[ARC5] BC-221 question

Fri May 22 21:35:49 EDT 2020

    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.

On 5/22/2020 5:42 PM, Bill Cromwell wrote:
> Hi,
>
> Ken Gordon who is a member of this list AND is dabbling in this 
> thread has a computer program to generate the cal book. If he 
> still has it you might find it on his webs site (assuming he 
> still has it). Ken might speak to that himself. If he no longer 
> has it I am sure I have it in my bitstash.
>
> I use a spread sheet and enter some reference points into it 
> relating the dial settings to some known frequencies. Then 
> graph the curve of those points with the spreadsheet. The more 
> points we enter the nicer the curve. I have not had to do that 
> for an LM but I did it for each band on my RAK.
>
> Tedious? For all of the bands on my RAK I spent a couple of 
> evenings playing with the radio. I once spent several weeks 
> plotting the response of a Hammarlund xtal filter in it's 
> various settings with a pencil and graph paper. Now *THAT* was 
> tedious. Later I found identical curves in an obscure manual. 
> By that time I already was wringing maximum performance from 
> that receiver. It worked exactly the opposite of the 
> recommendations in the ARRL Handbooks. No wonder they thought 
> the single xtal filter is not worthwhile. "Tedious" efforts can 
> give nice rewards.
>
> 73,
>
> Bill  KU8H
>
> On 5/22/20 8:05 PM, Richard Knoppow wrote:
>>     The crystals are in cans like metal tubes. I don't know 
>> the type as far as the crystal cut. I also have a BC-221A/K 
>> and use it. The A/K is the one with a modulator and an AC 
>> supply stuck in the battery compartment. The cabinet is 
>> supposed to be sealed but getting the crystal on frequency is 
>> a PITA unless you drill a hole to access the trimmer from 
>> outside. Before you do any drilling make sure there is not 
>> something in the way. I think there is on mine would have to 
>> open it up and look.
>>      The key to the BC-221 and the similar LM is having the 
>> calibration booklet. Its possible to make one with a modern 
>> counter but its a tedious project. I saw an article (and may 
>> even have it) on how the calibration charts were generated 
>> originally.
>>
>> On 5/22/2020 4:31 PM, Kenneth G. Gordon wrote:
>>> On 22 May 2020 at 23:19, Hubert Miller wrote:
>>>
>>>> I do not have a BC-221, so can someone remind me, this 
>>>> crystal frequency and package
>>> type?
>>>
>>> I have, and use, two BC-221-AKs. Both have 1 MHz crystals in 
>>> them. I can't remember the
>>> type.
>>>
>>> Ken W7EKB
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>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL