[NLRS] 706MkIIg + DEM TIB/222-28 PTT problem

[Brent Casavant]

Hello everyone,

Well, for the second VHF contest in a row I've been unable to use
my DEM 222-28 transverter for the want of being able to key the
thing.

I'm using an Icom 706-MkIIg as the IF radio, and a DEM TIB to both
drive the ALC level and split the IF signal for the transverter.
I've used this once to check into the Sunday 222 net, and KC0IYT/N0RPM
used it for the August UHF contest, all without problems.

However both in June and last weekend, I had problems that prevented
the use of this setup during the VHF contest.  The symptom is that
the 706 fails to drive the PTT sufficiently to cause the TIB and
222-28 to switch into transmit.  When I manually ground the PTT/HSEND
line the TIB, transverter, and 706 will all switch into transmit mode.
It's only when the 706 itself tries to drive the HSEND/PTT line that
TIB and 222-28 fail to switch to transmit.

Wondering about the 706's ability to sink enough current, I tried
experiments hooking up only one or the other (TIB or 222-28), all to
no avail.  We also tried keying a DEM 1296-144 and failed.  The
problem has manifested itself both on a 13.8V power supply and a 12.?V
battery.  And of course everything always works fine at home on the
tabletop, making this a bear to diagnose.  The only idea I have left
that I haven't tried is to switch out some of the IF BNC/LMR-240 cables
from the TIB to the 222-28; I don't think they'd matter but I'm quickly
running out of ideas.

I know I've mentioned this problem on the list before, but having
been unable to solve it I'm hoping someone might have a few more
ideas for me.  At this point I'm even game for wild ideas!  I'm
not even opposed to constructing some sort of additional interface
circuit to insert between the 706's HSEND line and the DEM equipment's
PTT line.

I thank you in advance for any ideas or assistance.

Brent

P.S.:  If anyone has a lead on a good used oscilloscope I'd sure like
to hear about it.  It would certainly make some of these problems easier
to deal with.  At least dual trace; at least 50MHz, would greatly prefer
150MHz, and would be ecstatic with a 500MHz scope; digital storage not
required but would definitely be a bonus.  Prefer Tektronix, but open
to anything whose brand I recognize.

-- 
Brent Casavant			http://www.angeltread.org/
KD5EMB				-.- -.. ..... . -- -...
44 54'24"N 93 03'21"W 907FASL	EN34lv