[R-390] decreasing signal touching kilocycle knob

Mon Feb 17 17:24:53 EST 2020

Jose, To check the end play, grab the vfo shaft with your fingers and try
to move it in and out with a little pressure.  Not too much, though (just a
few pounds).

The vfo shaft ground is a 90 degree metal stock that is mounted on the vfo
shaft collar by the manufacturer and contacts the shaft.  Verify that it is
in contact with the shaft with a little bit of pressure and the contact is
clean.  Just apply a drop of deoxit to the contact point.

Regards, Larry

On Mon, Feb 17, 2020 at 4:58 AM jm <josemic at gmail.com> wrote:

> 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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>>> > than "Re: Contents of R-390 digest..."
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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
>>> > Home: http://mailman.qth.net/mailman/listinfo/r-390
>>> > Help: http://mailman.qth.net/mmfaq.htm
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>>> > This list hosted by: http://www.qsl.net
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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
>>> >
>>> >
>>> >
>>> >
>>> > ------------------------------
>>> >
>>> > _______________________________________________
>>> > R-390 mailing list
>>> > R-390 at mailman.qth.net
>>> > http://mailman.qth.net/mailman/listinfo/r-390
>>> >
>>> >
>>> > End of R-390 Digest, Vol 190, Issue 2
>>> > *************************************
>>> >
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>>
>>