[NLRS] FT-857D PTT out?

[Gerald]

On Wed, 2005-09-14 at 20:36 -0500, Chris Elmquist wrote:
> On Wednesday (09/14/2005 at 07:02PM -0500), Gerald wrote:
> > 
> > Unless the 857 is indeed very unique, you don't want to key the
> > transverter with a PTT output. You want to have a sequencing circuit
> > that keys the transverter, preamp, and main antenna relay, THEN when
> > those relays have had maybe a tenth of a second to settle in the
> > transmit side you can key the exciter. Most often the exciter puts out
> > RF at the same time or faster than it puts out a control line. There
> > should be a PTT output on the linear connector though.
> > 
> > I know in the FT-726 that RF appears before the control output by a few
> > milliseconds. That's hard on relays and receiving stuff that gets hit by
> > transmitter power before the control signal arrives. I know it cost me a
> > GaAsFET in one preamp. The control output of the 726 comes from a relay
> > that's powered by a TX+12 volt. The same TX+12 that runs the transmitter
> > stages with a diode antenna switch. That just about guarantees a 10 ms
> > delay for the control output. Hence the need for external sequencing.
> 
> I am quite familiar with this issue but I am curious how people actually
> do it in practice.  Do you forego the use of the PTT switch on the mic
> and have some other XMIT button?  What happens if the radio is accidentally
> put into VOX mode and you sneeze?  How do you make this sequencing work
> in all modes SSB, CW, VOX, etc?

I've lost the GaAsFET in my 2m preamp when I forgot and went to FM with
the preamp in the antenna line. 100 watts of RF to the output of the
preamp melted the transistor without letting the smoke out of the case.
That was with the FT-726 that I still use on 2m.

You practically have to disable VOX if the radio has it, and let the
sequencer do all the switching driving by the microphone and key. You
could add VOX to the sequencer but just can't use the radio's VOX. Its a
fundamental problem with many if not most ham gear working with preamps
and transverters. Though you could let the sequencer switch audio too so
the rig VOX wouldn't see any audio until the sequencer was ready.
> 
> As I have mentioned to others, my dream radio for transverter use would
> have an RTS (request to send) output and a CTS (clear to send) input.
> (hey, I'm a computer guy :-)  ) RTS would go true when you initiate
> transmit from any method-- PTT, VOX, CW, etc. but no RF would be produced
> until CTS went true.  If you were running without a transverter, you just
> loop RTS to CTS and the radio works normally.  If you are sequencing
> then you start the sequence when RTS goes true and the last thing the
> sequencer does is drive CTS true... and then RF comes up and off you go.

TenTec radios sometimes do that, For sure the Corsair and Corsair II.
Probably others of that same vintage because their linears of that era
did that to make relays last better. I have a Corsair II that I'm
thinking of selling to look for a 1296 module for the 736 to replace its
6m module.

> 
> But I haven't found a radio that works like this yet.  I mod'd my FT-736
> so it does... but it's a lot of screwing around.
> 
> But I seem to see most people running DEM transverters without any
> sequencing.  They just key the transverter with the PTT-L output from
> the IF rig and seem to get by.  It sure makes it easier because then
> the PTT switch on the mic still works... as does VOX, CW, etc.
> 
> Chris NØJCF
> 

It is possible to protect the transverter that doesn't have too much
going with it (like preamp and a big power amp) with the transverter
interface. First thing is that the transverter has excess gain on
receive so there can be a significant attenuator between transverter and
the exciter. If that gain uses an MMIC like the MAV-11 its good for a
couple watts applied to its output without damage (1 dB compression
point as an amplifier is nearly 200 milliwatts). Then 10 or 20 dB
attenuation to make up for the excess gain in the receiver line raises
that damage threshold.

Preamp and power amp raise the problem at those stages, though generally
the DEMI transverters need an external antenna relay so the two
amplifiers are permanently connected to the independent RF ports on the
transverter. It should be possible to delay RF out of the transverter
long enough to let the antenna relay(s) get switched to protect the
preamp from the big PA and to protect the relays from being switched
hot. Relay contacts will carry a lot more current than they can switch.
That delay shouldn't take more than a capacitor or two in the drive to
the transmit +12 volt switch transistor to delay it. That might not be
sticking out on the schematic even, but it would be easy to add.

It could be that I'm not giving enough credit to the modern DEMI
transverter controls. I've not dug into them lately. I built my own rig
for 1296 using a tripler from the 432 output of the 726 and receiving
with the 2m input. Worked for CW and narrow FM only. But I think its
ill, I've not heard any signals not generated by me for several years so
when I got the 220 module working (I bought it broken, turned out to be
a broken coax in the connecting cable) I took down the 1296 antenna and
put up a 220 antenna. The 1296 stuff sits in a pile under other piles of
things to work on waiting on the work bench. I suspect the local
oscillator multiplier chain has lost most of its output due to corrosion
of variable capacitor contacts. Might be a drop of DeoxIT on each with a
little twist will fix it.

-- 
73, Jerry, K0CQ
Dr. Gerald N. Johnson, electrical engineer
All content copyright, Dr. Gerald N. Johnson