[Hammarlund] SP-600 IF mod, is it worthwhile? OTHER notes.

[Al Parker]

Hi folks,
    Tom, W4PG, has agreed to share his recent observations on the subject.
I've cc'd below.  He also has given permission to include it on The
Hammarlund Historian website, and is interested in any comments/discussion
which may lead to addtional info to be edited in.  He is not a list
member, but has been a good friend for several years.
73,
Al, W8UT
New Bern, NC
BoatAnchors appreciated here
http://www.thecompendium.net/radio/
http://www.hammarlund.info

----- Original Message ----- 
From: "Warren, W. Thomas" <wtw at rti.org>
Sent: Thursday, May 26, 2005 10:41 AM
Subject: RE: [Hammarlund] SP-600 IF mod, is it worthwhile? OTHER notes.

Al Parker asked me to comment on the IF gain modification since I have a
recently restored JX26 on the bench and am still fussing with it.

I installed the IF Gain control per the Government modification because
I don't need the dial lock and the IF gain control is probably a more
useful application of that panel hole.

What I did notice (and call these preliminary results) is that by
reducing the IF gain by about 10 dB, then the overall sensitivity of the
receiver or comparably, the output SNR is substantially increased.  I'll
do a better job of measurements later as I get my JX26 more settled down
and I gain more experience with it.  However, at the stated conditions
(4 microvolts signal, 400 Hz modulation, 30%, this benchmark thanks to
Roger Ruszkowski, KC6TRU), the worst output SNR I get is 17 dB or so at
50 mHz.  Down on the lowest two bands, I get as much as 29 dB SNR.
These numbers are comparable to a properly tuned up R-390A.  (Please
read the measurement sequence below.)

This business of reducing the IF gain and getting better output SNR is
the same as the R-390A trick.  Obviously I'm rebalancing the gains
between the RF stage (the RF gain should be the same before and after
reducing the IF gain), the IF gain (obviously reduced), and the AF gain
(increased to compensate for the reduced IF gain).  This seems to
potentiallly indicate a design error by Hammarlund and Collins (if
indeed they were designing the receivers for max sensitivity, a very
dubious assumption) in that the IF's of both receivers are too noisy and
aren't overcome by the noise figure and gain of the RF stages.  Further,
with reduced IF gain and in the AVC mode, there almost certainly
(haven't measured this yet, but would be completely nonplussed if not
true) will be less AVC voltage, thus running the RF stages at higher
gain and extremely likely more susceptable to overload up front and
increased intermods up front.  I don't have the capability to measure
intermods up front or in the IF stages, so don't know the tradeoffs the
design engineers may have made among the gain distribution in the
various stages versus the intermods in the various stages.  I saw one
reference to the Collins engineers measuring the intermods in the 390A
IF and some comment about design considerations there, but I don't
remember the conclusions.

My conclusions from the preliminary experiments are:

1. The SP-600 is nearly as hot a receiver as the R390A.
2. It's very interesting to me that the 6BA6 front end and the 6BE6
mixers of the SP-600 perform as well as the 6DC6 (a less noisy tube than
the 6BA6) and the 6C4 mixers (much, much better than 6BE6's).  There
must be lots of other design considerations and experimental results the
Collins and Hammarlund engineers took into account in choosing those
tube sets.
3.  For my money (at the moment), if the bands are absolutely quiet so
that it's the noise of the receiver setting the SNR and I'm really
trying to dig out a signal, then I'd set the receiver as in the
measurement sequence to allow the receiver to get the best SNR output.
Under other receiving conditions, probably doesn't make much difference.
It's just a very fun and interesting measurement game to see how
sensitive these old girls are, and that answer is, they're very, very
good. (But NOT 0.07 microvolts for 10 dB SNR as claimed by one expert.)

Oh, one more observation.  I recapped all the BBODs (including the RF
stages) with Orange Drop caps with the thought that their resonant
frequencies couldn't possibly be any worse than the BBODs.  The usual
recommendation is to use disc ceramics for bypasses over 455 kHz.  I was
a little worried that by using Orange Drops in the RF stages that I may
not achieve optimum results.  But obviously based on the output SNR I'm
measuring, the use of Orange Drops isn't hurting a thing.  Let's chalk
that up to really excellent engineering by the Hammarlund crew to use
those lousy BBODs with low resonant frequencies and getting superior
results in 1952.  So when we substitute modern Orange Drops (with what I
think and hope are higher resonant frequency resonances since they are
being designed and manufactured about 50 years later), we shouldn't be
surprised to have superior results.  BUT THE NEXT TIME I DO A SP-600 RF
DECK, I will use disc ceramics since they are physically smaller and the
leads aren't as stiff as Orange Drops.

The whole setup and SNR measurement sequence is:

1. AVC/MAN switch to MAN.  RF gain at max.  Bandwidth control to 3 kHz.
Signal generator set to 4 microvolts indicated, 400 Hz modulation at
30%.  I'm using a URM-25D that has a 50 ohm output resistance and I'm
NOT putting a 100 ohm resistor in series with the sig gen as recommended
by the technical manual - maybe later when I do final tuneup and
measurements.

2. Measure the AF output across a 600 ohm resistor.  Set the AF Gain
control for 10 dBm output. Then reduce the IF Gain so that the AF output
drops about 10 dB.

3. Then increase the AF Gain control to bring the audio back up to 10
dBm.

4. Turn the modulation off and observe that the AF output is now minus
19 dBm indicating a 29 dB SNR at 4 microvolts input.

That's it.  Much too long an explanation, but still fun to see how great
these old receivers are when brought back from the precipice of death.

Tom, W4PG
=====================