[NLRS] T/R on a TR-751

[Chris Elmquist]

This has always been a challenge with older radios as they were never
designed with external transverter (or even power amplifier/preamp)
setups in mind.

Gary, W0GHZ and I modified our FT-736 radios to add what amounts to an RTS
(request to send) output and a CTS (clear to send) input.  We found the
spot in the radio where the drive is ultimately enabled once ANY transmit
mode is activated.  This includes SSB, CW, VOX, etc.  There is usually
one signal that finally goes active when the radio is supposed to make RF.
We cut this signal before it goes to the drive circuit and it became the
RTS output.   Then the input to the drive circuit, which enables the RF,
becomes the CTS input to the radio.

If you don't have any external equipment connected to the radio, you
jumper RTS to CTS and the radio works as normal.

If you do have external equipment, with some sort of sequencing, then
the RTS starts the sequencer, all the external goodies get to the right
place for transmit and finally you assert CTS back to the radio and it
starts making RF.

The external sequencer can be as simple as a timer if you just wish to
hold off the RF for a short time while mast head preamps get out of the
way or whatever.

This is of course a little more work because you need to mod the radio
but in the end, I think we decided this is the most reliable way to
ensure preamps and transverters don't get smoked.

Chris N0JCF


On Tuesday (08/14/2012 at 08:24AM -0500), Bruce Richardson wrote:
> 
> 
> Ok, for sequencing on my 10GHz set-up, I always intended to build a
> sequencer and put the microphone and the PTT switch on the mic at the
> start of the chain.
> By that I mean, the mic switch would get depressed and the sequencer
> would detect this and:
> 1. switch the SMA relay to ON (xmit)
> 2. ground the xverter for xmit
> 3. ground the 2&8 watt amps for xmit
> 4. Finally pass the PTT and mic signals to the TR-751 (turning the rig
> on last)
> 
> The plus to this is that the rig really is turned on last.  That's fine
> in SSB.  But what about in CW mode where if I bump the key, the rig goes
> into transmit???
> 
> An alternative scheme that is possible:
> The TR-751 has an aux port on the back of the rig that shows "ground"
> whenever the PTT is depressed OR if the rig goes into CW tranmit
> (complete with break-in delay holding it in transmit on CW).  I could
> run this "ground" to the sequencer and detect it and:
> 1. switch the SMA relay to ON
> 2. ground the xverter for xmit
> 3. ground the 2&8 watt amps for xmit
> 
> The plus to this latter scheme is simplicity but the minus is that the
> IF rig really is putting out signal down the IF line to the xverter
> before the xverter is switched over.  Yeah, I know, this is what I've
> been doing for years but....
> 
> The reason I want to do the SMA relay first is that the LNA pre-amp is
> always powered on and I want it out of line first for protection.
> 
> Will the latter scheme be ok?  What do you think of my order of switched
> items?
> 
> A busy time at the FZ household with major 10GHz overhaul in progress.
> 73
> Bruce W9FZ
> 
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-- 
Chris Elmquist