[R-390] AGC problem

[James Miller]

One more suggestion:  Tighten all the hardware, screws, nuts, etc.
especially those that hold the tube sockets and ground lugs down.  Again,
they loosen with age due to heating/cooling cycles.  And don't discount a
few cold solder joints or even joints where the mfgr forgot to solder and
QC.  I had an IF deck by Clavier once where at least 5 pins had absolutely
no solder on them at all.  Anything can happen.

----- Original Message ----- 
From: "Bill Abate" <wabate at dandy.net>
To: <r-390 at mailman.qth.net>
Sent: Saturday, December 11, 2004 12:55 AM
Subject: Re: [R-390] AGC problem


> Win a new Icom IC-756PROIII and help QSL/QTH.net
> Details at: http://mailman.qth.net/index.html
> OK guys, thanks for the info.  First off I have the gain pot a little on
> the low side for a better s/n ratio.  If anything that should help.  My
> sensitivity is between 0.6 and 1.1 microvolts for 10 db s+n/N.  All the
> tubes are correct and have been tested in a Hickok before aligning the
> rig.  It really is perking quite well.  I have that darn linearity
> adjustment to do for the Cosmos PTO, but I got sidetracked on this SSB
> thing.
>
> I pulled the IF deck and played around with it for most of the day.  I
> replaced a few resistors and caps but not because they were that bad.
> Same results.
>
> I got one more piece of data that makes no sense.  The suppressor grid
> of V504 and V508 (pin 2) has +6 V on it instead of the -2.5 that should
> be there according to my manual.  The voltage does not change with AGC
> or MGC selected with the function switch so I conclude that it is coming
> from V509 and not the AGC bus itself.  Now the no signal voltage on pin
> 1 or 2 of V509 is -.5 V and should be -1.9 V.  Maybe that is why I am
> not getting the AGC voltages that Ed, WB2LHI, said his manual indicates.
>
> One thing that I think has helped is to lower the B+.  The military put
> diodes in the place of the 26Z5's and the voltage was 255 which is a
> little high.  I inserted a 10W 40 V zener and got the voltage down to
> 215.  The distortion is not that bad now.  I can copy some stations with
> full RF gain but it still does not have good fidelity.  Maybe I moved
> the overall gain of the rig to where the AGE has more of an effect?  I
> am still blaming the AGC voltage.  Z503 has been peaked but maybe there
> is something wrong with it?
>
> Not ready to give up yet.
>
> Thanks again,
>
> Bill, K3PGB
>
>
>
> Bill Abate wrote:
> > My 390A is almost finished its restoration.  Looks pretty good and works
> > well.  Well, except for SSB reception.  If I try to tune in a ssb signal
> > I get a lot of distortion in the audio and the signal can't be tuned in
> > properly.  If I try the old method of max audio gain and reduced RF
> > gain, all is well.  Of course this is not the way other R-390A's work.
> > BTW, AM reception is pretty good but has some distortion in the audio.
> >
> > I figured that the AGC was not working properly so I ran the AGC test in
> > the manual.  I adjusted the ,local audio for 1.7V of audio with 5
> > microvolts of RF.  I then increased the RF sig gen to 1000 microvolts
> > and the audio output went to 20 volts.  Its supposed to increase to 3
> > volts, so I found the problem.
> >
> > The problem is I can't seem to find the cause.  I have AGC voltage on
> > pin 2 of v506 (AGC line).  I got these AGC voltages with my sig gen:
> >
> > 5 mv    ->     .04VDC
> > 50mv    ->     -.2 VDC
> > 500mv   ->    -3.8 VDC
> > 5000mv  ->    -7.1 VDC
> > 50000mv ->    -9.6 VDC
> >
> > I also found the AGC voltage on all the tube grids it was supposed to
> > control.  The only thing I can conclude is that the AGC voltage is
> > insufficient but the manual does not say what the voltage should be with
> > varying inputs.  At least I could not find it.  I also tried slow, med
> > and fast AGC and the results were the same.
> >
> > Anybody have data on the AGC line?  Any ideas as to the cause??
> >
> > Thanks,
> >
> > Bill, K3PGB
> >
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