[R-390] Noise when warm ('390A)

[Jim Miller]

Correction to previous post, I meant to say "Diode Load Voltage", not AGC. 
The AGC is not carried on the mini-ax cable, but it is proportional to the 
diode load voltage on the mini-ax.  As signal increases, so does diode load 
voltage up to the point of dielectric breakdown and the popping noise.  It 
tended to go away on weak or no signals.  You will have to open the 
largemultipin connector on the IF deck to get to the end of the cable there. 
I forget the pin number, it's on the schematics.  If you disconnect that 
end, and run a temporary cable from the connector to an AF amp, and if that 
cures it, then there you have it.  Just one idea, however, that worked for 
me.
----- Original Message ----- 
From: "Jim Miller" <jmiller1706 at cfl.rr.com>
To: <sacramento.cyclist at gmail.com>; <r-390 at mailman.qth.net>
Sent: Monday, September 12, 2005 11:56 AM
Subject: Re: [R-390] Noise when warm ('390A)


> Here is what happened to me Stewart Warner 390a few years ago.  I would 
> get snap/crackle after warming up just like you said.  And it tended to go 
> away when no signals were present (such as antenna disconnected, hence no 
> AGC). I discovered that as the AGC voltage increased when a signal was 
> present, the noise would start up.  No AGC voltage (no signals) and the 
> noise went away.  I tried everything, it drove me crazy.  Then someone on 
> the list (forgot who) said this could be caused by a breakdown of the 
> center dielectric in the miniature coax cables that carry the diode load 
> voltage from the IF strip to the AF deck and some of the front panel 
> controls.  Even with the low voltages there (7-10 volts) the little coax 
> cables would start to break down causing the popping.  Sure enough, I 
> replaced the mini-ax cable from IF deck to the terminals on the back, to 
> the front panel, etc. and cured it.  You can verify this by temporarily 
> running a good cable from the IF deck to the AF deck or an external audio 
> amp, isolating all other cable runs.  Replacing the cables can be a pain 
> if you are a purist and want to pull the old cable from the wiring 
> harnesses, or you can just run the new cables along side the harness.
>
> Jim N4BE
>
> ----- Original Message ----- 
> From: "Dennis Wade" <sacramento.cyclist at gmail.com>
> To: <r-390 at mailman.qth.net>
> Sent: Sunday, September 11, 2005 11:50 PM
> Subject: Re: [R-390] Noise when warm ('390A)
>
>
> Update.
>
> First thing I did was start unplugging modules to try and see when it went 
> away.
>
> Guess what.
>
> It goes away when the antenna is unplugged  (sigh).  Sorry guys.
> Should have checked that first, although it still doesn't sound like
> its from the antenna.
>
> Is there any failure mode anyone can think of that would only be
> apparent when the antenna is connected?  Haven't changed the antenna
> configuration here by the way.  Its a 120' dipole fed with open wire
> line to a balun.  Coax to the balanced antenna jack fed according to
> Rippel's instructions.
>
>      Thanks for all the suggestions from all.
>
>               Dennis
>
> On 9/11/05, llgpt at aol.com <llgpt at aol.com> wrote:
>> The voltage regulator tube OA2 has been known to have those symptoms.
>>
>> Les Locklear
>> Gulfport, MS.
>>
>> -----Original Message-----
>> From: Dennis Wade <sacramento.cyclist at gmail.com>
>> To: R-390 HF Receiver List <r-390 at mailman.qth.net>
>> Sent: Sun, 11 Sep 2005 15:07:08 -0700
>> Subject: [R-390] Noise when warm ('390A)
>>
>> Hi everyone,
>>
>>       I've noticed lately that after a few hours of operation, a
>> "frying bacon" or "snap-crackle-pop" noise begins to occur, slowly
>> increasing as time goes on.  No real difference in band or mode.
>>
>>       Any quick troubleshooting hints to isolate to a stage?
>>
>>       What is the consensus...more likely a cap or a tube?
>>
>>             Thanks for the advice,
>>
>>            Dennis
>> --
>> Dennis L. Wade
>> KG6ZI
>> Carmichael, CA
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>
>
> -- 
> Dennis L. Wade
> KG6ZI
> Carmichael, CA
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