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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 1st Nov 2003, 1:39 pm   #1
jim_beacon
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Default BC 348 Crystal Filter

Hi,

I've just realigned my B-348-Q, with excellent results, apart from one thing. The crystal filter seems to have an insertion loss of about 20dB - is this normal, or have I made a mistake somewhere?

I've aligned the IF to the center frequency of the crystal, as specified in later versions of the manual.

With the filter switched out, a signal of 35mV applied to the grid of the mixer, at the IF frequency, gives 10mW out, which is as per the book, but switching the filter in requires an increase of signal by 20dB to get the same output.

RF sensitivity is about 0.5uV for 10mW out, except on long wave, where it is about 6uV (specification is <7uV for 10mW out on all bands).

If any one has one of these sets, can they do a comparison measurement for me?

Thanks

Jim.

Last edited by Station X; 28th Dec 2004 at 7:42 pm. Reason: Import
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Old 4th Nov 2003, 11:05 pm   #2
octode
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Default Re: BC 348 Crystal Filter

20 db seems excessive, upto about 6db would be considered normal. I'm not familiar with the set but is the filter just a single xtal with phasing control or is it a lattice or sealed unit.
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Old 4th Nov 2003, 11:10 pm   #3
jim_beacon
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Default Re: BC 348 Crystal Filter

Hi,

it's a single crystal, there's no phasing control, but the maching transformer has a neautralising capacitor, adjusted by snipping the two pieces of wire that it is made from. I haven't altered this, but I'm begining to think I may have to.

I have also made some swept frequency (wobbulator) tests, and I can see some ringing on one side of the response curve when the filter is switched in - this also leads me to suspect the neautralising needs checking.

Anyone got any thoughts?

Jim.
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Old 6th Nov 2003, 6:44 pm   #4
octode
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Default Re: BC 348 Crystal Filter

If the xtal is one of the older FTxxx types, you might try taking it apart and cleaning it (soap and hot water is fine but make sure you rinse it and dry it well) as they become sluggish after a while.

What is the xtal frequency?. I've a couple of boxes full of xtals of all shapes and sizes I might have one that you could try as a comparison.

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Old 6th Nov 2003, 7:03 pm   #5
jim_beacon
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Default Re: BC 348 Crystal Filter

the crystal is in a special bolted up wire ended holder, with the whole thing being dipped in wax! It ends up as a unit about 0.25" square by 0.5" long.

The IF centre frequency is 915KHz, so its not even a common value.

I will try the neuatralising adjustment tomorrow, if I get time, and see what difference that makes.

Jim.

Last edited by Station X; 28th Dec 2004 at 7:43 pm. Reason: Import
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