[R-390] decreasing signal touching kilocycle knob

Mon Feb 17 07:57:57 EST 2020

Many thanks Robert and Larry for your help.

The Rx is grounded through the power cord and additionally via the terminal
16 GND on the back panel.

I have grounded with a wire the Kcs shaft and the problem remains.

Don't know how to check the end play of the vfo shaft 'there should be
none' Sorry for my technical ignorance.

Regards

Jose

El lun., 17 feb. 2020 a las 6:26, Larry H (<larry41gm at gmail.com>) escribió:

> Hi Jose,  Yes, that would be bothersome to me also.  Two things, is your
> rx grounded correctly?  The 2nd could be the external grounding finger on
> the VFO shaft - they can get to where the slightest movement will change
> the contact and thus the grounding of the shaft and therefore the vfo
> frequency.  Also check the end play on the vfo shaft - there should be none.
>
> Regards, Larry
>
> On Sun, Feb 16, 2020 at 1:53 AM jm <josemic at gmail.com> wrote:
>
>> 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:
>> >
>> >    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
>> > ______________________________________________________________
>> > R-390 mailing list
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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
>> > *************************************
>> >
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