[R-390] AGC problem

[James Miller]

Get the ohmmeter and check the values of all resistors, replace any out of
spec. ones.   Start with the IF module.  The old carbon resistors tend to
drift higher as they age.  Especially check cathode, plate and screen
resistors (they tend to change from being in the line of power flow).  The
resistor aging problem is more common than you would think.  They can "look"
good cosmetically and still be very bad.  JM

----- 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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