[R-390] Lightning crashes in audio

[gary.biasini at shaw.ca]

Folks,

The following from Hollow State News No. 27 may be of some use.

R-390A THUNDERSTORM NOISE: One of the R—390As I worked on had a bad noise problem which sounded like a nearby thunderstorm was raging with no antenna connected. I traced it to the IF subchassis by disconnecting the output of the RF deck (P 213 and P218) and by switching AF decks. Within the IF deck I used "Freeze—it" and found the culprit, an Intermittent mica capacitor, after about 10 minutes work. I replaced the mica cap with an identical one from my "parts unit", and all was well. (Shaun Merrigan)

This is a wonderful discovery which Shaun has made. I had an intermittent "thunderstorm" problem in an IF deck which I never was able to isolate. For a while I suspected a bad tube, but repeated efforts to isolate the bad tube failed. It never occurred to me that I might have a bad or intermittent mica capacitor. Now I know what to do if I ever encounter the problem again. However, let me add that some of these "thunderstorm" problems are bad tubes, so you should check for bad tubes first before you proceed to try to isolate a bad or intermittent mica cap. (Ed.)

Regards,

Gary

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of Stan Gammons
Sent: August 15, 2020 9:46 PM
To: Gary Biasini <gary.biasini at shaw.ca>
Cc: Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
Subject: Re: [R-390] Lightning crashes in audio

Hi Larry and all,

Thanks for all the replies.

With the cables disconnected from J513 and J518, the problem is still present.  With the 1st IF amp tube removed, problem is all but gone. Have to crank the volume wide open in order to just barely hear a crash now and then.  Tried another 5749. Made no difference. Perhaps it's a resistor or capacitor around the 1st IF amp.

What happens to silver micas when they age?  Does the ESR or leakage change?  Just wondering if one can determine a bad one by measuring the ESR or leakage.

73

Stan
KM4HQE

On 8/15/20 9:41 PM, Larry H wrote:
> Hi Stan,  I think you are right about it probably being a cap going 
> bad.  However, it could also be a tube, the band switch, or resistor.
> You can try tapping on the side of the tubes.  The bad cap is probably 
> in the RF deck, though.  You can narrow it down a little by pulling 
> the tubes in the signal path 1 by 1 and see where it stops or starts.
> I'd start at the RF amp and work forward.  Also, it could be related 
> to a particular band, and perhaps above 8 MH or not.  If it's related 
> to a band, you can try pulling the tuning cans and see if it's related 
> there.  Have fun.
>
> Regards, Larry
>
> On Sat, Aug 15, 2020 at 7:13 PM Stan Gammons <sg063015 at gmail.com 
> <mailto:sg063015 at gmail.com>> wrote:
>
>     Hi All,
>
>     I have an R-390A that I recently put back on the bench and noticed it
>     has developed a lightning crash sound in the audio. It's much more
>     noticeable with no signal.   It appears the problem is around the
>     1st IF
>     amp.  With a scope on the plate of the 1st IF amp I can see the
>     signal
>     change when I hear the lightning crash sound.  Some time ago I
>     replaced
>     all of the black beauty caps with orange drops. The cap that goes bad
>     and takes out the mechanical filters has been replaced too.  I didn't
>     replace the disc, which I assume are ceramic disc caps or the green
>     rectangle shape caps which I assume are mica caps.  I'm guessing
>     one of
>     the ceramics or mica caps is bad and causing the lightning crash
>     sound
>     in the audio?
>
>     Thanks for any help.
>
>     73
>
>     Stan
>     KM4HQE
>
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