In fairness, before anyone runs out and buys an R4C, the R4C's we
used here on 160 that Bill talked about are far from stock.
For first mixers they have either dual-triode single-balanced mixers
(6J6 tubes) in the "tube-mixer" radios, or double-balanced diode
mixers in the solid-state conversions.
The mixers all require much higher local oscillator injection than
stock R4C's, several times the stock injection levels! One receiver
has over 100 milliwatts of local oscillator injection to high level
diode mixers, and that one was the favorite everyone liked. Our
own transmitter was barely above the blocking point of these
receivers, even when only a few kHz away!
They all have double-balanced diode second mixers. This is a
particular weak point of R4C's, and the early S/N's (those without a
1.5 kHz selectivity position) are absolute junk in the second mixer!
A low-noise solid-state amplifier after the last crystal bandpass
filter adds 10-15 dB of IF amplifier gain as well as reducing filter
blowby.
It's not correct to assume you can run out and buy a R4C, throw in
some simple mods, and have something substantially better than
an IC781 or any other good receiver. As a matter of fact early R4C
performance (S/N below 18500 or so) is extremely poor, even with
filter mods. Early R4C's are absolute junk unless almost gutted
and rebuilt at the second mixer.
73, Tom W8JI
w8ji@contesting.com
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