[R-390] Z-503

[hamfish at comcast.net]

Ross,

Just for grins & giggles check the IF gain value with the receiver cold.
Then check again later as it warms up. Any large change could be due to
those silver mica caps in the IF transformers (T501 & T502). Values for C557
through C560 are 200pF, 2%, 300VDC. As a side note those 2% silver mica caps
are like hens teeth. Larger % caps work just fine.

Regards,
Craig

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
Behalf Of ROSS HOCHSTRASSER
Sent: Sunday, February 16, 2020 7:21 AM
To: Charles Steinmetz <csteinmetz at yandex.com>; r-390 at mailman.qth.net
Subject: Re: [R-390] Z-503

Hi Charles, The frustrating thing about the AGC problem is that  it seems OK
when the radio is cold for a few minutes to a few hours then fails on strong
stations. I used 2 cans of freeze spray  and couldn't isolate  the problem.
I am suspecting  that I may have an alignment issue. I just set up the
sweeper and scope to do a sweep alignment, hoping that it may solve the
problem. I must have missed your troubleshooting post and maybe didn't need
to read it because this radio didn't arrive here with this problem till a
month ago.  How do I find the post?? I installed a new 82 PF  cap yesterday
and was able to see a dip in volume  running through the range of the
adjustment, but the problem is still there.  I'm letting the radio warm up
right now to sweep it.  It is possible that someone peak aligned the IF's
previously. THANKS for your help!! 73 Ross W1EKG
> On February 15, 2020 at 10:08 PM Charles Steinmetz <csteinmetz at yandex.com>
wrote:
> 
> 
> Ross wrote:
> 
> > Hi all, Pardon me for not shuffling through endless pages of R-390A
info, but I need to know the value of the capacitor in Z-503.  I am having
AGC problems and am coming to my wits end.
> 
> As I recall, C563 is typically around 82pF, though there is some 
> variation.  Note that Z503 is a *low-Q* plate load for V508, so it's 
> pretty non-critical.  Also check C543 (the V508 cathode bypass 
> capacitor), C544 (the screen bypass capacitor), and the value of R541 
> (the cathode resistor).
> 
> Of course, low gain anywhere in the IF section would produce similar 
> symptoms.  Is there any specific reason, other than the previously 
> defective (and now replaced) Z503, that you suspect Z503 and/or C563 now?
> 
> Have you been through the AGC troubleshooting procedure I have posted 
> here several times (most recently on 10/6/2018)?
> 
> Best regards,
> 
> Charles
> 
> 
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