[Boatanchors] W2EWL VFO range?

[Gary Schafer]

Hi Bob,

Part of the problem may be the drive level of the VFO signal to the mixer.
If you look at the central electronics 10B, they had a trap in the mixer
plate circuit tuned to 15 MHz and another at 13 Mhz. in the 20A they added a
series trap at 5 MHz to the grid of the 6AG7's also.
Trap coupling and Q is rather critical too so as to get proper rejection
without killing the wanted signal.

But the VFO drive level is critical to keeping down the spurious output. The
way that central electronics said to set the VFO drive level, by adjusting
the rotary link in the VFO, was to turn it up until the transmitter output
just stopped increasing and then back it off slightly. If you look at the
output of the transmitter on a spectrum analyzer you can see that even a
little overdrive from the VFO greatly increases the spurious outputs of the
transmitter, even with the traps etc.

73
Gary  K4FMX


> > I suspect that the problem of 3rd harmonics is a result of the
> > builder using too much L in the output tank and not enough C
> > resulting in Q that is too low.
> Maybe - it was always an issue as noted in follow-up articles in QST.
> The original circuit coupled the 5 mhz from the VFO tank coil into the
> grid and the 9 mhz SSB signal to the cathode of the 12A6 (changed to
> 12BY7 in the '58 version) which services as combination mixer and driver
> for the parallel 1625 finals.   A toggle switch selects the appropriate
> driver tank for  75 or 20 meters.   Since I had  50-60 watts output on
> 75 I figured the exciter and VFO levels were correct, and  tried
> everything I could come up with, including double-tuned circuits, traps,
> and even reversing how the two signals were fed to the driver/mixer.
> But I haven't been able to develop more than a few watts output on 20
> meters, and with the benefit (or curse) of a spectrum analyzer, see
> enough garbage I choose not to try it on the air in the present state.
>