[Collins] Intermittant tuneable BFO problem 75S-3C

[billriches]

Intermittent tunable BFO problem.  (I am using 15 nov 1975 page 7-9/7-10
schematic)

 

After bfo is turned on in 10 minutes it drifts and then quits.  Turn off
radio for an hour or so and it will do the same thing.

 

Changed V11 6DC6, plate and screen voltages ok.  Control voltage via CR5 56
v zener ok.

 

All fx determining parts are in L10 can.

 

The only 2 parts that can be checked without opening up can are R79 and
C146.  R79 ok.

 

Disconnected wire from pin 3 of can that goes to R81 bfo tuning pot. Hooked
up Sencore Z meter  (cool cap checker) between pin 3 and ground (pin 4) of
can.  Cap read 9800 pf (close enough for .01 cap).  Then checked for leakage
by setting voltage on z meter for 100 volts.  No leakage.

 

No other parts in can are able to be tested without removing and opening up
can - don't want to do that!

 

Re-connected control wire back on pin 3, fired up radio and bfo has been
working now for 8 hours!  I am thinking that the 100 volt jolt maybe cleared
leakage in C146.  

 

Anyone have any experience with this problem?  l know it will probably come
back again.

 

(At first I thought it might be CR4 that was bad - hope not as I could not
find any replacement for it.)

 

73,

 

Bill Riches, WA2DVU

Cape May

 



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