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

[Bob kb8tq]

Hi

The resistors are there to “de-Q” the coils. The coil as wound has a higher Q
that what makes sense for the passband of the IF. They add the resistors to
bring the Q to the “proper” level to get the correct passband. Since the main
selectivity is done by mechanical filters, this is not as big a deal in a 390 as in
some other radios. 

It is likely that different manufacturing techniques (and possibly different cores)
gave coils different Q “as wound”. That would drive different outfits to use different
resistors. Since the IF gain is manually adjustable (and often set way to high) there
isn’t much value in using them to modify the gain. 

Looking at your data, it would appear that the Capehart coils came in with higher
Q than the Motorola coils ….

Fun !!

Bob

> 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.
> 
> This one came with a Stewart-Warner IF deck (Order # 42428-PC-59-A1-51).
> 
> When I began to check everything into, it became obvious 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.
> 
> My question now is: what are the proper values of those resistors ??
> 
> I checked for the ones used in my Moto '56 and in a spare IF deck from
> Capehart '61 contract and the results were:
> 
> Moto '56: R511, 512, 553, 554 : 47k ohms R522: 82k ohms.
> 
> Capehart: R511, 512, 553, 554 : 39k ohms R522: 68k ohms.
> 
> 
> 
> So, why there is a difference between the different makers / contracts ?
> 
> Which values should I use in the Stewart-Warner ??
> 
> I was unable to find anything in the Wu Li's "Pearls" section.
> 
> Does somebody already had documented these values somewhere ?
> 
> 
> 
> 73, Jacques, VE2JFE
> 
> 
> 
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html