[Hammarlund] SP-600 IRecapping Finished)

[Richard Knoppow]

----- Original Message ----- 
From: "James A. (Andy) Moorer" <jamminpower at earthlink.net>
To: <hammarlund at mailman.qth.net>
Sent: Saturday, January 28, 2012 3:49 PM
Subject: Re: [Hammarlund] SP-600 IRecapping Finished)


> Alignment tools - "all" you need is a standard plastic 
> alignment tool
> with a metal tip that will turn a trimmer capacitor or a 
> slotted
> threaded shaft. One of those plastic tools with the small 
> square of
> sheet metal on the end works as well as anything.
>
> Note that the high band is so twitchy that even putting 
> the tool in the
> hole makes it go crazy. You end up just turning maybe 1/8 
> turn, then
> taking the tool all the way out to see if it made things 
> better or worse.
>
> BE SURE AND FOLLOW EXACTLY THE ALIGNMENT PROCEDURE FOR 
> L37. That one is
> wacky - it is a very broad peak, but it makes a huge 
> difference in the
> operation of the receiver. The given alignment procedure 
> is the only one
> that comes up with good results.
>
> All the other coils are easy. Crank n' peak n' move on. Be 
> sure and let
> the radio warm up thoroughly before starting. And go easy 
> on the signal
> generator - don't use too strong a signal or you will find 
> peaks everywhere.
>
> On the audio mod, there are no downsides. I don't think it 
> makes much
> difference in the resulting sound, however (imho). There 
> is so much
> distortion in the audio final that it swamps all the other 
> issues. The
> manual is quite proud that they kept the distortion below 
> 10%(!).
>
> -A
>
> -- 
> James A. (Andy) Moorer
> www.jamminpower.com
>

     If possible a non metallic tool should be used, 
however, the one I mostly use is a very old combination tool 
with a metal blade on a plastic shaft.
     The first oscillator _is_ somewhat sensitive so you 
must tweek it and then remove the tool to see where you have 
gotten. With a little practice its not difficult.
     L-37 is the loading coil for the crystal filter. Its 
tuned to get the _broadest_ response from the crystal when 
its in its broad position. The way to tune it is to use a 
modulated signal of about 2khz and tune for maximum.
     Note that the entire IF is aligned at whatever the 
crystal filter resonant frequency is. You can find that 
easily by setting it in its narrowest position and using the 
BFO to find the peak by ear. Once you have that tune the IF 
for that frequency.
     I tune using a modulated signal. Set the receiver for 
manual RF gain and set the signal generator for the minimum 
level that gives you a relatively noise free signal. The 
audio modulation for all but the crystal can be any fairly 
low frequency, I use 400 hz. The amount of modulation should 
be fairly low, say about 30%. Us an audio meter to find the 
peaks.
     Before starting make sure the crystal phasing capacitor 
is centered, you may have to remove the can to see it. mark 
the shaft so you can find this position again. Align the 
crystal with the phasing control centered. Note that that 
the Hammarlund crystal filter is excellent. It is a 
Hammarlund patent and was used later by Collins and TMC 
among others.
     The RF alignment is simple but the antenna input stage 
is sensitive to reactance so should be adjusted using a 
signal generator with fixed resistive output impedance. 
While the input impedance of the receiver is around 100 ohms 
it can be aligned fine with a 50 ohm generator. However, the 
calibration of the RF meter will not be right without a 
correction. BTW, over much of the range this meter is 
surprizingly accurate.
     About audio quality: If you want the best quality use 
an external amplfier fed from the detector output. You will 
need a DC decoupling cap and a high value resistor to avoid 
loading. Most of the distortion in the normal output is from 
the AVC trying to follow the low frequencies in the 
modulation. The easiest cure for listening to broadcast 
stations is to run the thing in manual. That will get rid of 
the intermodulation distortion. One can see the AVC working 
by looking at the RF level meter, it bounces with modulation 
because the carrier level is actually changing. Slowing down 
the AVC will also make a noticable difference. One can 
simply hang a high value cap across the AVC terminals on the 
back. This is OK for broadcast reception but not for general 
use. The AVC in the SP-600 is actually very good but is 
designed for following rapidly varying RTTY signals rather 
than Hi-Fi audio.
     As far as the audio output stage itself, just leave it 
alone. It is a single-ended pentode stage with a series-fed 
transformer and no feed back. Since you don't have to rely 
on it just leave it. The detector stage is actually a very 
good one with low distortion so using an external amp will 
be quite pleasing.
     Note that most broadcast stations currently use a very 
great deal of processing so are not sending out anything 
resembling a high fidelity signal.
     Another note:  If the tuning capactor has not been 
damaged by plate bending and the dials are aligned correctly 
the frequency calibration is very good. Hammarlund never 
really published a spec but mentions one in a couple of 
places namely 2%. I think actually they are a bit better 
than this.
     The tuning capacitor stators are held in place by clamp 
screws with insulating fiber washers. Evidently the stators 
can drift off after many years. Its vital that the stators 
be exactly centered with relation to the rotor and that they 
be exactly in line at the edges. This is not difficult to do 
but requires removing the capacitor. The rotor blades should 
NEVER be bent. I stress this because I have enountered a 
couple of bent ones and there is a web site with elaborate 
instructions on bending them. DO NOT bend the plates, its 
not necessary and will screw up the cap beyond repair.
     If anyone is interested in the method I use for setting 
up the dials write and I will send it to the list. Its not 
difficult at all but it took me a while to figure it out.


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