[R-390] AGC voltage issue

Sun Sep 14 20:31:59 EDT 2014

Ryan,

The 2uF might test at a value of 2uF, but that is just part of the issue.
Poor insulation resistance at rated voltage will equal a short. No matter
the position of the AGC switch or if the Function switch is at AGC or MGC;
one end of C551 is connected to the grid of V506A. A bad C551 will cause
problems.

The IF decks played with here in Springtucky, OR; the IF gain pot R519 is
somewhere/just about/near 80% counter clockwise when life is good. (-7VDC @
150microvolts/455KC)

The "Too Loud Amelco" in the shack has been a problem child! The quality
control of assemble & component quality are lacking, nuf said. Go thru that
IF deck with a fine tooth comb. Break out the soldering iron and melt every
connection possible. Take the covers off of the transformers and look for
wires that might be touching the cover or the wrong component under said
cover. Find a set of tube extenders for the joy of working on boat anchors.

Craig,  

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Ryan Scott
Sent: Sunday, September 14, 2014 4:42 PM
To: Larry H; R-390 at mailman.qth.net
Subject: Re: [R-390] AGC voltage issue

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
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net Please help support this email 
> list: http://www.qsl.net/donate.html
>
>
>
>
>

--
Ryan P. Scott
n7qj at arrl.net
503-880-1214
______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html