[R-390] The R-390A IF transformers internal resistors.

[Roy Morgan]

Jacques,

I don’t have info on the values of the resistors, but I do have a guess:

Those resistors are there to “spoil the Q” of the IF transformers, and in general widen the passband.  It could be that in the two other radios you tell about, the different manufacture coils have slightly different winding resistance and thus need different parallel resistors to get the desired bandpass.

Indeed, there were articles that suggested removing those resistors to “make the radio hotter”.  It was of course bad advice, and the resulting changes in the actual performance were both un-necessary and bad.

Note: in the MIT Radiation Laboratory series volume on “Vacuum Tube Amplifiers” by Valley and Wallman, Volume 18, there is an extensive explanation of the engineering involved in bandwidth control in amplifiers - as applied to radar IF amplifiers (presumably mostly 30 mc).

The entire 28 volume set is available in PDF form here:
https://www.febo.com/pages/docs/RadLab/

Roy

> On Sep 25, 2020, at 11:14 AM, Jacques Fortin <jacques.f at videotron.ca> wrote:
> 
> Hi all,
> 
> I am still working on the last R-390A that followed me home. ... that a previous
> "owner" removed all the resistors within the IF transformers.
> 
> The windings test all OK and the caps looks like the original ones, so it
> may have been an attempt to "boost" the IF gain.

Roy Morgan
K1LKY since 1958
k1lky68 at gmail.com