[R-390] C-551 Replacement/AGC trouble?

[Roy Morgan]

At 01:43 PM 6/28/2005, you wrote:
>      From looking at the AGC line, I think there may
>be some leakage somewhere as there doesn't seem to be
>enough there.  Any quick troubleshooting hints?


Dennis,

Put an ohmmeter capable of reading high resistances on the AGC line to 
ground (notice if it reads the same with the nagative polarity on the line 
as with the positive polarity on the line)..then unplug each module (RF, 
and IF) in turn to see where you might be getting leakage. Knowing how the 
AGC wires run from teh source in the IF module to the rear panel terminal 
and the AGC switch and to the RF module will help figure out where any 
excess leakage is.

(One way to measure low levels of leakage is to hook up a 9 volt battery to 
simulate AGC voltage with a DMM on low voltage (or current) range in series 
plus side to ground, negative to the AGC line.. You'll be able to detect 
microamperes of leakage and normal AGC line currents.  If the tubes are all 
cold, you may not detect a leaky tube, but you will find any leaky caps or 
cable leakages.  A warmed up tube may leak more than if it's filament is cold.)

THEN, with the offending module connected, remove each of the tubes 
involved to see if you have a leaky tube.  If you find one, you are lucky. 
If you don't, remove the module and find the (likely) leaky cap in there.

Cable wires can be leaky, too, so don't discount that possibility.  Teflon 
wire in the AGC line would be a good idea.

Roy



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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