[R-390] Z-503

[David Olean]

I am having a similar problem with two R-392 receivers here. They work 
great when first turned on, but after some warmup, the AGC voltage 
starts dropping, and the signals start to distort as the AGC voltage 
does not limit the gain in the various stages. The lower AGC voltage is 
the problem. How the AGC voltage drops is a mystery for now.

Rather than individually test the capacitors, I got a can of freeze 
spray, and will apply some freezing spray while monitoring the AGC 
voltage.  I should be able to zero in on the problem quite fast with 
this method. (I hope!)

I would not suspect alignment at all.  You could have a bad mica cap in 
the RF stages, but these will only affect a single band. (2-4, 4-8, 8-16 
etc)  Trying the radio on different bands should show a pattern if an RF 
cap is defective. I had a bad cap in the 1-2 MHz band that started 
shorting out with about 8 volts P to P across it. It was located in one 
of the 1-2 MHz cans.  I saved the cap to demonstrate how capacitors may 
fail. It tests OK on many cap testers. The only way I found it was to 
put some DC voltage across it and look for noise on an oscilloscope. 
Above 8 volts DC, I started to see DC voltage appear on the opposite end 
of the capacitor. Below 8 volts, there was no leakage!

I just bought a GenRad 1657 in an effort to test capacitors.  I have a 
feeling that some mica caps will be hard to isolate with such testers. 
I'll fill in more when I get some more experience.

73

Dave K1WHS

On 2/16/2020 3:20 PM, ROSS HOCHSTRASSER wrote:
> 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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