[Boatanchors] First radio

[Richard Knoppow]

----- Original Message ----- 
From: <djed1 at aol.com>
To: <kf5evv73 at gmail.com>; <boatanchors at mailman.qth.net>
Sent: Saturday, August 11, 2012 6:10 AM
Subject: Re: [Boatanchors] First radio


>I have both, and if I had to pick one, it would be the 
>R-390.  A good one is not hard to tune, and the benefit of 
>an accurate frequency readout is significant.  The one 
>other choice with the R-390 is whether to get the older 
>(and rarer) R-390 with the conventional IF filters, or the 
>more common R-390A, which has mechanical filters.
> As far as antennas- both will work, and both have about 
> the same input impedance.  The R390 has an antenna 
> trimmer, which will help match a random wire.  Both were 
> generally intended to operate with good antennas tuned to 
> the operating frequency.  I operate mine with an antenna 
> tuner, so that my dipole can be matched at most any 
> frequency.  For receiving, a small tuner doesn't cost 
> much.
> Enjoy   Ed  W2EMN


     A couple of comments:  The audio system of all of these 
receivers was designed for communications purposes with 
relatively narrow band and not much attention to distortion. 
All use single-ended (single tube) pentodes.  The frequency 
response is limited on the low end mostly by the output 
transformer.  While some extension is possible by increasing 
the time constants of the coupling circuits is likely to 
increase distortion due to the DC in the transformer core. 
Both the R-390 and SP-600 have means for connecting to the 
diode load on the back panel. For really good fidelity an 
external amplifier can be connected.  The output of the 
detector is very good and both receivers have wide enough IF 
bandpass to yield decent results on AM.
     The SP-600 is a more stable receiver than its generally 
given credit for.  I think part of the reputation for 
drifting is due to most of them having decentered tuning 
capacitors.  The stators of the tuning caps are held in 
place by screw clamps and seem to move with time. They must 
be exactly centered on the rotor for minimum drift and for 
the dial calibration to be accurate.  If all of the sections 
are not centered it the tracking of the RF stages will also 
not be good and it should be right on. One of the most 
pernicious problems is people who bend the plates of the 
capacitor to adjust calibration. There is actually a web 
site with elaborate instructions for plate bending.  NEVER 
bend the plates of an SP-600, it is NEVER necessary.  I 
think it is usually done because the stators have become 
decentered and the person does not know how to center them.
     Usually its possible to return to a given frequency 
quite reliably by means of the logging dials.
     The SP-600 has three IF stages and a very good crystal 
filter. In its narrowest non-crystal position the IF is 
adequate for SSB use but its not a good SSB receiver without 
an external adaptor like the TMC MSR series.
     The R-390 is very complex. In fact one reason for the 
re-design that resulted in the R-390A was to make it more 
maintainable.  It takes a great deal more specialized 
knowledge to work on them than for the SP-600.
     There were many versions of the SP-600, the JX-17 was 
one of the last and is designed specifically for use in 
diversity applications without modification.  The diversity 
facilities are, in general, of no value for general ham use 
but are also not in the way.  These receivers, if original, 
have three red knobs on the front.  There web sites with 
lists of the variations of the receiver.
     In all about 25,000 SP-600 were built and about double 
that number of R-390A's, who knows how many have survived 
because many military sets were demolished when removed from 
service.
     The Collins 51-J series especially the 51J-3 and its 
military counterpart the R-388 is in many ways a better 
receiver for many purposes than either of the above.  Its 
simpler than the R-390, has Collins tuning method which 
makes it extremely stable with good calibration and 
excellent repeatability of tuning and is fairly easy to work 
on.  However, all three of these sets require some skill and 
you are _going to have_ to do some maintenance if you have 
one; that's true of any boatanchor radio due to age. The 
main problem with the A line Collins sets is aging of the 
PTO. These are not too difficult to repair and instructions 
are on the web but all will eventually require adjustment 
which requires removing them from the set. The 51J does not 
have the wide band IF of the other two but its wide enough 
for most AM purposes.  A product detector and slow AVC mod 
is available but again none of these sets were designed for 
SSB and require an external detector or modification to work 
well in that mode.


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