[R-390] AGC voltage issue

[Ryan Scott]

Hi Larry,

Many Thanks for the advice.  Sounds like you played this game before!  I am
surprised as the 2 other EAC IF decks the resistors are in tolerance
(although close to 10%) but that AMELCO every resistor was way off.  Guess
they used another component vendor.

For C553 I'm using a Vishay Poly Film 0.01 @ 630V. (yellow jacket type)
 (Newark MKT1813310635G) I'll check out your recommendation and get some on
my next parts order.

I found that I replaced C548 with a 0.01 instead of a 0.1... That's in the
AGC circuit!  but, no joy on that.  Still the same, really low audio.  I
did compare voltage readings with the good IF vs. the bad IF and my trouble
seems to start at V504, pin 2.  (3.8V on bad IF, -2.1V on good IF)  Of
course the detector voltages are different as well, likely due to low gain.
   But if the gain is low, and the detector voltage is low, then one would
expect to have AGC off a bit..   I'm going to study up and tackle it again
soon.

Time to wind down and get ready for another full week of work and
everything else.

Best Regards,
Ryan, N7QJ

On Sun, Sep 14, 2014 at 6:45 PM, Larry H <dinlarh at att.net> wrote:

> Hi Ryan,  Sounds like you solved 1 of your problems.  Great.
>
> As for the overall gain, the pot r519 should not need to be at max -
> something wrong if so.  Check gain in the 1kc position (no filter).  If its
> better, look at the filters, check 2, 4, 8, and 16.  If all bandwidth
> positions are about the same, then the filters are probably ok.
>
> In ref to c553, there are some .01's you should not use.  See the
> discussion in this list in August by Charles Steinmetz.  Allied and Mouser
> carry a good one 715P10356KD3 for a few $.  Or I could send you one.
>
> Also, the test cable connectors are available at Fair Radio and other
> places - no need to cannibalize anything good.
>
> 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