[R-390] decreasing signal touching kilocycle knob

Sun Feb 16 04:53:13 EST 2020

Hi fellows

I have a little problem with my r390a that being not important, sometimes
is a bit annoying.

When I touch the kilocycles knob for tuning a station, the strength of the
signal decreases slightly. When a broadcast station is tuned the issue has
not relevance, but in the case of tuning cw or ssb signals with low signal
levels, the effect is noticeable and a decrease in the intelligibility of
the signal and the increase of the background noise happens.

Especially, the issue is aggravated when the AGC/MGC is selected.

What can I do? Is there a faulty component? This is the only (minor I
suppose) grievance I have with my receiver.

Many thanks and regards

Jose

EA8DCP

El dom., 16 feb. 2020 a las 3:30, <r-390-request at mailman.qth.net> escribió:

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> Today's Topics:
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>    1. R-391 Motorboating with RF Gain beyond 7 (Mack McCormick)
>    2. Re: Z-503 (hamfish at comcast.net)
>    3. Re: R-391 Motorboating with RF Gain beyond 7 (Larry H)
>    4. Re: Z-503 (Charles Steinmetz)
>    5. Re: R-391 Motorboating with RF Gain beyond 7 (Charles Steinmetz)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sat, 15 Feb 2020 16:31:11 -0500
> From: Mack McCormick <w4ax.mack at gmail.com>
> To: R-390 Group <R-390 at mailman.qth.net>
> Subject: [R-390] R-391 Motorboating with RF Gain beyond 7
> Message-ID:
>         <
> CAFzyUt53P2S+a5TGFwrxFTdpYp6HsyzfAJuVvpm8N1YqX8UN0Q at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> My R-391 has developed motor boating (audio oscillation) when the RF gain
> control is advanced beyond 7.
>
> If you're run across this issue I'd appreciate knowing how you fixed it.
>
> I have rebuilt about a dozen R-390a and have a full test equipment bench.
> I'm just hoping for a head start before I begin troubleshooting the IF
> stage. Yes, I tried the usual searches but didn't find anything.
>
> Thanks in advance.
>
> 73,
>
> Mack
> W4AX
> https://www.qrz.com/db/W4AX
>
>
> ------------------------------
>
> Message: 2
> Date: Sat, 15 Feb 2020 14:40:07 -0800
> From: <hamfish at comcast.net>
> To: "'ROSS HOCHSTRASSER'" <bavarianradio at comcast.net>,
>         <r-390 at mailman.qth.net>
> Subject: Re: [R-390] Z-503
> Message-ID: <[email protected]>
> Content-Type: text/plain;       charset="us-ascii"
>
> Hello Ross,
>
> Digging through Wei-Li's Pearls......................
> https://www.r-390a.net/Pearls/index.htm and under IF Decks, seems values
> between 82pF to 88pF. Also stated on page 340 those parts might have been
> mixed & matched for resonance at the factory for 455KC.
>
> Regards,
> Craig
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
> Behalf Of ROSS HOCHSTRASSER
> Sent: Saturday, February 15, 2020 12:12 PM
> To: r-390 at mailman.qth.net
> Subject: [R-390] Z-503
>
> Hi all, Pardon me for not shuffling through endless pages of R-390A info,
> but I need to know the value of the capacitor in Z-503.  I am having AGC
> problems and am coming to my wits end. When  this receiver landed on my
> bench, Z-503 was open. I obtained one from K1ANX which has a good coil and
> may be A-OK, but I can't get a satisfactory response when tuning it. I am
> unable to get full action out of the AGC on strong stations. Swapping the
> IF
> module with my other receiver, the problem follows the IF module. I have
> recapped it, changed all of the tubes, checked all of the resistors, even
> changed (most) of the silver micas to no avail.  I am suspecting that Z-503
> is not at resonance due to  an intermittent cap.( I could very well be
> wrong
> on this...)  The  manual I have shows the cap, but offers no value, in the
> parts list it lists the cap as part of Z-503, but gives no value. HELP!!
> Thanks, and 73 Ross W1EKG
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>
> ------------------------------
>
> Message: 3
> Date: Sat, 15 Feb 2020 17:37:48 -0800
> From: Larry H <larry41gm at gmail.com>
