[ARC5] BC-221 question

[Bill Cromwell]

Hi Richard,

Thanks for that info. I already had some of it but you nicely filled in 
some details. I regretted letting my HQ-145 go the same day I sold it. 
Some of the ladder filters we can make today do a pretty good job so I 
have abandoned the single xtal filter. However, having one of those in a 
receiver would not put me off.

73,

Bill  KU8H

On 5/22/20 9:35 PM, Richard Knoppow wrote:
>     FWIW, Hammarlund had a patented crystal filter. It was designed by 
> Donald Orem or Oram who was chief engineer of Hammarlund at the time. It 
> was introduced on the HQ-120-X and used on all Hammarlund receivers with 
> crystal filters afterward. A similar filter is used on Collins receivers 
> and the TMC GPR-90, probably elsewhere too.  It is not like the prevous 
> crystal filters which were mostly versions of James J. Lamb's filter, 
> also patented and first used by National Radio. The Lamb filter changes 
> resonance with  bandwidth and has a different sort of phasing adjustment 
> which requires retuning the receiver when its changed. The Lamb filter 
> was an enormous improvement over receivers without filters. It was 
> described in detail in early 1930s editions of QST. I think the Handbook 
> method of adjusting probably applies to the Lamb filter. It is tedious 
> and may have errors.
>     The Hammarlund filter is easy to adjust but there is a trick that is 
> left out of the procedure. Namely, that the loading coil is adjusted for 
> _maximum_ bandwidth using an AM signal. The generator is set for the 
> exact crystal resonance frequency with the selector set for minimum 
> bandwidth. Then adust the input transformer for maximum. Then check the 
> phasing capacitor for centering. When centered the bandwidth should be 
> minimum. You can usually find the center by listening to the noise. This 
> center should not change with bandwidth.  Then set the filter  for 
> maximum bandwidth. Leave the generator on frequency but modulate it at 
> some fairly low level, maybe 30%, with a tone of about half the filter 
> bandwidth. For most filters around 2K or even 2.5K is about right. Then 
> adjust the load coil for maximum output. You can re-check that you are 
> at the exact resonance of the crystal by resetting the generator to CW 
> and recheck that the input transformer is still peaked. There is not 
> usually much interaction.  The Orem filter does not change center 
> frequency with bandwidth or with the phasing setting. The phasing 
> control should be pretty much symmetrical. Check the position of the 
> knob for nulling out an interfering signal at about 1 Khz on either side 
> of the desired carrier, the displacement of the knob should be about the 
> same.
>      It is a very good filter. Modern mechanical filters have better 
> skirt selectivity and a modern notch filter can be tuned throughout the 
> bandpass but the Hammarlund type crystal filter will cut though most 
> interference.

-- 
bark less - wag more