[R-390] Further AGC measurements

Wed Oct 8 12:20:28 EDT 2014

Craig,

You ask about order of events.

If you understand the theory and can trouble shoot then you can do the 
cosmetic bathing and cleaning first. If your skills are not yet well
developed then you sort of want to keep the receiver running and learn things
from it in steps. Back in the school house you started every hour with a 
working receiver and ended every hour with a working receiver.
Within the hour some thing would go bad ( a bug got inserted while you were out smoking)
you trouble shot the problem, provide a fix and again had a working receiver to end the hour.
Of course you were working on a clean receiver that did not need cosmetic cleaning.

But you do mention cosmetics and dirty switches and oxidized contacts and grounds.

So there is a two edge problem.
In the process of doing a good full tear down clean and lube you get to cure / reset / avoid a bunch of 
contact problems, dirty scratchy pot problems, oxidation problems an discover any obvious bad resistors and caps.
In the process of doing a good full tear down clean and lube God only knows what you will break and thus have to fix.

We are blessed as the R390 R390A has wonderful systematic back audio end to front RF end trouble shooting 
procedures that will let you find and fix any and all single or multiple problems.

Forty five years latter I favor the bath first, Swap all the caps, reset all the mechanical ground lug, clean up the past bad solder, clean up the
tube socket pins and switch contacts, re clean / lube the pots, test all the tubes and then put it all back together. 

Then with a prayer and good ground just apply power watching for smoke and spontaneous part disassembly.
Then you can check the fuses, check the B+, eye ball all the filaments and do a front panel diagnoses.

Now you have a clean machine and you can go about debugging and repairing one problem at a time with
faith that the problem is not an oxidized contact or bad ground or bad mechanical alignment or obvious broken resistor or cap.

I have found that you shotgun more small problems out than you introduce new problems.

The mechanical size of the receiver is such that you do not have to worry a lot about introducing shorts as you go 
about soldering in new parts.

 I have gone after signal to noise problems one solder joint and ground lug at a time.
I have learned to save a lot of time by just doing all the ground lugs in the IF deck at once.
Then troubleshoot the IF deck for the one point I screwed up to return the IF deck to operation.
Just believe there is sixty years of oxidation in every mechanical connection and repair it.

You are only going to do this level of overhaul to a receiver once when you take possession
of it and then you are just going to use it for the next ten to twenty years with the minimum 
of tube testing / replacement and IF - RF deck alignment. Blow the dust out every fall and wash the 
face plate every spring.

Only when the oxidation on the contacts again get to the point where the receiver does not 
operate on all the bands reliably as you tune around will you want to repeat a deep overhaul and cleaning.

You will know when its time to service you receiver.

Roger Ruszkowski  33C4H 68 - 75  AI4NI

-----Original Message-----
From: Craig Heaton <hamfish at efn.org>
To: 'Alan Victor' <amvictor at ncsu.edu>; r-390 <r-390 at mailman.qth.net>
Sent: Tue, Oct 7, 2014 10:49 pm
Subject: Re: [R-390] Further AGC measurements

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,
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-----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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