[NLRS] Subject: DEM 222-28 Interfacing ?

[Brent Casavant]

On Mon, 30 Jul 2007, Melvynkc0p at aol.com wrote:

> Subject: DEM 222-28 Interfacing
>  
> I now have a DEM 222-28 transverter that only requires 1mW of
> drive. The  Icom 706 shows 4 W out key down on a Bird Wattmeter
> at the lowest RF drive  setting. How do we make up for the
> difference ? That sounds like more than 30  dB of attenuation
> required and then relays around the pad on receive.
>  
> What have others done ?

Mel,

I have a DEM 222-28 and drive it from a 706-MkIIg.

Originally I used the DEM TIB to control the ALC input, setting
the pot in the TIB to cause the 706's output to be just less that
1mW of drive.  However, I regularly had problems with this setup
for multiple reasons.

The biggest individual problem was that I used this setup mostly
in field operations, sometimes running off of commercial power,
and sometimes off of battery.  As the TIB was only providing an
adjustable voltage divider circuit for setting the ALC point,
the output power of the 706 varied depending on whether I was
running off of mains or battery (and varied as the battery was
discharged).  Given that only a slight adjustment in the pot
would cause a significant change the 706's output power, this
just seemed to be asking for trouble.

Second, the 706 had barely enough current sinking ability on the
PTT line to drive the relay in the TIB.  This wasn't a huge
deal once I figured it out and stopped trying to drive the
222-28's relay directly from the 706 as well (instead doing what
I should have done at first, relaying the signal through the TIB).
It did point out to me that there probably wasn't a lot of
headroom in the 706's current sink.  I also have the sneaking
suspicion that the relay switching was also affected by the
power supply levels, but never confirmed as much.

A third concern was all the headache of the split IF configuration,
particularly all the extra cables involved.  This probably wouldn't
be a deal-killer for a fixed station setup, but for field work it
was a significant pain.

After about a year I ordered the high-power (10W) common-IF drive
kit from DEM and installed it without too much difficulty.  I found
that setting the 706 to power level "1" produced the correct drive
level, and the interfacing was greatly simplified (two cables as
opposed to six plus a box).  I've been very happy with the results
and don't think I'd ever consider going back to using a TIB.

After that glowing endorsement, if anyone's looking for a working TIB,
just drop me a line.

Brent

-- 
Brent Casavant			Dance like everybody should be watching.
www.angeltread.org
KD5EMB, EN34lv