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#7
October 13th 13, 07:02 AM
posted to rec.radio.amateur.boatanchors,uk.radio.amateur
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Variable selectivity?
WOW!
A very full response, thank-you. "Richard Knoppow" wrote in message ... "gareth" wrote in message ... I wonder if anyone can offer a picture of the variable selectivity arrangement of the Eddystone EA12, which I believe was also used by some other manufacturers? This was the scheme whereby coupling between the primary and secondary of IF transformers was varied nechanically by distancing the two halves of the transformer from each other. I have access to photos showing the operating levers, but what interests me for the moment is to have a peek, or peeks, inside the IF cans themselves. This is the same arrangement used by Hammarlund in the Super-Pro series. Its been too long since I had a Super-Pro IF can open to remember exactly how the movable coils were fastened and guided but the variation was done by a post coming out the bottom of the can and operated by a cam on a shaft turned by the front panel "selectivity" knob. Very simple mechanism. This type of variable selectivity is the only one with completely symmetrical expansion of the passband. Those using tapped inductances coupled to the IF coils, such as used in the SP-600-JX and RCA AR-88, are nearly as good. Capacitance coupling usually results in some asymmetry as seen in some Hallicrafters receivers although the variable pass band in the SX-28 works quite well. Later in this thread someone asks about variable crystal filters. There are essentially two forms: the original Lamb filter and the later one patented by Hammarlund in 1938 and first used in the HQ-120-X. The Hammarlund filter has the advantage that the center frequency does not vary with the bandwidth adjustment or when the phasing null is moved around. The Hammarlund circuit was used in the SP-200, 400 and 600 Super Pro receivers as well as the HQ series and, in slightly modified form, by TMC in the GPR-90 and by Collins in the 75A-1,2,3 and 51J series. National and Hallicrafters mostly used the earlier Lamb circuit. as did Hammarlund in the SP-100 Super-Pro. -- -- Richard Knoppow Los Angeles WB6KBL |
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