[Hammarlund] SP600 Characteristics

[Richard Knoppow]

----- Original Message ----- 
From: "David Wise" <David_Wise at Phoenix.com>
To: "Wes Bolin" <k5apl at yahoo.com>; 
<hammarlund at mailman.qth.net>
Sent: Thursday, May 30, 2013 2:19 PM
Subject: Re: [Hammarlund] SP600 Characteristics


If you go back through the message archive, you should find 
a post containing a cure for strong-signal distortion.  I 
modified the bias on the last IF amp to make saturation less 
likely.  The details vary depending on the production level 
of your radio, so you'll have an opportunity to get very 
familiar with it.

Dave Wise

     The last IF has fixed bias using a cathode resistor. 
It should not be difficult to change it and see if it makes 
any difference.  First thing is to see if the distortion 
goes away when using manual gain.
     Many broadcast stations use very heavy processing. They 
run close to 100% all the time, plus current practice is to 
tip up the high end. The result is a raspy sounding 
distortion.  I hear this on my RCA AR-88 though not quite as 
bad.  I tested by listening to the output of a 
Hewlett-Packard 606A generator modulated from a high 
fidelity source, the result on the RCA receiver is pretty 
clean so I think what I am hearing is on the stations.
     I recapped the AR-88 with film caps because I could not 
find a source of good ceramics at the time.  I might change 
to ceramics in a couple of places.
     I have one SP-600 that had all original BB caps, all 
were bad. Some were cracked, some had pieces missing.  All 
were of value nearly all had high dissipation factor.  These 
were oil filled caps. The filling was done by means of a 
tube in one end which later held the lead on that end.  If 
the leads were overheated when the cap was installed it 
would melt the seal and the oil would leak out.  None of the 
caps I dissected had any oil in it.  Also, the windings in 
all of them were distorted.  I think they had a problem with 
the molding process.  At the same time the BB series was 
made Sprague also made another series called Orange Drops. 
These were described as having the same plastic impregnated 
paper dielectric as the BB but were not oil filled and were 
in dipped epoxy cases.  AFAIK they were quite reliable so I 
strongly suspect it was the molding process that caused the 
trouble.  In any case, even the best paper dielectric caps 
of the time would have degraded by now.
     Hammarlund and the military changed the 0.02uf to 
0.01uf when replacing the BB's with ceramics.  This may have 
been due to the cost factor and to the very low dissipation 
factor (same as ESR) of the ceramics.  The lower value 
ceramics may have been as effective as higher value paper 
caps.
     My measurements, made on a GR impedance bridge at 1khz 
indicate the plastic caps have such low dissipation factor 
that it does not register.  Now, the effectivness of a cap 
as a bypass depends on the parasitic reactances and 
frequency.  The idea of the cap is to provide a very low 
impedance path to ground for any AC to prevent coupling 
between stages or feedback.  All capacitors have some 
inductance. Modern film caps have a lot less than the old 
paper variety and ceramic and mica caps have very little. 
At some frequency the capacitor becomes resonant.  Since it 
is series resonant its impedance actually becomes less then 
goes back up on the other side. Since the original paper 
caps had higher resonance than modern film caps its hard to 
see how this could cause the audible difference.  Neither 
plastic film caps or ceramic caps cause distortion despite 
the audiophool myths about ceramic caps. This is especially 
true of low K ceramics of the type sometimes known as 
Class-1, AKA NP0 or C0G.  These are the best of the ceramics 
but are large compared to the hi-k type. For boat anchor 
radios it makes no difference since they are still smaller 
than the original paper caps. They are probably the optimum 
except for some RF applications where one wants silvered 
mica caps.


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