[R-390] AGC voltage issue

Sun Sep 14 19:42:22 EDT 2014

After reminding myself I do this for fun a few times I managed to have the
IF deck hooked up where I could poke at it from underneath.   I found C553
(someone else before me replaced, but I removed to replace a few parts then
put it back) was really sensitive to tapping, even lightly.  The audio (and
subsequently the AGC voltage) would come up to about 0.5V then would drop
if tapped again.  Really sensitive on the lead to the inductor.  (I think
my removing one lead to move it out of the way and then putting it back may
have had something to do with it).  So I replaced the cap.  The issue where
putting the function switch in standby and then moving back to AGC killing
everything went away and it's no longer sensitive to tapping and prodding,
but compared to 2 other IF modules, the gain is really low.  (low audio) AM
BCST is acceptable, but WWV @15MHz on this IF Deck is weak.  (ACG voltage
on strong AM BCST is ~ 0.5V while WWV is 0.2V) .  It's possible to that
someone was messing with the transformers as well so it probably needs an
alignment which I will read up on.

I removed the wire from R507 to verify the filters are not pulling down the
AGC voltage (I read this in Y2K R3 manual) and the results were the same
with the resistor connected and if it was not connected.

I checked the 2UF cap of course not at the rated voltage but it seems OK.
 The schematic shows if I have the AGC set to Fast, the cap is not
connected to anything.  Moving the AGC switch and even MGC seems to make no
change.

I agree at this point I'll need to scrutinize everything I did which was to
replace almost every resistor (I think 10 or so checked OK) and all the
brown beauty caps...  I'll put it aside for another day when I feel like
circuit tracing it to try and find the reason for the low gain.  I have
found that I can put a piece of cardboard and rest the IF deck on it's side
near the back of the radio and get to the underside.  I'm not yet ready to
sacrifice another radio to make an extender cable yet.  (I have one
connector, but not the mating connector)

Oddly, on every IF deck I've looked at (3 so far), I've found the gain
control fully cranked.  is this common?

Thanks again,
Ryan N7QJ

On Sun, Sep 14, 2014 at 3:36 PM, Larry H <dinlarh at att.net> wrote:

> Hi Ryan,  Since you have both audio loss and agc loss, it's probably in
> the 1st to 3rd IF.  Any problem in the agc only circuitry that reduces the
> agc voltage only, would normally cause the audio to increase.
>
> BTW, did you put in a good cap for C553, the filter killer.
>
> Regards, Larry
>
>
>   On Sunday, September 14, 2014 2:32 PM, Larry H <dinlarh at att.net> wrote:
>
>
> Hi Ryan,  It sounds like you've got a bad solder connection or an
> unintended short somewhere.  I'd closely inspect all of the connections you
> did, which sounds like quite a few.  If that yields nothing, then try
> measuring some voltages or signal levels in appropriate places.  If the agc
> line is getting loaded to ground, you'd probably see louder and distorted
> audio, so its probably loss of IF signal before the agc amp.
>
> Do you have the 'test' cables so you can operate it outside the chassis?
>  If not, you can use 'tube extenders' to get at most of the points you need
> to check.
>
> Regards, Larry
>
>
>   On Sunday, September 14, 2014 11:46 AM, Ryan Scott <n7qj.rs at gmail.com>
> wrote:
>
>
> Greetings to the group.
>
> Having been a lurker for the last 7 years I am finally getting around to
> working on the 12 or so R-390a's I have.  I have one R-390a that I use as a
> test bed for testing the rebuilt AF and IF modules I have worked on.
>
> The original IF module in my test bed works fine, (EAC '64 which I did
> replace all brown beauty caps).  I recently completed working on an Amelco
> IF module which I replaced all the capacitors (brown beauty) but also found
> many of the resistors had increased in value well beyond the 10% tolerance
> so I replaced those.  So after all that work, I place the module in the
> test bed and it seems to work fine for about 10 min.  Then the audio drops
> out.  Still have reception, gain and RF gain make some difference in audio,
> and BFO works.
>
> Subbing several tubes have not solved the problem.
>
> Here's what odd.  From a cold start, the IF deck works good.  If you move
> the function switch to standby and back to AGC, the audio drops out (again
> still have reception) and the AGC voltage drops to ~200mV.  When the radio
> is cold, the AGC voltage is 2V..So why does moving the function switch kill
> the audio??  Could it be the big 2uF cap on the IF deck?  Slow/med/fast AGC
> do not change anything, same with MGC.
>
> I'm certain it's the IF module since swapping the test bed IF module back
> into the radio the problem goes away.
>
> I have triple checked the parts I installed and R5's tube sockets and
> tightened some of the grounding points.  The IF deck is using a 12BH7 in
> place of RT-150 but since that is for the BFO and PTO I don't see how that
> can be an issue.
>
> I'm going to start looking in the archives and the Y2K R3 manual (Thanks to
> all who wrote that!) for clues, but in the meantime has anyone seen this
> when moving function switch from AGC to STBY and back to AGC?
>
>
> Thanks and Regards,
> Ryan
> N7QJ
>
> --
> Ryan P. Scott
> n7qj at arrl.net
> 503-880-1214
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-- 
Ryan P. Scott
n7qj at arrl.net
503-880-1214