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

[Fred Moore]

Yes, and please let me add the following.  Its a 'tale of two cities.' 

1.  In the R-390A the intent is to have the IF section have a flat response
across the bandwidth of the 16 KHz mechanical filter.  Then filter selection
determines the overall bandwidth -- not the IF transformers.  If you sweep (or
even graph on paper) the IF response for the various filter selections you can
confirm that you have the correct choices of Q swamping resistors and that the
transformers are are tuned to achieve the desired flat response.  The stages
are stagger tuned.   (There should be a discussion of stagger-tuning the IF
section in the manual.)  

2.  The R-390 has a completely different approach.  The design is based on a
1930s General Electric paper that asks, how many stages can be cascaded that
are sharply tuned, before it becomes impractical?  That was six.  Then the
bandwidth selection switches Q swamping resistors across several IF
transformers at the same time to broaden the response.  But all the stages are
tuned to the same frequency, not like the R-390A. 

-Fred WN9I

------ Original Message ------
Received: Fri, 25 Sep 2020 03:29:23 PM GMT
From: Bob kb8tq <kb8tq at n1k.org>
To: fred_moore at usa.netCc: Collins R-390 HF Receiver List
<r-390 at mailman.qth.net>
Subject: Re: [R-390] The R-390A IF transformers internal resistors.

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
> 
> 
> 
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