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

[David Wise]

I would like to reinforce Tim's point about the S-meter.
When you switch from triple to dual-conversion, the first
mixer, which *has AGC on it* is removed from the signal path.
This will change the receiver's AGC response curve.  Don't
expect your S-meter to read the same, and don't expect the
same audio level.  To really get the picture, you'll have
to measure input for a given S/N.

As he alludes, if .5-6 are okay and it's just 7 that's weak,
that's a different story; either switching troubles, bad RF or
1st VIF top-end alignment, weak 2nd crystal, or misaligned 2nd
crystal oscillator.

73,
Dave

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of 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.