[TenTec] QRP mod for TenTec Omni B?

[Neil Martinsen-Burrell]

As a follow-up on my message from earlier this week, I hooked my Omni B up
to a dummy load and measured the power out using a DVM and a diode
detector.  Besides verifying that the max power was around 104 watts on
both 40m and 80m, I was able to turn the power down essentially as low as I
wanted just by turning down the DRIVE knob.  I was able to get down
reliably to around 3V peak (approx. 1 watt).  It did not take much turning
of the knob to get down to sqrt(5)*10 = 22.4 volts (approx. 5 watts).

I mistakenly trusted the manual which specified that the DRIVE knob will
only turn the power down to 25 watts.  That does not appear to be the case
on my Omni B.  It is possible that my radio is mis-adjusted or already
modified in some way, but I think that is unlikely as the radio has been
serviced at the factory a few times in its life already.

I didn't hear any of the 80 meter foxes on my low dipole, but I was ready
with just 5 watts to call them back!

-Neil KD0UKC

On Tue, Nov 11, 2014 at 4:05 PM, Neil Martinsen-Burrell <
neilmartinsenburrell at gmail.com> wrote:

> With the upcoming QRP Fox Hunt season (one of my favorite weekly operating
> opportunities) I am interested in the possibility of using my main station
> rig at QRP power levels.  My QRP radio is an HB-1A with RIT that does NOT
> remember the offset settings when RIT is switched off and then on.  This
> means that it take a lot of fiddling to listen to a split pile-up to find
> the station currently being worked.  I would prefer to use the Omni with
> its separate offset knob and a RIT on/off switch so that I could easily
> switch between the offset and non-offset frequencies.
>
> There are google mutterings from the late 1990s in this list's archives
> about a mod that might change the range of the ALC knob from 0-20 instead
> of 25-100 watts.  Does anyone know if such a mod exists as more than a
> rumor?  On my previous Kenwood TS-520SE, there was an external ALC pin that
> could be driven with a negative voltage to reduce output power.  Is there
> something similarly simple for the Omni 546B?
>
> I can see from the schematic that R21 on the 80743 low level driver
> controls the effect of the ALC control pot, but it seems like that isn't an
> easy solution to getting down to QRP power levels, at least without some
> fiddling to figure out what value would give the appropriate minimum and
> maximum power levels.
>
> Thanks in advance for any help you can offer.
>
> -Neil KD0UKC
>