[R-390] R-390A Noisy Ant. Trim and Raspy Calibrator

[Larry H]

Hi Ken,  

On the trimmer, the gear mounted (pinned) on the top of the trimmer cap is at agc potential.  Check to see that it is and the mating gear on the horizontal 6" shaft is also (it should be isolated from the 6" shaft).  You should measure about 420k ohms from both gears to ground.

On the cal osc, Bill is probably right on the micas.  You might have a bad pin contact on one of the two tubes - try wiggling them quite a bit and see if that helps.  If not, check the voltages.  The cathode follower should smooth off the corners of the square waves from the multivibrator a little via L210.  Can you scope the cal output at E208?

Good luck.  Regards, Larry


On Saturday, July 5, 2014 6:11 PM, Bill Riches <bill.riches at verizon.net> wrote:
 


In the cal crystal circuit there are a few low value mica caps that become
leaky and make a racket.  Look for the ones going from plate to grid of the
osc/mult tube.  I do not have a print in front of me.  Suggest you change
the 3 or 4 that are there.  An ohm meter will not measure the leakage but
when you put voltage on the cap you can see the leakage.  You have to remove
the rf deck to do this.  Yippee.

73,

Bill, WA2DVU
Cape May, NJ

My second issue is on an EAC model. The Calibrator is very raspy (dirty)
sounding, not the nicer, more sine-wave like sound from the Teledyne. Work
in this area has been limited to visual inspection and re-doing potential
cold solder joints. The crystal oven has been swapped over from the Teledyne
with no difference...still raspy and the EAC crystal oven put in the
Teledyne and the note is clean, so I'm sure it is not the Crystal oven.
V-205 and V-206 have been replaced...nope, still raspy. Of note is when I
wiggle V-205 the pitch of the note changes slightly (this does not happen on
the Teledyne) so I re-soldered all tube socket pins...no change. I suspect I
have a bad mica in this area. All resistors are within spec and so far no
components have been replaced in this deck.

Any ideas or advice would be much appreciated!

Kind regards,
Ken Harpur
ZL3AA


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