[R-390] AGC problem

[JamesMiller]

AGC voltage for the front end RF Amp tube exists on the tuning shaft for the 
antenna trimmer ... if there is oil or grease on the fiber coupler and 
washers associated with the antenna trimmer, the oil could show as a path to 
ground for the AGC voltage at this stage.  This can reduce AGC effectiveness 
for the front end.  Spray with a non-oil based degreaser cleaner such as Big 
Bath.  I have also found that sometimes the .005 bypass capacitors at the 
AGC points tthroughout the radio can go bad and pull down the line.  I have 
had to lierally replace all of them in one radio.  Finally, AGC is developed 
in the IF Module ... there is a tuning coil there that looks like an IF can 
Z-something, I forget the number) that you have to tweak to get the right 
AGC action.


----- Original Message ----- 
From: "Bill Hawkins" <bill at iaxs.net>
To: "Bill Abate" <wabate at dandy.net>; <r-390 at mailman.qth.net>
Sent: Friday, December 10, 2004 3:42 AM
Subject: RE: [R-390] AGC problem


Don't have voltages for you, but here are a few ideas:

Do you have the right tubes? No sharp cutoff where they should
be remote cutoff.

Does the S meter read correctly?

Do the resistors in the AGC circuit have the correct measured
values? How about the cathode resistors of the controlled tubes?
They should be within 20% of the schematic value.

Is the resistance to ground of the AGC line greater than 100 meg
if you isolate the line? Can you measure the resistance with
something that applies 20-30 volts to the line? Or just apply
voltage to the line with three 9 volt batteries and see if it
propagates down the line correctly.

Regards,
Bill Hawkins


-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of Bill Abate
Sent: Thursday, December 09, 2004 11:31 PM
To: r-390 at mailman.qth.net
Subject: [R-390] AGC problem


I also found the AGC voltage on all the tube grids it was supposed to
control.  The only thing I can conclude is that the AGC voltage is
insufficient but the manual does not say what the voltage should be with
varying inputs.  At least I could not find it.  I also tried slow, med
and fast AGC and the results were the same.

Anybody have data on the AGC line?  Any ideas as to the cause??

Thanks,

Bill, K3PGB

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