[R-390] Please spare us

[Dan Merz]

Hi, thanks for the correction,  "signal signal" was a typo - that'll teach
me to do late night replies!!  Should have been single signal.  My first
real ham band receiver was a simple homebuilt thing with 1700/1700.5 Khz
crystal pair front end filter for the i.f.  It was better than most
commercial inexpensive sets with only 2 or 3 i.f. transformers.  The R-388
even when operating with only i.f. transformers has better selectivity. The
single crystal filter when implemented well does a nice job,  the SX-28,
early Super Pro's, SP-600 come to mind within my own experience.  I spend
little time using receivers that were actually designed for ssb though I
listen to ssb most of the time.  I must actually prefer to twist knobs,
which probably orignates from using an S-38 Hallicrafters to discover and
receive ssb - not even an external bfo adjustment.  Radios designed for ssb
are somewhat boring to operate in comparison,  though I have a couple. I
must admit at times I miss the HQ-180 I had for a while but opted to keep
the SP-600 instead and ride the r.f. gain.  The SP-600 kept the phasing
control, so it's not that phasing control wasn't still regarded as a useful
front panel control in 1950. Maybe the Hammarlund engineers were just better
at implementing it than the Collins guys (better here means either it was
cheaper, quicker, or proven in their shop so don't think I'm belittling the
C guys). Occasionally,  I have used the crystal filter in the SP-600 for
ssb.  I haven't studied the single crystal circuit in the 390 enough to
comment other than to say that 2 degrees of selectivity in that mode were
considered sufficient.  Best regards,  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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