin 7 of
V507.  It only dropped about 0.01V.  Given the fact that it takes 4V p-p on
the plate of the detector to see 0.15V on pin 7 of V507, I sort of expected
to see a good drop in voltage when I included the diode-load connection.

I suppose I'll try subbing the 150pF to see what effect it has, but I'm
beginning to think this isn't the problem.  I have some 180pF so until I get
the right value, this should at least give me an indication if this is the
problem point.

Thanks guys.  I do hope to get to the bottom of this...

Barry - N4BUQ