[R-390] Further AGC measurements

Wed Oct 8 15:41:52 EDT 2014

Thank you David. I will take a look at these items in addition to others.

I appreciate your time taken and I will let you know what I find.

The assumption, the radio is not corrupted with mods etc... Is valid. It is
quite original but not a pedigree.

On Wed, Oct 8, 2014 at 3:17 PM, David Wise <David_Wise at phoenix.com> wrote:

> First, let's assume it hasn't been butchered.  If it has, then all bets
> are off; anything
> could be miswired to anything.
>
> From a systems point of view, there are three possible causes for
> subnormal AGC voltage.
>
> 1. Signal chain gain is subnormal.  Less signal means less AGC.  This is
> pretty likely.
>  Standard troubleshooting techniques apply.
>
> 2. AGC chain is bad.  No matter how much signal you have, AGC will be
> low.  Symptom is
>   excessively loud (maybe distorted) output, since the signal chain will
>   be running with excessive gain.
>
> 3. Something is loading down or pulling up the AGC line.  Standard
> suspects are listed below.
> You probably have more than one.
>
>  a. Leaky C551.  Look for a change when switching between FAST, MED, and
> SLOW.
>   If this cap has not been replaced, I guarantee it's bad.
>  b. Leaky mechanical filter(s).  The foam rubber goes bad.  With power
> off, look for
>   a change in resistance when changing bandwidth.
>  c. Leaky antenna trimmer.  This variable cap is tied to AGC.  It's
> insulated from the
>   chassis, but the insulation is exposed to contamination and can easily
> get leaky.
>  d. One or more tubes have grid emission.
>  e. One or more coils, switches, etc are leaking to the AGC line.
>
> We've taken care of item a.
> Items b, c, d, and e can be diagnosed in a number of ways, but here's what
> I do.
>
> First, we will partition the AGC system into "the controlled" and "the
> controlling".
> We will analyze the former first.
>
> Disconnect the AGC NOR jumper between TB102 terminals 3 and 4.
> Except for R201 and R234 in the RF deck, and a number of tube grids
> which will conduct if driven positive, the controlled side of the AGC
> net should be a perfect open circuit.
>
> Measure the voltage at TB102-4 with the radio running.  It should be
> around 0V or slightly negative.  (The negative comes from the Contact
> Potential effect.)  If it's more than about 0.5V positive, you have B+
> intrusion,
> grid emission, or both.  Pull the following tubes, one by one, until you
> see the voltage fall: V201, V202, V203, V204, V501, V502, V503.
> If the voltage stayed positive even with all these tubes out, you
> definitely
> have B+ leakage.  (You might also have grid emission; you will have to
> check again after you have eliminated the leakage.)
>
> I had much more but I got tired.
> Post your results and I will direct you further.
>
> Regards,
> Dave Wise
>
> ________________________________________
> From: R-390 [r-390-bounces at mailman.qth.net] On Behalf Of Alan Victor [
> amvictor at ncsu.edu]
> Sent: Tuesday, October 07, 2014 11:34 PM
> To: Craig Heaton
> Cc: r-390 at mailman.qth.net
> Subject: Re: [R-390] Further AGC measurements
>
> Hi Craig,
>
> excellent question and you are probably right! My approach was driven by...
> Is this radio electrically sound or is there something so bad that a
> mechanical tear down is an absolute necessity? And lets start ripping out
> stuff. I looked at CAM alignment, no binding, no slug hangup, decent
> sensitivity above 8 MHz. Checked for bad mechanical filters, AH yes,
> replaced the .01 uF Vitamin Q (which I subsequently measured and it was
> spot on and not leaky), pulled the power supply filter caps, a-ok, and the
> 8uF acid leaky cap in the audio section, was pristine! That all said,
> decided to look at troubelshooting what was to me obviously not right
> electrically. That process found several of the 2nd LO xtals way off in
> spec. Fixed. The PTO over range and end points are nearly spot on. And so I
> continued along the electrical route with this final bit with the AGC. At
