[Hammarlund] SP600 Perormance

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: "Wes Bolin" <k5apl at yahoo.com>;
<hammarlund at mailman.qth.net>
Sent: Thursday, May 30, 2013 10:13 AM
Subject: Re: [Hammarlund] SP600 Perormance


Wes:

"Cleaner" is a perfect description. That is what I
experienced when I changed out the caps in mine. You're
right. It IS hard to quantify or describe but it is
definitely there.

Joe Connor

     This is surprising if you had modern film caps and
changed them to ceramic caps.  Ceramic caps may have an
advantage at higher frequencies but not at lower ones.
Modern film caps have very low ESC, not measurable on my
General Radio bridge. I don't have instruments to measure
the actual reactance of caps at high frequencies.  At some
point the inductance of the cap equals the capacitance so
they become series resonant circuits. For bypass purposes it
makes no difference since they then look like a short
circuit to ground for AC, which is exactly what you want for
bypass purposes.  The early production sets had the
notorious Black Beauty caps which were evidently defective
from manufacture. These were sold as high quality caps and
were used in a lot of deluxe equipment. Disc ceramic caps
may have some faults but are generally very good for FR
purposes and almost never fail although the High K type do
age some.
     The SP-600 has an AV. problem which causes
intermodulation at low audio frequencies. This results in
quite audible distortion on broadcast stations. The
distortion goes away when used in Manual RF gain.  A fairly
large capacitor across the AVC terminals on the back will
help but slowing the AVC down. You can see the effect on the
signal strength meter because it is deflected downward on
strong modulation. This meter measures the carrier strength
at the detector so the variation reflects an actual
variation in the carrier.
     Much of the distortion in distant short wave stations
(and medium wave AM too) is from selective fading. This
happens when the signal comes in by two paths at once. It
causes something like the "flanging" effect used in records.
A synchronous detector will eliminate the distortion but not
the frequency flanging.
     Alignment of both IF and RF are very simple.  You need
an insulated tool to work the trimmers in the RF. The kind
with a recessed blade make life a lot easier.  You may have
to cock the bandswitch slightly for some bands since the
adjustments don't always line up with the access holes.
     I've tried painting tube shields with Krylon Ultra-Flat
Black paint.  I don't have means for measuring envelope
temperature but it probably helps to absorb and re-radiate
the heat.  The main concern is the envelope temperature.
Since tubes have a very good vacuum inside they are pretty
good heat insulators. The heat from the electrodes reaches
the glass by radiation.  Some may be radiated back to the
plates but its not very efficient.  I am not sure how much
difference envelope temperature really makes.
    In many cases the bottom part of the usual tube shield
provides enough shielding since it shields the leads.
    The SP-600 is an excellent receiver.  I think a lot of
them are working but not working right so they have a bad
reputation for the wrong things.
    Dial calibration should be quite accurate but often the
stators of the tuning capacitor have moved and someone has
bent the plates to try to compensate. If you cap has bent
plates and they are not bent a lot you can try straightening
them but often they have been bent too much and will break
loose.
    The stators and rotors must be exactly centered. The
stators are held in place by screw clamps.  Adjusting them
is not difficult but the cap must be removed to do it.


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