[R-390] Z-503

[ROSS HOCHSTRASSER]

Hi all, I just finished replacing all of the mica caps in the IF cans... no difference!! I was able to see a peak in the response  tuning through the range of the cores, but still distorted signal on strong stations. This time no warm up time, bad right out of the gate! Thanks for all of your help!! 73 Ross W1EKG
> On February 16, 2020 at 1:55 PM hamfish at comcast.net wrote:
> 
> 
> 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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