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Re: 736r help



At 08:12 PM 03/25/1999 -0800, Alan Bethel KE6QIS wrote:
>hi the name here is alan and i recently picked up a 736r. i hooked up my
>mast mount
>mirage kp mast mount preamps and notice something weired.
>  only when the preamps are on, when the rig first keys up it will imediatly
>jump to
>about 3/4 power then 'ease' the rest of the way to full power. again this
>does not
>happen when the amps are off.

This is a standard problem with the 736R.  What's happening is that the
radio's solid-state switching is faster than the bypass relays in your
preamp.  So the radio "sees" an open circuit for an instant after you press
the transmit button.  The SWR foldback circuitry in the radio immediately
runs up the ALC voltage to lower your power to a safe level (which may
actually be zero--it was on my setup).   Yaesu made the foldback circuit
*very* sensitive and fast to avoid blown finals.

As soon as the preamp's bypass relay contacts close and settle in, the
radio sees a reasonable SWR, and lowers the ALC.  This happens more
slowly--sort of like the "slow" AGC setting on a receiver, so the power
creeps up slowly to full over a few seconds.

There are a number of cures.  First of all, if you are relying on the
preamp's RF sensing to bypass itself, I strongly recommend that you "hard
key" the preamp--it's both faster and safer.  But this alone won't
necessarily solve the problem, because the radio is still faster than the
preamp relay.  

If your coax is an exact multiple of an electrical half wave (taking
velocity factor into account), it "fools" the foldback circuit into
thinking everything's OK.  I initially used this trick by making a ~19 inch
length of coax between the radio and my "brick amp," thanks to somebody on
this list.

But I consider this a temporary measure.  You still risk premature failure
of your amp/relay contacts because they are arcing RF each time you start
to transmit.  And if your rig isn't protected by fast foldback like the
736R, you may prematurely age your final transistors.

The best solution is to buy or build a sequencer.  Such a device keys a set
of relays or solid-state devices sequentially.  You push a button, and the
sequencer keys your preamp first, then your linear amp (if any), then your
rig.   This way, RF doesn't come out of the radio until the rest of the
system is ready for it.  We're talking about only a large fraction of a
second's delay here.  

Another solution is to build the delay into the radio.  There is a mod for
the 736 floating around.  I think it's by a Swedish ham.  It makes the 736R
key the control socket on the back an adjustable fraction of a second
before the rig transmits.  This method means one less device and its cables
on your operating desk, and you can use the 736's PTT switch. But it voids
a new radio's warrantee, and you may or may not be comfortable building the
circuit and doing surgery on a complex rig.  Sequencers is more
versatile--usually they control the radio plus up to three devices.

I went the sequencer route.  If you want to build one yourself, there are
designs floating about on the net and in old issues of QST (?) and
VHF-oriented publications.  An easy route (which I took) is to buy the
sequencer board from Advanced Receiver Research (ARR) in Connecticut.
Their Model TRS04VD board is complete--you just add connectors, a box, and
a pushbutton, and 12v DC.

ARR's address:

P.O. Box 1242
Burlington, CT 06013
(860) 485-0310

I'm sure there are other sources, too.  Check the AMSAT Web pages for
articles about this stuff, manufacturers addresses--I believe the Swedish
736R mod may be on there, too.    http://www.amsat.org

Hope this helps!
73 from KD7MW

--- Peter
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