[R-390] Detector tests followup

[Cecil Acuff]

Need to try that same test and back up to pin 1 on T503.  If .33V at that 
point gives you proper level at Pin 7 of V507 then the problem is either in 
T503 or a bad V506 the 5814/12AU7. (swap it with V206 out of the calibrator 
for a test)

Check for a 6 ohm reading across pins 1 and 2 of T503 and an open circuit 
between pins 1 and/or 2 and the remaining pins of T503.  It's possible L513 
in T503 could be open...or shorted to a turn of L512.

As a long shot you could lift one end of C535 and see if isolating the BFO 
circuitry has any effect.

Getting close...

Cecil...


----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, September 19, 2006 9:17 PM
Subject: Re: [R-390] Detector tests followup


>
> ----- 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
>
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