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Re: FT-847 band-specific PTT disable

>From: Estes Wayne-W10191 <Wayne_Estes-W10191@email.mot.com>
>Hello everybody,
>I apologize if this question has already been answered recently.
>I have an FT-847 but I don't have the service manual.  Has anybody with a
service manual found points in the schematic that can be opened or pulled
high/low to disable PTT for a particular "band" (HF, VHF, UHF)?  The
objective is to automatically disable 144 MHz PTT when I energize a coaxial
relay that connects a 2400->144 MHz downconverter.
>If so, can that person document it in a PDF document and put it on a WWW


Well, what you want several eme operators would also like!  I have the
service manual, since I knew before purchasing my FT-847 that I would need
to modify it for use with low-noise preamps.

>From what I can determine with only a cursory inspection of the diagrams,
PTT is common to all bands {no help here}.  Looking at the VHF and UHF PA
assemblies, they are served with separate AVC control lines.  So I suppose
one could cut these lines [or possibly bridge onto them] and supply the
correct voltage [?] to cause the AVC to bias off the PA stages.  I have not
heard of anyone doing this.  There is an exterior AVC input jack, but it is
not freq. specific [probably operates on the band selected for xmit...not
what you want in satellite mode].  

In the alignment procedure, you could posibly experiment by adjusting the
AVC circuits so power out was minimized [unknown how low the adjustment
will go].  This requires using a precise audio generator/ac voltmeter to
adjust {you would have to re-align the AVC everytime you wanted to go back
and forth...not very realistic}.

What is probably a better approach is separating the VHF [and/or UHF] Rx
antenna input from the Tx/Rx antenna jack.  That way you have no risk of
transmitting into your convertor or preamps.  Use your relay to re-connect
the separate Rx ant port back to normal operation.

This has been well documented by DL4MEA:
http://www.qsl.net/dl4mea/ft847mod/ft847mod.htm   {yes "ft847mod" is twice
in the correct address}

Now a caveat:  this is a very sophistcated radio using mostly surface-mount
components.  I really sugggest you get or borrow the service manual before
trying to go inside it.  Then, only if you are an experienced technician or
engineer.  Otherwise buy a case of beer or other "poison" for a buddy that
has the requisite skills!  (give to him after he operates on your "baby" ;-)

My solution is to also use an outboard T/R sequencer thru which both the cw
key and the mic passes before connecting to the radio.  You may not need
this for satellite, as Rx is porbably using a different antenna than Tx.

Hope this helps!
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