[Boatanchors] Sp-600 info

[Richard Knoppow]

----- Original Message ----- 
From: <manualman at juno.com>
To: <boatanchors at mailman.qth.net>
Sent: Thursday, June 27, 2013 10:43 AM
Subject: Re: [Boatanchors] Sp-600 info


> Hammarlund web site:
> http://www.hammarlund.info/SP600mods.htm
> There were a number of articles in all the ham magazines 
> back in the 60's
> for adding product detectors to receivers.
>
> Pete, wa2cwa

    A couple of comments on this site. He says the meter is 
not accurate and there is too much delay in the AVC. The 
meter measures carrier strength at the detector and is 
actually pretty accurate considering its really meant as a 
tuning indicator and is approximate.  The accuracy depends 
on the tubes since variations will vary the effect of the 
AVC.  AVC delay is a fixed bias at some minimal level that 
prevents the AVC from desensitizing the receiver for weak 
signals. In other words the incoming signal has to reach a 
certain strength before it activates the AVC and gain 
reduction begins.  Removing the delay will reduced the 
receiver sensitivity although some writers seem to have that 
backward.
    The SP-600 has a rather simple single loop AVC system. 
Its worst fault is that it follows low frequency modulation 
in AM signals causing a very noticable intermodulation 
distortion.  This distortion goes away when the set is in 
"Manual" RF gain and can be reduced by increasing the AVC 
time constants. If you take the audio from the diode load 
terminal on the back and feed it through an outboard 
amplifier of good quality you will find the audio quality is 
very good. The AVC must be turned off or modified for 
because of the above, but the detector is actually pretty 
good and is isolated by a driver so that it does not load 
the IF.  If you take off the diode signal you must use a 
decoupling capacitor to remove the DC from the amplifier 
feed and must use a fairly high value ressistor in series, 
say 100K, to prevent loading of the diode by the external 
circuit.  You will also need a pot since the voltage from 
the detector is quite high.
     The BFO injection is fairly low, very typical of 
receivers of the time. The idea was that for CW excessive 
injection would desensitize the receiver. On receivers with 
an injection adjustment on the back SSB performance can be 
improved by putting it at maximum but "normal" setting is 
about where it drives the signal strength meter to zero, 
that is just at the AVC delay level.  The JX-17 has fixed 
injection at about this level.
     The TMC MSR-5 or MSR-9 will work well with the SP-600 
and has a suitable AVC which connects to the receiver via 
the AVC terminal in the back.  The AVC and diode terminals 
were intended for use in diversity receiving systems but 
prove useful for other things.
     There was a very late version of the SP-600, later even 
than the 21, which had a modified AVC and some other needed 
changes but they are extremely rare. Hammarlund had a better 
AVC circuit in the old Super-Pro but I think they simply ran 
out of room on the SP-600. It is interesting that Hammarlund 
began advertising the SP-600 about 1948. The ads feature a 
drawing which is recognizable but different in panel details 
and describes a rather different circuit with calibrated 
band spread dial, octal tubes, push-pull audio among others. 
AFAIK this never go beyond the planning stage although I 
suppose there must have been a prototype.  The receiver as 
finally produced used miniature tubes which were not 
available at the time of the original announcement and was 
probably simplified.  It would be intresting to know more 
about the original plan.  One problem obviously was the need 
to have a unified chassis rather than a separate power 
supply. A separate supply solves some problems such as real 
estate on the chassis and heat. My suspicion is that 
Hammarlund smelled Collins on their tail and rushed the 
receiver to the market.  It would have been nice if they had 
done a little more refining but if they had delayed the 
receiver would probably never have been made.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com