> this point I think I am finally convinced I must pull units to begin a
> probe for out of spec items. The tube extender and playing modules out of
> the main frame is attractive to me for troubleshooting but has mixed
> reviews. I do have evidence now that the MC bandswitch needs attention,  as
> the sensitivity below 8 MHz degraded last night only to return after
> changing to say 21 MHz and then returning back to 7 MHz. So, yeah, now I am
> seeing some mechanical ugly issues. Finally, the AGC fault is not going to
> get resolved without pulling the subchassis. Hence, now its time to do what
> you suggested. P.S. the power supply is not converted, not solid state,
> supply caps look and play like new, and operation for now is on variac.
> . .
> On Tue, Oct 7, 2014 at 7:49 PM, Craig Heaton <hamfish at efn.org> wrote:
>
> > Alan,
> >
> > A couple of questions, sort or wondering if the cart is in front of the
> > horse? Going back thru this thread on the R390 e-mail reflector & AM
> Forum,
> > have you replaced the BBOD's & electrolytic caps yet in the entire
> > receiver?
> > Have all the switches been cleaned? There are so many of those little
> > got-ya's which should be addressed first. Have all the variable IF & RF
> > transformers been removed/cleaned so there is good pin contact?
> >
> > I'm rather new playing with these R-390/A's, it takes me about two weeks
> of
> > cleaning, replacing old caps, etc., before any mechanical or electrical
> > alignment. After which comes all the bugs to track down. (poor grounds,
> bad
> > pots, leaky caps, more out of speck resistors)
> >
> > Should the module pull & mainframe breakdown come first?
> >
> > Craig,
> >
> >
> ----------------------------------------------------------------------------
> >
> >
> ----------------------------------------------------------------------------
> > --------------
> >
> >
> >
> > -----Original Message-----
> > From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Alan
> > Victor
> > Sent: Tuesday, October 07, 2014 6:54 PM
> > To: r-390 at mailman.qth.net
> > Subject: [R-390] Further AGC measurements
> >
> > I completed some more AGC measurements and have some clues.
> > However, bottom line, ready for a module pull and mainframe breakdown.
> For
> > the record though, I found the following:
> >
> > Monitor the AGC TP3-4 points with VTVM. That AGC voltage tracks AGC feed
> to
> > the RF amp, and the three mixer grid control AGC voltages within 0.2 V.
> > Still the AGC value always low even on large RF input of 150 uV/30%
> > modulation.
> >
> > First a careful Z measurement again on TP 3 shows ~ 380K. Clearly less
> than
> > the desired 500 K. Possible issue here.
> >
> > Next, look at all the series grid R values from RF amp and all mixers.
> The
> > values are all on target, except for the RF amp, its 50 K to large!
> > Measured
> > over 500 K. Possible issue here.
> >
> > Finally, the most interesting measurement, with the AGC loop open.
> Function
> > switch is set for the AGC ON, MEDIUM time constant.
> > The AGC control line is fed via an external power supply with series 10K
> R
> > to pin4. The RF input level is 150 uV/30% AM mod, now I can easily
> control
> > the AGC line so that -7 V (AGAIN AGC OPEN LOOP) is generated at TP3 and
> the
> > carrier meter comes alive.
> > Larger input RF level can readily drive the AGC line pin3 to over -12 V.
> >
> > So the AGC circuit (OPEN LOOP) is functioning. Clearly, with the AGC loop
> > closed it is not. Either the AGC source Z is not nearly low enough to
> > source
> > the required current (includes leakage) or the leakage which is present
> is
> > larger than desired for proper AGC action.
> >
> > I think it would be helpful to measure the actual power supply current
> when
> > this experiment is run. That would confirm that the current required by
> the
> > AGC loop closed is just to high to support a functioning AGC.
> >
> > Comments?
> >
> >
> > Alan
> > ______________________________________________________________
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