[R-390] Noise when warm ('390A)
Jim Miller
jmiller1706 at cfl.rr.com
Mon Sep 12 17:37:21 EDT 2005
My story about oil on the antenna trim shaft reminds me,... any oil anywhere
on tube sockets or rotary switch materials can cause problems, even deoxit
and contact cleaner. If the oil soaks into the tube socket or switch
phenolic, you have a nice high impedance path formed, and it doesnt take
much to disrupt a high impedance circuit like the AGC line. I fought with a
Collins KWM2 radio once thinking I was doing good to clean the tube sockets
with contact cleaner... NOT... messed up performance and AGC big time. Have
to use deoxit very sparingly, q-tips or tooth picks just where you want it.
If you suspect oil intrusion into any sockets or switches, use Big Bath
http://www.newark.com/product-details/text/catalog/59473.html or similar to
displace it. These are high impedance circuits, everything you learned in
solid-state books doesn't matter here. It just gets funner and funner
doesnt it?
Jim N4BE
----- Original Message -----
From: "Jim Miller" <jmiller1706 at cfl.rr.com>
To: <sacramento.cyclist at gmail.com>
Cc: "R-390A Receiver List" <r-390 at mailman.qth.net>
Sent: Monday, September 12, 2005 5:29 PM
Subject: Re: [R-390] Noise when warm ('390A)
> Dennis
> If I were you I would replace the problem caps, C553 especially,
> regardless of how new they look. It's still a 40+ year old deck and they
> can still start leaking with time. The AGC bypass caps in my radio were
> disc types also, but even they developed leakage apparently as I was able
> to remove some problem behavior by replacing them as well. I don't recall
> any disassembly required in the IF or RF deck to replace these, I used a
> small pencil type soldering iron with lots of solder wick to get the
> solder out first, then some dentist picks to loosen the wires and remove
> the capacitors. A magnifying lamp helps also. Your past posts seemed to
> indicate that the AGC to ground resistance dropped when you plugged in the
> IF deck, so these are suspects.Look also at caps C547, C548 (0.1) around
> the AGC time constant tube V506A. An earlier post mentined seeing
> 350-400K on the AGC line when the ID deck is plugged in. This is probably
> normal since R547 220K and R545 100K form a path to ground. And then
> there's also the story that the antenna trimmer shaft is actually at AGC
> potential in the RF deck. Too much oil and grease on it can create a path
> to ground there and mess up the AGC in the RF stage. A VOM/VTVM test will
> never detect that, only actual operation of the radio. Not sure I have
> helped, unfortunately I have found that part by part replacement may be
> the only way to finally isolate a problem, starting with the mosr likely
> culprits firts. The best indicator of a bad part is how it misbehaves in
> the radio, not a VOM test. Jim
>
> ----- Original Message -----
> From: "Dennis Wade" <sacramento.cyclist at gmail.com>
> To: "Jim Miller" <jmiller1706 at cfl.rr.com>
> Sent: Monday, September 12, 2005 4:58 PM
> Subject: Re: [R-390] Noise when warm ('390A)
>
>
> Thanks for the notes Jim.
>
> I may have said this in previous posts, but in case I didn't...the IF
> deck is a recent vintage EAC deck that was "factory" modded for SSB
> (Rippel on his site has a short description). It had no black/brown
> beauties, and all the "problem" caps were the West-something sealed
> variety. Therefore I took the "only replace if needed" approach with
> the radio. My primary concern was physical access to the caps in the
> IF deck, especially the ones near V501 and V503.
>
> The bypass caps in there now look like disk ceramic (in shape). Are
> they really paper? And yes, of course they test good.
>
> Do you remember having to go through any disassembly to get to those
> caps to replace them? That is my major reluctance...incurring damage
> in getting to them.
>
> I am beginning to think wholesale replacement (at least in the IF
> deck) is indeed warranted even in a late EAC deck.
>
> Anyway, thanks for your input..I appreciate it.
>
> Dennis
>
> On 9/12/05, Jim Miller <jmiller1706 at cfl.rr.com> wrote:
>> 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
>> > >> _____________________________________________________________
>> > >> R-390 mailing list
>> > >> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> > >> Help: http://mailman.qth.net/faq.htm
>> > >> Post: mailto:R-390 at mailman.qth.net
>> > >> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>> > >>
>> > >>
>> > >
>> > >
>> > > --
>> > > Dennis L. Wade
>> > > KG6ZI
>> > > Carmichael, CA
>> > > _____________________________________________________________
>> > > R-390 mailing list
>> > > Home: http://mailman.qth.net/mailman/listinfo/r-390
>> > > Help: http://mailman.qth.net/faq.htm
>> > > Post: mailto:R-390 at mailman.qth.net
>> > > Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>> > >
>> > > _____________________________________________________________
>> > > R-390 mailing list
>> > > Home: http://mailman.qth.net/mailman/listinfo/r-390
>> > > Help: http://mailman.qth.net/faq.htm
>> > > Post: mailto:R-390 at mailman.qth.net
>> > > Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>> > >
>> >
>> >
>>
>>
>> --
>> Dennis L. Wade
>> KG6ZI
>> Carmichael, CA
>>
>>
>
>
> --
> Dennis L. Wade
> KG6ZI
> Carmichael, CA
More information about the R-390
mailing list