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

Fri, 19 Sep 2003 20:27:09 -0500

Hi Brent.
I had the same problem with my DEM TIB and 222 transverter when I put my
station back together. My external band controller (that is slaved to my
FT-847) detects which band is selected and repowers the 4 PTT select lines;
one of which goes into the TIB. My switching transistor (Darlington) that
drives the repowered PTT saturates with a Vce of approx 0.6 VDC. If you look
at the PTT input circuit on the TIB schematic you'll see that it's just a
diode (CR2 1N914) to ground from the base of the relay driver transistor
(Q1). In order to activate the PTT function you have turn Q1 Off by sinking
current away from its Vbe junction. Unfortunately, both the Vbe and Vcr2
voltage drops work against you if you don't have a low voltage output from
you PTT driver. Of course a relay contact would have a zero voltage output
when shorted to ground. I can't get my PTT driver darlington to saturate at
less then 0.6 volts. My solution was to put a jumper across CR2 so that its
voltage drop is removed from the circuit. Then Q1 switches OK when the PTT
driver turns On. The only downfall of doing this is that if you change to a
different rig someday that has a relatively low value of pull up resistance
on the PTT output you could burn out Q1s Vbe junction with too much current.
I used a temporary jumper with an external label on the TIB box.

So, you can put a VOM on the PTT output of your 706 with a pull-up resister
to +12 VDC (if not already provided within the 706) and measure what your
PTT On voltage is. If it's higher then +0.3 (or so) then this is your
problem. Otherwise, punt.

73, Jim, K0MHC/0
----- Original Message ----- 
From: "Brent Casavant" <[email protected]>
To: "Northern Lights Radio Society" <[email protected]>
Sent: Thursday, September 18, 2003 11:01 AM
Subject: [NLRS] 706MkIIg + DEM TIB/222-28 PTT problem

> 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
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