[Boatanchors] Hallicrafters Crystal Polarity?

[McDonald, J Douglas]

Building a homemade xtal filters that works right will take careful experimentation
to get it to work right. I had day-long wars with both an SX-25 filter and a National
NC-400 filter which had replacement crystals. Since you will be using a 
different transformer, it will be worse. First you have to get the phasing control
to balance correctly, then get all the resistances and tunings of the load after the
xtal proper. You can copy circuits if the coil inductances are close, but in general
both source and load are tuned off-resonance depending on the desired 
bandwidth. 

I eventually got both radios to work per spec, but in both cases 
the values of the small mica caps were wildly off the schematic values,
often by factors of more than two. The caps over 60 pF were OK or close. 

Simulation of the before-after cases showed that the replacement 
xtal inductances were off by as much as factors of four or five. 

Before fiddling neither filter worked even close to spec. One setting of the
National filter had a measured bandwidth of 12 Hz, yes you read that right,
6 dB down at 6 Hz off center! That's not useful. 

Doug McDonald

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> On Behalf Of Michael Bittner

... lots cut

Dave,
The only thing I can think of is that there may be different stray 
capacitances between the two sides of the "specially designed polystyrene 
holder" that could unbalance the filter circuit.  Probably the use of the 
word "polarity" in the manual is just a short hand way of referring to this 
capacitance effect.

I am interested in hearing any further thoughts on this topic as I am 
presently working on a homebrew receiver based on the AN/GRR-2 (military 
SX-28-A).  It will use an original Hallicrafters 455 kHz plug-in crystal in 
a circuit using Meissner crystal filter transformers.
Mike, W6MAB