[R-390] Yet another question... sigh...

[Cecil Acuff]

Barry and group,


Which tube pin on V507 2 or 7 is resulting in normal results....? (hopefully 
pin 7)

How much signal on pins 6 and 7 of V506B gives you some output? (crank it up 
and see) The IF may be driving much more than the .1V that you are injecting 
resulting in the radio working somewhat.  Should be able to put a scope on 
it and see what the level is there with a strong signal on the antenna port 
and the radio tuned to it.  Are you using something like a .01uf DC blocking 
cap in series with your generator probe? (should be)

Try injecting your 455 at pin 1 of T503 then at the diode load point....any 
change?

I see opportunities at C531 the .1 coupling cap, R 527, some of the limiter 
circuit components, L502, C562 in T503.  Possibly C530 leaky....

Looks like shielded cables out to the diode load terminals on the rear panel 
and back.  Pinched or damaged cable?

If you can inject and get normal results at the grid (pin 7 of 507B) I would 
back up and open the diode load jumper on the rear panel and see if I could 
do the same from terminal 15.  That would eliminate the cable from the back 
panel, the influence of the limiter circuitry, R527, C531 etc.....

Unless you have been injecting on pin 2 of V507A which then adds several 
other possibilities.

If you can inject your signal at the diode load with just a reasonable 
decrease in signal level due to R527, R527 and C531 then I would look at 
what's going on between pins 6 & 7 of V506B and the diode load terminal 14.

Have you tried using the NOS tube somewhere else in the radio to be sure 
it's good.  NOS tubes are sometimes bad.....

Are you sure your signal generator is outputting .1uv and that it's on 
frequency?  An HP 3400A RMS voltmeter pays for itself here...(for the level 
part anyway)

Put a scope on the same point you put the RF generator and see what you 
measure while injection your 455 Kc RF signal....

Just some thoughts after looking at the schematic....and considering other 
opportunities you might be facing....:-)

The modulation frequency of your RF generator won't matter at this point...

Cecil....
----- Original Message ----- 
From: <n4buq at knology.net>
To: <r-390 at mailman.qth.net>
Sent: Monday, September 18, 2006 8:51 AM
Subject: Re: [R-390] Yet another question... sigh...


> Still mulling this over.  Injecting a 0.1V modulated 455kc signal at the
> plate of the detector (V506) isn't giving any output.  If I inject 0.15V
> audio at the grid of V507, I get the expected results so the table says 
> the
> problem is in the detector circuit.
>
> What I can't figure out is the radio does play -- perhaps not to
> specification, but it does play so I'm curious how the problem could be in
> the detector.
>
> Not sure where to go from here.  There's very little componentry around 
> the
> detector (and I'm running a NOS tube there just in case) so it should be
> easy to spot if anything's amiss there.
>
> Weird.
>
> Barry - N4BUQ
>
>
> On Sun, 17 Sep 2006 22:00:02 -0500, "Barry" <n4buq at knology.net> wrote :
>
>> Well, some of my test equipment actually agreed for a change.  42mW
> measures
>> 5V on my DVM.  Cool.  The audio tests were spot on.  Following the table,
>> when I inject 0.1V of modulated 455kc on pin 7 of the detector (V506), I
>> don't get the expected results.  The only difference in my test is my
>> internal modulated signal is 800cps whereas I think the test states
>> something like 400cps but I don't think that would really matter.  I
> suppose
>> I can hook the audio signal generator to the RF signal generator and use
> the
>> external modulator position, but, again, I doubt that would change
> anything
>> very much.
>>
>> According to the table, I have problems in the detector circuit.  I'll
> start
>> looking there for problems.
>>
>> Thanks, guys!
>>
>> Barry - N4BUQ
>
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