"MoiInAust" wrote in message
...
Lots of snipping here, very long quoted thread.....
Do any of you guys know the SP400? I had one in the
sixties. Nice glow to the meter!
I am more familiar with its predecessor the SP-200 and
210. The SP-400 is essentially identical except it covers
the range of 540Khz to 32Mhz rather than the 2:1 frequency
range bands of the earlier models. It contains the broadened
out RF stages for the broadcast band of the standard SP-200
and the shunt-fed RF of the high frequency model that went
to 40Mhz. Shunt feeding the RF makes it slightly more
selective which is desirable for reducing image response in
the highest band. Otherwise the 400 is the same receiver. At
the time the 200 was made it was probably the best receiver
on the market. I used a BC-779 version as my original
station receiver. I modified it to use an electron coupled
LO with voltage regulation. I later returned it to the
original configuration and found that the RF gain or AVC
would cause serious freqency pulling. The cure for this is a
voltage regulator. The ECO can be made to stay put despite
considerable variation of B+ and filiment voltages but it
had too low an output and loaded the tuning circuit
differently than the original causing some innacuracy in
calibration. I think a regulator on the LO and possibly on
the mixer screen will fix the voltage drift in these guys.
Otherwise they are very nice. Hammarlund had a patent on the
type of crystal filter used in their receivers. The
Hammarlund filter does not detune when the phasing control
is adjusted and has a much wider range of bandwidth than the
original Lamb type filter used by National and
Hallicrafters.
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Richard Knoppow
Los Angeles, CA, USA
