[R-390] Z-503

[ROSS HOCHSTRASSER]

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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