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LF: 500 - Preamp conversion.

To: [email protected]
Subject: LF: 500 - Preamp conversion.
From: [email protected]
Date: Sat, 21 Apr 2007 12:10:14 EDT
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Hi all.
If anyone is having sensitivity issues with their receiver, then it is possible to convert the G3YXM / G0MRF pre-amp from 136kHz to 503kHz.
This is not an 'ideal' solution as the inductors used for 136k are quite large.  However, a little testing earlier today has shown this works.
You will need   2 x 470pF and 1 x 47pF
Remove C1 to C7     keeping the 2 x 4n7
Replace / add the capacitors as follows:
C1 + C7       4n7    - As removed from the 136kHz pre-amp
C3 C6          Not needed
C2 + C5      470pF
C4              47pF
The pre-amp can be peaked to give 11dB gain at 502 kHz with a bandwidth of about 25kHz.
The response is slightly over coupled in that it has signs of a double hump response but the overall performance is quite good. it certainly cuts out all the intermod from the big medium wave transmission on 558kHz that caused problems here and compensates for  the attenuators in the radio.
David  G0MRF
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