[R-390] Low sensitivity on 7-8 Mhz band

[Tim Shoppa]

> I checked my Amelco and saw a difference of about 3 dB in calibrator
> signal between 8.000 and 7.999+.

Well, that is the switchpoint where the radio goes from being triple
conversion to double conversion. You switch out a mixer stage,
switch out the 17.5-27.5 Mc tunable IF. And the crystal oscillator
frequency jumps
a lot (27Mc output on the 7Mc band to 11Mc output on the 8Mc band).
With all those changes, I'd say a change in 3dB of the meter reading is
entirely normal (and it's in the range of what I see with my EAC RF
decks.)

So, not only check the RF stage peaking, also check the 17.5-27.5Mc IF
peaking
if you really think you have a problem.

That said, S-meter reading is not the same as "sensitivity" (which
is usually quoted as uV at a given S/N).

If the local oscillator output for the 7Mc band were weak, it would
also be weak on the 24Mc band. Not much to listen to on 24Mc right
now. (Guys, check me if I'm right, I'm not sitting at the radio with my
frequency counter right now...) The base crystal is 9Mc, and harmonics
are picked off to make 18 and 27 Mc. 18Mc is used on the 15Mc band
and there's stuff to listen to there in the daytime. 27 Mc is used for
7 Mc (triple conversion) and 24 Mc (double conversion).

If I look at the crystal oscillator output on my scope, I see roughly a
factor
of two in output voltage between different bands. To over-generalize the
higher osc frequencies are lower in amplitude than the lower
frequencies,
there is also some clear change between different crystals, although
I think it has more to do with strength of harmonic.

If the peaker capacitor for the crystal oscillator is out of whack (say
you only
get one peak instead of two) that's not a good thing (although the radio
will
work). According to my reading about mixer design, having the oscillator
input
to a mixer down by a factor of two can really cut conversion efficiency
by a lot
more than a factor of two.

Tim.