[Hammarlund] Crystal Filter Alignment

[Carl]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <k2cby at optonline.net>; <hammarlund at mailman.qth.net>
Sent: Monday, February 07, 2011 4:39 PM
Subject: Re: [Hammarlund] Crystal Filter Alignment


>
> ----- Original Message ----- 
> From: "k2cby" <k2cby at optonline.net>
> To: <Hammarlund at mailman.qth.net>
> Sent: Monday, February 07, 2011 11:04 AM
> Subject: [Hammarlund] Crystal Filter Alignment
>
>
>>I would be grateful for any more information on aligning
>>the SP-600 crystal
>> filter, starting with the procedure for orienting the
>> little arrowhead on
>> the dial (which never seems to be explained in the service
>> manuals).
>>
>>
>>
>> Although I have followed the standard 455 kHz and 3.955
>> MHz procedures
>> outlined in both the commercial and military service
>> manuals with apparently
>> good results, I have never been able to square the results
>> with the very
>> roughly described procedures for sweep oscillator
>> alignment. Is there a more
>> complete description of the sweep oscillator procedure?
>>
>>
>    All of the crystal filters used on Hammarlund receivers
> since the HQ-120-X are the same basic circuit and is one
> Hammarlund had the patent on. It is far superior to the
> original Lamb filter used in National and Hallicrafters
> receivers.
>    The entire IF must be adjusted to the actual crystal
> filter frequency. While it will be close to the nominal IF
> frequency it will not be exact. The crystal is then used
> either to set the signal generator to the correct frequency
> or to establish a center marker if a sweep generator is
> used. Once the IF is set up the crystal filter is done using
> a modulated signal. There are two adjustments: one is a
> peaking adjustment, the other establishes the loading on the
> crystal to broaden it out. Both are set with the crystal
> bandwidth at maximum. The first is set using a low
> modulation frequency, say 400 hz, and peaking for maximum
> audio level with the AVC OFF and input kept fairly low. The
> second is then set using a higher modulating frequency, say
> 2 khz, and again adjust for the maximum output. What you are
> doing is setting the crystal filter _wide_ bandwidth for its
> maximum, the narrower ones will follow. Once set check that
> the phasing is centered. The easiest way is by ear. Tune to
> plain noise with the BFO OFF and the crystal in its
> _narrowest_ setting. The noise will have its narrowest
> bandwidth, that is the least highs, at the center. You must
> first check to make sure the phasing capacitor is centered.
> It is a butterfly type. In the HQ-129-X the shaft has a flat
> on it so the knob is automatically in the right place but
> this is not true of many other receivers using this circuit.
> The minimum noise should occur with the phasing knob
> centered. If it does not check the trimmer capacitance in
> the filter can. In the HQ-129-X this is a short piece of
> insulated wire bent into a U shape, you may have to move it
> or bend it to center the phasing knob. In the older
> Super-Pro there is a compression trimmer in the can for the
> same purpose. Some RX using the Hammarlund circuit, for
> instance the Collins 51J series do not have adjustable
> trimmers but do have a fixed capacitor serving the same
> purpose which can be padded or changed.
>    When set correctly, the null will also be reasonably
> symmetrical. To test for this set the filter for its
> narrowest bandwidth. Then set the generator for the exact
> crystal frequency. Then set the filter for its _widest_
> bandwidth and then off set the generator to one side by a
> known amount, say 1 khz. Null it with the phasing control
> and note the position of the control. Then offset the
> generator the same amount in the other direction, again null
> the signal and note the position of the phasing control. It
> should be approximately the same distance from center in
> both directions. Note that the position of the phasing
> control does NOT follow the frequency of the null: the
> control will be at its maximum displacement for frequencies
> close to the crystal center frequency and move toward the
> center as the null moves away from the center. It is not
> completely symmetrical but will be close for frequencies
> around the middle of its range in either direction. The
> Hammarlund filter is the best of the single crystal filters,
> it was used in Hammarlund receivers and later, by contract,
> or when the patent expired, by TMC, RCA, and Collins and I
> think later by National and Hallicrafters as well. It has
> the advantage over the original Lamb circuit that the
> bandwidty control and phasing control do not vary the center
> frequency or gain of the circuit. I suspect that the reason
> the original version of the RCA AR-88 did not have a front
> panel phasing control was to avoid infringment on the
> Hammarlund patent.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com


National introduced a decent filter in 1938-39 on the NC-200 that is very 
effective and better than what they started with on the AGS, FB7-X and early 
HRO.

The alignment directions in the HRO-60 manual (Hammarlund copy) is the best 
Ive seen and doesnt require a lab to do it either plus it has many more 
tuned IF circuits than other makes.

The Hammarlund, and others can also be adjusted for mode specific use such 
as CW or AM only.

Ive used a stable signal generator and an audio output meter when aligning 
any brand filter since the 60's and its rather simple after youve done a 
few. Nor do I use a modulated signal as its possible to get quite 
confused...at least for me. Instead use the BFO with AVC off and RF gain 
down and step the BFO offset as you decrese the selectivity from 5 to 1. 
Here is where you can either align as built or customize the selectivity for 
a particular preference.

There are at least 3 ways to skin the cat with the HQ/SP filters and they 
all seem to work as advertised.

In a pinch Ive also done the full IF and filter alignment by ear using just 
the BFO and backround noise.....its a bit tricky but once you get the feel 
for the process its pretty fast and reliable. Ive known several OT's that 
never did it any other way including one at National who was there since 
1938 and taught a young kid fresh out of the Navy a lot of tricks that were 
never in the manuals.

Carl
KM1H