[Hammarlund] SP-600 HFO Alignment

[Richard Knoppow]

    A couple of things. The caviet about the plate bending is 
important. However, it is possible for the stators to drift out 
of centering. Easily fixed without damaging the capacitor. The 
tuning cap must be removed. Tedious and one must be careful of 
the parts used to link the capacitor to the tuning mechanism. 
There is a spring which will fly off into the forth dimention. I 
usually tie it with some fine solder. The stators can be centered 
with sections of feeler blades. They must be exactly centered 
laterally and vertically and exactly parallel with the rotor. 
While most obvious in the oscillator sections its worth making 
sure the RF sections are also centered. The stators are clamped 
in place with screws and fiber washes.
    The IF is centered on the crystal filter frequenc. It will be 
very close to 455Khz but not exactly on. Find the crystal 
resonance and use it to tune up the rest of the IF system. The 
455Khz IF is done first, then the 3955 IF on the higher bands 
where it works.
    The IF transformers are all critically coupled at the 3Khz 
position so can be just peaked there. This is easier than the 
overcoupled transformers in the HQ series and the RCA AR-88 where 
one must use a sweeper.
    In most receivers with a crystal filter the actual crystal 
frequency is used to peak the IF. An important exception is the 
Collins 51J where the IF  be _exactly_ at 500Khz. The crystal 
filter may be off center by a little.
    IMO the SP-600 is a much maligned receiver. I think there are 
a lot of them which work but are really broken. They should have 
very good dial calibration and low drift.
    It would be interesting to know the whole story behind the 
SP-600 design. An earlier version was advertised with octal tubes 
and, evidently, a vernier dial with separate band spread 
calibration for the ham bands. I think it got completely 
redesigned. There are some things about its design that puzzle me 
because I think Hammarlund had a better way of doing the same 
things which were used in earlier receivers.

On 3/24/2020 8:11 AM, k2cby at optonline.net wrote:
> I would repeat in boldface underlined Never bend the plates of the tuning
> capacitor! It is undoable and a death sentence so far as proper alignment is
> concerned.
>
> That said, there are several points to consider in trying to match the HFO
> frequency to the tuning dial.
>
> 1.	On the frequencies below 7.4 MHz (the 3 lower bands) be sure that
> the IF is actually centered at 455 kHz, and on the higher bands be sure that
> the IF is aligned at 3,955 kHz and the crystal oscillator is putting out
> 3,500.00 kHz.
> 2.	Be sure there isn't some non-factory "improvement" hanging off the
> wire that runs from the plastic feed-through at the front of the RF strip to
> the VFO-Crystal oscillator switch. That wire leads directly to the HF
> oscillator, and any stray capacitance to ground will foul up the frequency
> calibration at the high end.
> 3.	Without touching the oscillator coils, use the proper size steel
> screwdriver to "exercise" the air variable capacitors of the oscillator
> section for each band. Especially if the boat anchor has been sitting
> untouched for a couple of decades, the capacitor shafts will stiffen and you
> will either bend or break a plastic alignment tool.
>
>   
>
> Miles B. Anderson, K2CBY
> 16 Round Pond Lane
> Sag Harbor, New York 11963-3821
> Phone: (631) 725-4400
> FAX: (631) 725-2223
>
> e-mail: k2cby at optimum.net
>
>   
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL