[Collins] Inrad 2.5 KC Mech Filter Assembly

[Bob and Sue Jefferis]

I am wondering why you would need to change BFO frequencies in a receiver. The original BFO XTAL frequencies are placed symmetrically above and below the 455 kHz filter center frequency.

A transmitter, like a 32S-3, would indeed require new BFO or 1st IF XTALS since they [Collins designers] relied on the mechanical filter to provide at least 20 dB of carrier suppression. Am I missing something?

I use Inrad/Collins  500 Hz filters in my 75S-3B receivers for CW ops with excellent results. I agree, the time domain transient behavior (ringing) is improved in the modern torsional filters.

Aside, from the possible side effect of increased 120 Hz or 60 Hz noise, I would suggest trying the 2.5 kHz filter before changing it. Again, am I missing something here?

73, Bob, KF6BC
On Jan 19, 2012, at 9:26 AM, Dr. Gerald N. Johnson wrote:

> Lot of people like the wider bandwidth. I'd think setting the BFO 
> crystals to the right part of the slope would be a bother since they 
> aren't adjustable and matching for transceive would be out of the 
> question. But then the S-3B has a tunable BFO and you can always use 
> manual spotting with the PTOs independent. I have a modern torsional 
> mechanical filter pair in a Yaesu and they don't ring on static nearly 
> as long as the original 75S filter F455FA21. But I use an external 
> passive audio low pass filter to narrow the pass band from 2.4 down to 2 
> from my years of using the 2.1 in my 75S-3B.
> 
> The original filter converts lightning summer times on 75 meters from a 
> click in other receiver but it comes of the ringing filter as a craaaaash.
> 
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
> 
> On 1/19/2012 10:24 AM, Byron Tatum wrote:
>> Hello-
>>     I am restoring a 75S-3B receiver and found the Inrad 2.5 KC mech filter
>> PC board assembly plugged in, in place of the usual Collins 2.1 KC filter. This
>> Inrad assembly has the Collins #526-8694-010 torsional mech filter. I thought I
>> would ask list members their opinion of this filter as compared to the standard
>> #526-9427-00 2.1KC filter. Is this a desirable filter to keep in your 75S-3B?
>>                                                         Sincerely,
>>                                                                 Byron WA5THJ
>> 
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