[R-390] Further AGC measurements more

Wed Oct 8 14:28:31 EDT 2014

Hello Roger and thank you for the feedback (no pun intended).

I certainly appreciate all comments and the recent correspondence from
Craig makes sense. However,
my approach in this "fix-it" process has always been to look for a
measurement or an experiment that
would hopefully find a clue as to what is wrong. With respect to the AGC,
opening the loop and conducting
measurements, certainly was an eye opener. Before doing a tear down,
perhaps this evening, I'll record the numbers
for a given RF input level. Specifically the 150uV/30% AM metric point. The
power supply controlling the AGC line is fed
to TP4 via a series 10 k R. I need to record the voltage right at the AGC
terminal, I suspect it would read the power
supply V IF THE LEAKAGE IS SMALL. As reported prior, say -3V to 0V in the
power supply V was sufficient to drive the
open loop AGC V as measured at TP3 from nearly zero V to over -12 V. At the
same time, the -3V on TP4 was sufficient to just about shut the audio down,
while at 0 V, your ears hurt! Sorry, did not record S/N ratio. Carrier
meter tracking quite nicely. However, upon closing the loop the range of
AGC V is severely constrained. It almost appears as though the reference V
for the AGC, obtained by the resistor bias divider (see the circuit
description in the TM) consisting of R544, 546, 545, and 547 is off base.
Or, the open loop gain of the AGC loop is way to LOW. That is to say, it
takes a 3 volt shift in AGC line V at TP4 to swing 12 V at TP3. In a
"proper" working unit, that voltage change may be substantially less! Weak
V506A, V509A; perhaps? A leaky cap... perhaps. The Z as stated before is
not 500k. And of course a part value (resistor) way out of spec. So the
real AHA moment will be when the problem is found correlating back to the
original problem and those symptoms.

Thanks, Alan
W4AMV

On Wed, Oct 8, 2014 at 12:45 PM, Roger Ruszkowski <
flowertime01 at wmconnect.com> wrote:

>
> Allen,
>
> In your past post you indicated that you thought the AGC of the  RF deck
> was OK, in that you could open the AGC loop and control the RF deck AGC
> gain
> with an external voltage.
>
> So now you are looking at the IF deck for the problem.
>
> Past practice has been to set the receiver up on end on the bench and
> operate
> the IF deck just swung out of the receiver laying on the bench. You can
> also
> operate the audio deck this way.
>
> You can thus  get some scope and meter probes into the circuits this
> way and get readings with all the tubes in place.
>
> Back in the days 68 - 75 we were not having AGC problems and
> so we never got to understand the problems in the AGC circuit as we did
> not have them
> back then.
>
> Now the parts are getting old and problems are popping up. We are learning
> to resolve the AGC problems.
>
> By way of understanding the AGC circuit you are out on the leading edge
> of measuring and reporting what the circuit has to offer up as operating
> values
> for both good and bad points.
>
> Thank you for putting the time into sharing your results here on the
> reflector and of course it all goes into he archives where your finding
> can be found again.
>
> I trust the reflector Fellows are giving you lots of input and feed back
> to encourage you to continue working your problem to a successful
> solution in your receiver.
>
> Thank you for sharing it all with us.
>
> Roger Ruszkowski AI4NI
>
>
>
> -----Original Message-----
> From: Alan Victor <amvictor at ncsu.edu>
> To: Craig Heaton <hamfish at efn.org>
> Cc: r-390 <r-390 at mailman.qth.net>
> Sent: Wed, Oct 8, 2014 4:30 am
> 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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