[R-390] Please spare us

[Dan Merz]

Hi,  since the 51J-3 (R388) has phasing control for the single xtal filter,
as does the Collins 51J-4,  my comment about Hammarlund vs Collins engineers
was nonsense.  Evidently,  phasing control for notching was still a
desireable feature on some Collins radios in 1950.  Perhaps the 390 and 390a
eliminated this feature to preclude the operator from notching out a desired
signal by mistake while having the frequency dial set on the known signal
frequency.  Others may know the story here.  I don't recall a discussion of
this but haven't looked for it.   Dan.  

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of David Wise
Sent: Tuesday, June 07, 2005 10:20 AM
To: R-390 at mailman.qth.net
Subject: RE: [R-390] Please spare us

> From: Dan Merz [mailto:djmerz at 3-cities.com]
> 
> Hi,  the crystal filtering in the radios you mention is of the type 
> that dates back to the early thirties and is not the passband type of 
> filter with a relatively flat top that passes a range of frequencies 
> uniformily.  Rather the top is very sharp and the passage of some 
> range of frequencies is further down on the skirts of the response.  
> This was early on called a "signal signal response" filter because the 
> bfo could be

Typo.  "Single signal".  You leave out the important fact that the crystal
had not only a sharp peak, it also had a sharp notch.
The Phasing control affected the spacing between them, so you could peak up
on the desired signal and simultaneously null out an interfering signal
nearby.  There must have been some shortcomings or side-effects, because
later crystal filter designs (such as in the R-390x) make a point of doing
away with the notch, by means of a neutralization adjustment.

[snip]

> very close.  I added a 500 khz ssb mechanical filter to my
> 388 to see the
> difference;  it helped some but wasn't world shaking.  Though it was a 
> Collins filter,  it may not have been as good as the ones used in the 
> 51J4.

It depends a lot on how you install it.  "Blowby" is a big issue, and you
have to take a lot of care with shielding to get anywhere near the
out-of-band rejection the filter is capable of.  That's why the R-390A
filters are in those double-ended flange-mount cans.
This is discussed in the Cost Reduction Report.

73,
Dave Wise
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