[Boatanchors] First radio

[Al Klase]

Richard Speaks the Truth.

A while back I worked out some worth while audio improvements for the 
SP-600.  These bacame lost on my website, and I had to use Google to 
find them.  Read the instructions carefully.

http://www.skywaves.ar88.net/commrx/Hammarlumd/SP-600/SP-600_Audio.html

Al

On 8/11/2012 12:53 PM, Richard Knoppow wrote:
> ----- 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
>
>
>       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
>
> ______________________________________________________________
>

-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/