> To: Mack McCormick <w4ax.mack at gmail.com>
> Cc: R-390 Group <R-390 at mailman.qth.net>
> Subject: Re: [R-390] R-391 Motorboating with RF Gain beyond 7
> Message-ID:
>         <CAOSJ_nuKUZBpMSJRUmd=
> tp+NqkT_OTufKG+joyCSFC7oayTdQg at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Hi Mack,  This is usually caused by feedback due to weak bypass caps
> (screen, plate, or cathode).  This is an easy check (grab a .1 - .5 mfd 400
> v cap and ground the neg end and touch the + end to all those appropriate
> points).  It can also be caused by a weak agc cap - try the slow position
> and see.  And it could be a weak audio out B+ filter cap.
>
> Have fun.
>
> Regards, Larry
>
> On Sat, Feb 15, 2020 at 1:31 PM Mack McCormick <w4ax.mack at gmail.com>
> wrote:
>
> > My R-391 has developed motor boating (audio oscillation) when the RF gain
> > control is advanced beyond 7.
> >
> > If you're run across this issue I'd appreciate knowing how you fixed it.
> >
> > I have rebuilt about a dozen R-390a and have a full test equipment bench.
> > I'm just hoping for a head start before I begin troubleshooting the IF
> > stage. Yes, I tried the usual searches but didn't find anything.
> >
> > Thanks in advance.
> >
> > 73,
> >
> > Mack
> > W4AX
> > https://www.qrz.com/db/W4AX
> > ______________________________________________________________
> > R-390 mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/r-390
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:R-390 at mailman.qth.net
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> >
>
>
> ------------------------------
>
> Message: 4
> Date: Sat, 15 Feb 2020 22:08:25 -0500
> From: Charles Steinmetz <csteinmetz at yandex.com>
> To: r-390 at mailman.qth.net
> Subject: Re: [R-390] Z-503
> Message-ID: <5E48B229.9090905 at yandex.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> Ross wrote:
>
> > Hi all, Pardon me for not shuffling through endless pages of R-390A
> info, but I need to know the value of the capacitor in Z-503.  I am having
> AGC problems and am coming to my wits end.
>
> As I recall, C563 is typically around 82pF, though there is some
> variation.  Note that Z503 is a *low-Q* plate load for V508, so it's
> pretty non-critical.  Also check C543 (the V508 cathode bypass
> capacitor), C544 (the screen bypass capacitor), and the value of R541
> (the cathode resistor).
>
> Of course, low gain anywhere in the IF section would produce similar
> symptoms.  Is there any specific reason, other than the previously
> defective (and now replaced) Z503, that you suspect Z503 and/or C563 now?
>
> Have you been through the AGC troubleshooting procedure I have posted
> here several times (most recently on 10/6/2018)?
>
> Best regards,
>
> Charles
>
>
>
>
> ------------------------------
>
> Message: 5
> Date: Sat, 15 Feb 2020 22:29:40 -0500
> From: Charles Steinmetz <csteinmetz at yandex.com>
> To: r-390 at mailman.qth.net
> Subject: Re: [R-390] R-391 Motorboating with RF Gain beyond 7
> Message-ID: <5E48B724.8050901 at yandex.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> > My R-391 has developed motor boating (audio oscillation) when the RF gain
> > control is advanced beyond 7.
>
> Most likely, unwanted feedback due to one or more defective bypass or B+
> filter capacitors (don't assume it's just one bad cap).  Could be in the
> IF and/or AF section, and could be plate and/or screen caps (much less
> likely, cathode bypass caps).
>
> Larry's suggestion to use a temporary parallel capacitor is a sound
> troubleshooting technique, and pretty quick.  I advise using spaghetti
> tubing or heat-shrink tubing on the "probe" lead, leaving just enough
> bare at the end to make contact with the target node.  This greatly
> reduces the risk of accidentally shorting the target node to somewhere
> unfortunate during the procedure.
>
> Best regards,
>
> Charles
>
>
>
>
> ------------------------------
>
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> End of R-390 Digest, Vol 190, Issue 2
> *************************************
>