[R-390] Why is it so ? AGC SOLUTION

[David Wise]

Sorry not to get involved until it was over.  I was busy with other things.

Anytime the symptom is low AGC voltage combined with high signal output, I consider C551 the prime suspect.

On my '58 Motorola, C547 and C548 are Sprague "Vitamin Q".  They are based on oil-soaked paper, which is notorious for deteriorating over time, but this model is hermetically-sealed and uses exceptionally good materials.  Mine are in good shape even 60 years later.

But my C551 was leaky.  Replacing it effected a complete cure.  I gutted and stuffed the original can, an operation I did not enjoy.  Later I wrote up a mod to fix an ergonomic problem (see "Ending The Moment Of Silence"), which included replacing C551 again, this time with a cap remotely located on the front panel.  The original refurbished C551 was abandoned in place.  If I had to replace a C551 today and for some reason did not want the Moment Of Silence mod, I would put the new cap on the front panel anyway.

73,
Dave Wise

PS: How is the Carrier meter mod working for you?  (Gary Gitzen came up with the idea; I just made it ready for prime time.)

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Pete Williams
Sent: Saturday, August 06, 2016 6:13 PM
To: r-390 at mailman.qth.net
Subject: [R-390] Why is it so ? AGC SOLUTION

​Hi  all..... My  thanks  to  Larry and  Charles  for their excellent
advice on  solving  my  AGC   problem  per  recent  email..The  solution
was relatively  simple  once  you know  all the  facts    and  I  apologise
for  not initially reporting that this  IF   module  had been modified
to mate  with  a  non  standard  Carrier meter (100 ohm)  using the  David
Wise mod with the  addition of the  1N914 diode from R546/R547 to   chassis
.It  might have made the  diagnosis   easier in retrospect.
However,   following  the   checks  suggested,  I  changed all the   AGC
capacitors  figuring  that  in this  case  a 'shotgun'  approach  would
eliminate   any  doubtful and  leaky  caps  seeing  as how  they  were aged
about 60+  years. A new  2 mFd replacing C551  nicely installed  under the chassis  finished  that  bit.
Surprisingly ,  reinstallation of the   module  produced   absolutely  no
improvement ! -what the   ? !
No  AGC  and the  high -45  V on pin 2 of  V509  not  getting to
V506-----if it  was  getting  somewhere   why  was it  being  siphoned
off   and to where?
An inspired(?) moment of  reflection  and  look at the  meter  mod
schematic seemed than it might be possible that  the   1N 914 might
be questionable.
A  check showed the  diode  had  a problem--- 1.5 K in  both  directions.!
The  future  beckoned  and a replacement  did  all that  was  expected of
the  module  and  faith restored with  situation and   voltages  normal.
Again , my  thanks to  Larry  and  Charles who  steered me in the  right direction.
Cheers
Pete  VK3IZ
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