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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Feb Mon 05, 2018 5:34 am 
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Joined: Aug Fri 29, 2014 6:17 pm
Posts: 2551
Location: Vincennes Indiana
I've never heard of the Collins style mecha filters being bent for wider bw or whatever. I saw a Murata ceramic mecha filter where a guy had gone into the circuit and used a switch to remove ground from some of the poles of the filter, thereby adding variable selectivity = all grounded means tightest selectivity. Worked but have no idea as to spurs or z fluctuation.


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Feb Mon 05, 2018 1:58 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5660
If you put a capacitor across the input/output of the filter you are effectively increasing leakage around the filter which is what we generally want to avoid; some of the higher performance modifications for the Drake R-4C involve rearranging some components to prevent coupling around the filter.

If you put a large enough capacitance around the filter it will become the prime coupling element and you will have plenty of bandwidth (probably more than you want). Small amounts of coupling capacitance are going to primarily create a lot of filter "blowby" and a very skewed response. The problem with trying to broaden the bandwidth while coupling around the filter is you will still have the increased response of the filter now coupled with a wider but greatly attenuated wider bandwidth so the audio will sound strange at best.

There are a number of mechanical and crystal filters available that were sold in USB/LSB pairs, KVG had a very popular set and a version of these is readily available on ebay out of decommissioned Israeli receivers. It would be interesting to set up a receiver using these to select either USB, LSB, or use them in a spliter/combiner stage utilizing both filters simultaneously to provide AM selectivity. The KVG crystals are at 9 Mhz. and the board on ebay also includes an AM filter; I used the AM filter in my SX-146. Collins and others made these USB/LSB pairs and Drake uses 9 Mhz versions in the TR-3/4 series and 5645 Khz. versions in the T-4X series for sideband selection. I originally bought the KVG board on ebay for the 9 Mhz. AM filter but I suspect I can use the SSB filters to replace those in my TR-3 if they become a problem like so many of those early Drake crystal filters.

Rodger WQ9E


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Feb Mon 05, 2018 3:25 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1104
Location: Santa Rosa, CA
Back in the day Collins built some nifty plug in units that contained
a mechanical filter and associated tubes and circuitry. They were
designed to be used in a number of other manufacturers receivers
such as the SP600 and later HRO's (50 and 60). I've got one around
here somewhere. The one I have is plug and play with some minor
tuning required but no mods.

A google search might turn up a schematic which might be very useful
for what you are trying to do.
Steve

_________________
'cell phones and the internet are tools, not a lifestyle'


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Feb Mon 05, 2018 3:46 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5660
I think someone on here recently had a NC-400 with the rarely seen optional mechanical filter module. I have two NC-400 receivers but they are both standard.

The basic adapter manual is on the Collins.org website: http://collinsradio.org/archives/manual ... Manual.pdf

Rodger WQ9E


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