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Re: Inhibit Xmit on 736



In a message dated 11/5/2001 10:59:33 PM Eastern Standard Time, 
r.sturgis@worldnet.att.net writes:

> Sometime back I saw a post explaining how to inhibit the 2M xmit function on
>  the FT736r.
>  
>  I would like to try this before attempting QSO on AO40.
>  
>  Could someone point me to the post or repost the message?

Hi Richard,
Here's one I posted:
==================================
I've received numerous requests for an explanation of the way that I protect 
my Drake 2880 from accidental destruction while using it with my (muTek 
modified) FT-736R.

What follows is from personal experience. I have been using my Drake since 
late 1997 with no problems. Your mileage may vary!

There are several documented ways to inhibit transmit on a Yaesu FT-736R. 
Most involve disabling drive output. (My favorite is OH5IY's TX delay mod.) 
This is a global procedure. i.e. it disables _all_ band modules. While this 
is fine for terrestrial work, if this is your only satellite radio, you need 
the drive for your uplink so you can't globally disable it.

The key is to isolate the TX-9v line to the band you wish to disable. In this 
case, 2M. For those of you with service manuals, it's easy to spot the TX-9v 
line. It's clearly labeled entering the 144 MHz P.A. Unit. There is, however, 
a rub! Or as a friend says, "Yaesu threw a spanner in the works". The 5 lines 
that enter the 2M P.A. do so through feed thrus. If you take the top off your 
radio and look at the left hand side of the rear heat sink, you'll see the 5 
feed thrus for 2M. 5 on the left side are 2M and 5 on the right side are 70 
cm. Now here's the rub. There is a small circuit board attached to 2 of the 
feed thrus and 5 wires going to the board!! What the heck is this???

Try as I might, I couldn't locate this circuit board in the service manual. I 
called Yaesu. They didn't know what I was talking about. Anyway, I finally 
obtained Yeasu's docs on this "performance modification". On early 736R's 
there was "heat buildup" problem in sat mode that was solved with this little 
board. It's called the "TX 9v switch unit".

If you put a voltmeter on the wire that is connected to the upper right hand 
corner of this little board (it's above the feed thru that is second from the 
left) you'll see this line go to 9v when you transmit. This is the line to 
open when you want to inhibit xmit on 2M. On my rig, this line is orange.

I did some quick and dirty tests of 2M output power with this line 
enabled/disabled.
First, with everything enabled:
Drive control at max = 27 watts
Drive control at min = 2.7 milliwatts
Disabled TX 9v line:
Drive control at max = 27 microwatts
Drive control at min = .0027 microwatts (I don't really trust my test gear 
this low :-} )

On my system, I use a 20 dB pad in between the Drake and rig to get the S 
meter near S0. Also, I run 75 feet of RG-58 down the tower. So in addition to 
the numbers above, subtract 24 dB. Worst case scenario for me is that I 
transmit full drive with inhibit on and the signal will be -84 dB down from 
27 watts or ~ .1 microwatt.

Similar procedures work for the FT-290R MKII. Hope this helps.

73,
Mike, N1JEZ
AMSAT #29649
Local Area Coordinator
"A closed mouth gathers no feet."
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