[R-390] R-390A calibrator oscillator

[Roger Ruszkowski]

John,

You are working with a 50 plus year old critter.
Clean the tube socket pins
Change the tube

You may have a pair of tubes that test good in the tester and
just are not up to par. We always put the two poorest noise 
5814 tubes into the cal oscillator. 

So V205 B is a grid leak stage with only 33 ohms in the cathode.

Pin 7 of the cathode follower stage has a 1 meg resistor to the
oscillator and divide  by two flip flop plate. It is a wonder the pin is not at 
at 150 volts. The 15 pf cap across the resistor couples in the cal tone
harmonics to the grid of the cathode follower.

The tube depends on the plate voltage sucking on the grid
with enough potential to leave the pin looking like it has a negative
voltage on it. As the tube changes, so the conductance changes,
so the voltage on the grid will change.

There is no specification for the cal tones beyond
the tone shall be audio able on each 100 KHZ to 31.+000

You are right to expect a nice clean tone.
See the other input from the Fellows and make sure 
you do in fact have a cal tone problem and not another 
oscillator problem.

If changing the tube does not clean up the problem,
you will need to pull the RF deck and do some inspection.
We often find resistors way out of wack in the cal osc tube sections
The crystal oven for the 200KHz and 17 MHZ crystals heats that corner of 
the deck and over works every thing in that
corner of the RF deck.

You could also have a bad cap and parasitic spurs in the cal tone
flip flop and buffer. 
You could have a bad crystal with spurs.

What ever it is, Its still an R390/A and thus can be fixed.

John, If all the mail does not help you get the receiver fixed, Post us another letter.
Do tell us what you tried and what you may or may not have been able to narrow down.
There is no reason to not fix the problem. We will share ideas with you for as long as it takes.

Roger 33C4H (68 - 75)

P.S. Fellows this is exactly why one R390 or R390/A  is not enough. You need a ready reference at hand.