[R-390] Noise when warm ('390A)
Jim Miller
jmiller1706 at cfl.rr.com
Mon Sep 12 16:20:54 EDT 2005
Dennis
I read some of your past posts on your AGC problem,.. wow that's a good one.
When I worked through my Stewart Warner I recall getting some questionable
AGC behavior. The bypass caps on te AGC line would of course read fine with
the VOM test. I finally just brute-force replaced ALL bypass caps in the
darn thing. I suspect they could behave differently under power. Went
through the whole radio, IF and RF decks, any bypass cap that touched the
AGC line got replaced with a new one from Mouser. It wasn't elegant, but it
cleared up a lot of problems. (I have used the brute force "replace 'em
all, don't ask questions" approach on plate, screen and cathode resistors
too.)
Jim
----- Original Message -----
From: "Dennis Wade" <sacramento.cyclist at gmail.com>
To: "Jim Miller" <jmiller1706 at cfl.rr.com>
Cc: <r-390 at mailman.qth.net>
Sent: Monday, September 12, 2005 2:58 PM
Subject: Re: [R-390] Noise when warm ('390A)
Hi Jim,
Well they say great minds think alike...<grin>. I'm going down
the same path.
After my last post, I continued poking around looking at
relevant voltages and resistances to ground.
Remember that bad filter with a partial short to ground? Well,
looks like something else is dragging the AGC line down again. I
thought I heard the return of the strong signal distortion.
I'm not looking forward to digging around in the IF deck again.
Troubleshooting that thing requires some time and effort for me, and
I'm not likely to get to it till the weekend unless I get really
lucky.
In the meantime...I have another "Poll" type question. How
likely do you all think that one of my two remaining filters has
developed a partial short to ground? How unexpected is that? I've
already isolated the fault to the IF deck by the way.
Would a leaky blocking cap contribute to a filter's slow demise,
or would that take it out in milliseconds?
Thanks to all...
Dennis, who wishes he could just send his IF deck to a
depot for overhaul...
On 9/12/05, Jim Miller <jmiller1706 at cfl.rr.com> wrote:
> 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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>
>
--
Dennis L. Wade
KG6ZI
Carmichael, CA
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