[R-390] R-390A Changing VFO PTO EP adjusting range

[Larry H]

Thanks for the comments and good question on the NTC of 200.  I didn't do any calculations to arrive at 200.  It was mostly an estimate.  Most of the VFO's I've seen drift on warm up and take quite a while to stabilize, and I believe more so now than when they were new.  This is due to a few reasons (seal leakage (humidity penetration), nitrogen depletion, and component degradation).  
These 2 Cosmos were drifting a little, so I estimated N200.  The 2 NTC caps in most PTO's are 10 pf at N100.  If I would have put in an NPO, that would have reduced the NTC effect a little, but probably would not be noticed.  Because they were drifting, I wanted to increase the effect slightly, but not a lot.  The drifting was improved, although they still did drift some.  The value to use depends on the PTO.
I have tried calculating the correct NTC value in the past, and have not had much success at it.  Once I tried a rather elaborate formula and it was not as good as an educated guess.  If someone has a good method of  estimating or calculating it, I'd be interested.  Before I work on a working PTO, I measure the drift with a counter to get an idea of how much drift is there.  You can't use the BFO as a basis because they drift quite a bit sometimes (unless you've measured it with a counter and know what you have).  It should be installed in the rx, because of temperature considerations.
All things considered, I like a good working Cosmos better than the Collins because it's easier to linearize.  I too, make the adjustment while it's installed.  The Collins liniarization stack is very hard to work with.  It's difficult for me to determine which shim needs to be moved and how much.
Regards, Larry