[R-390] R-390 VFO Question

[Jordan Arndt]

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" <r-390 at mailman.qth.net>
Cc: "R-390 Mailing List" <r-390 at mailman.qth.net>
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