[R-390] Z503 Failures

[Gordon Hayward]

Z503 is the AGC peaking coil. Mine was open, so I used a slug tuned coil (an old loop stick) that I had lying around and a small parallel capacitor.
I found that the inductance has to be big and the  cap has to be small as possible to get a high enough Q. R goes up with the number of turns
while L goes up with the number of turns squared.  I put the replacement in the original can and installed it. The AGC now works as it should
and the radio meets the manual's specs. Not original, but cheap.
Cheers, Gord VE3EOS.
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of ROSS HOCHSTRASSER <bavarianradio at comcast.net>
Sent: Thursday, March 16, 2023 2:02 PM
To: Jacques Fortin <jacques.f at videotron.ca>; r-390 at mailman.qth.net <r-390 at mailman.qth.net>
Subject: Re: [R-390] Z503 Failures

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Hi all, As it turns out I did successfully  repair one a few years ago and it did work. This one was out of a Collins R-390 (S/N1501) and it was open circuit. I figured I had nothing to lose, so I took a 1,000 ohm resistor and my variable bench DC supply and cranked it up to 500 volts. I drew an arc from the resistor lead to the  coil lead(after disconnecting the capacitor) and  it arc welded the break inside the coil. It is definitely possible that I lost a turn or maybe a few turns  in the welding process, but it tuned up and worked nicely until I discovered that the 390A also uses the same coil and stole one out of a parts set and replaced it. I still have the one I "fixed" and it checks OK with an ohmmeter, but I didn't want to have a failure, so I left the replacement in the radio.  It may be worth the try!! 73 Ross W1EKG
> On 03/16/2023 1:24 PM Jacques Fortin <jacques.f at videotron.ca> wrote:
>
>
> About those Z503 failures: does anyone tried to repair a defective one already ??
> Please share your experience.
>
> 73, Jacques, VE2JFE in Montreal
>
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