[Hammarlund] Re: Phasing control null on crystal filter for SP-600

[Bob Young]

I assume to use the AC voltmeter method like the manual you need to keep a 
speaker connected to the output also?
Also what bandwidths did they change to in issue 2? I have had trouble 
aligning mine after I changed faceplates and screwed up the setting,

Bob Young
KB1OKL


>From: "Les Locklear" <leslocklear at cableone.net>
>To: "David Wise" <David_Wise at Phoenix.com>,<hammarlund at mailman.qth.net>
>Subject: Re: [Hammarlund] Re: Phasing control null on crystal filter for 
>SP-600
>Date: Tue, 3 Jul 2007 15:15:21 -0500
>
>FWIW, I have aligned it this way for years....................take off the 
>screws holding the can to the chassis. Set the air variable at 1/2 mesh and 
>center the phasing knob at the mid-point. Of course you have to peak it.
>
>Close 'enuff for me or guvmint' work. It seems to work just fine that way. 
>Incidentally, Collins used that method for the 51J series.
>
>Les Locklear
>Gulfport, Ms.
>DX'ing Since '57
>http://www.hammarlund.info/
>
>----- Original Message ----- From: "David Wise" <David_Wise at Phoenix.com>
>To: <hammarlund at mailman.qth.net>
>Sent: Tuesday, July 03, 2007 1:15 PM
>Subject: [Hammarlund] Re: Phasing control null on crystal filter for SP-600
>
>
>Andy Moorer writes:
>
>>L37 is always a problem to align. It is a very broad peak at the best of
>>times. The ONLY way to align it is exactly the procedure in the manual -
>>including the AC voltmeter on the audio output. You have to put the
>>bandwidth select to 1.4 (the most narrow non-xtal setting) and then you 
>>can
>>sorta get a peak.
>
>Small correction: 1.4 is the widest xtal setting, not the
>narrowest non-xtal setting.  Selecting 1.4 is in the procedure.
>
>I was tweaking L37 the other night, and I noticed that
>with a 400Hz modulated carrier, the broad peak had pure
>400Hz audio at the center, with 800Hz harmonic on either
>side.  (I ignored the S-meter; it didn't correlate with
>any of this.)  Maybe that pure spot is the target.
>At least it's sharper than the peak.
>
>Tektronix and HP used a similar crystal filter arrangement
>in some of their older spectrum analyzers, using a crystal
>as the upper leg of a voltage divider.  At series resonance,
>a crystal's impedance is minimum.  It rises sharply on either
>side.  You can affect the bandwidth of the overall circuit
>by adjusting the load placed on the crystal.  For reasons
>that escape me, Hammarlund and Tek used a parallel-resonant
>LC circuit for the load.  (HP used a resistor.)  A low Q tank
>is a heavy load, and the crystal's own resonance dominates,
>giving a narrow bandwidth.  A high Q tank is a light load -
>even when it's slightly off resonance, because it's broader
>than the crystal.
>
>Back to the SP-600.  As you go from 1.4 to .5 to .2,
>the LC tank is spoiled to a lower and lower Q.
>If it's tuned exactly to the crystal series peak,
>you get a symmetrical response.  If it's mistuned,
>well, you don't, and I can't figure it out any further,
>nor explain how the phasing control does its thing.
>
>Hammarlund changed the spoiler resistors in Issue 2.
>They didn't change the crystal part number or anything
>else around the filter, so the bandwidths must have
>been wrong in Issue 1.
>
>Dave Wise
>SWL in Portland Oregon
>R-390A '56 Motorola
>SP-600 R-274A (Issue 2)
>HQ-100
>SX-96
>FRG-7
>Tektronix 535/535A/545/547 o-scopes
>HP 141T spec-an
>GR 1001A
>HP 8640B
>Etc
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