[R-390] Further AGC measurements

[Alan Victor]

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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