As I discussed with Jim a few days ago, I replaced the PTO in my 390 with
one from a 390A, but that was ~4 decades ago and I don't recall all that was
necessary. I recall making some component changes, and I did not need to
have any mounting hardware fabricated.
There was no discernable loss of sensitivity or any other operational issues
after the swap. Linearity was excellent as was the PTO stability after the
swap.
I guess I should have kept notes, but I didn't...The B+ feed was changed to
use only the 150Vdc required by the 390A PTO, and I'm wracking my brain to
see what else might shake loose up there...!!!
73...Jordan VE6ZT
Ralf's Law: Murphy was an optimist..!
----- Original Message -----
From: "Jim Whartenby via R-390" <[email protected]>
Cc: "R-390 Mailing List" <[email protected]>
Sent: Tuesday, September 17, 2024 12:24 PM
Subject: Re: [R-390] R-390 VFO Question
It appears that mounting wise, there is no physical difference between the
two PTOs. Electrically, there is the screen grid of the 70H12 which is
separately regulated while the 70H2 is powered by a regulated B+ supply
but other then this, B+ appears to be the same. The output of the tanks is
different but I am not sure it that matters since both have similar output
impedances and voltage ratings.
I am not sure if the formula for the ferrite core remained the same.
Perhaps there were lessons learned from the earlier builds that influenced
the ferrite core formula? I am under the impression that later build
R-390A/URR receivers had less of a problem with frequency effects due to
aging.JimLogic: Method used to arrive at the wrong conclusion, with
confidence. Murphy
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