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From: ke7gh@primenet.com (Brian K. Short)
Date: Fri, 26 Jul 1996 22:40:14 +-100
I have recently added an Ameritron AL-1200 amplifier to my contest station
and have been using it with my Yaesu FT-990.  In the course of operation,
in CW and RTTY, I have noticed the occasional fast flashing of the HI SWR
light on the FT-990 when first keying for a transmission.  This I came to
recognize as the amplifier relay not activating as quickly as the FT-990.
Using the N6TR logging program, this was NOT the case because of the PTT
signal used by the program to prevent such timing problems.  Since I don't
always use the N6TR logging program (e.g. in RTTY mode) I became concerned
for two reasons: 1) long term effect on rig and relay, and 2) effect on
Dunestar band pass filter in line to the amplifier.

No solution appeared obvious at first.  My solid state QSK FL-7000 amp
had employed the FT-990 TX inhibit signal (INH) present on the BAND DATA
jack to interlock the amp and rig.  Alas, I thought a simple RC network
on the INH signal could disable the rig for the desired amount of time,
allowing the amplifier relay to ALWAYS completely energize before enabling
the transmitter.

What follows is a "simple" circuit for accomplishing the desired goal.
The precise time constant needed is tricky!  There are two conflicting
requirements: 1) switch slowly enough to allow the amp relay to close,
but 2) switch fast enough so that the first dit, even QRQ is not missed.
For this reason, others may want to tailor slightly the values of the
two 1k resistors in parallel comprising the R of the RC network.  These
values seem to work for me...

Again, what this circuit does is disable the FT-990 transmit signal for
a very small amount of time to allow the AL-1200 TX relay to ALWAYS key
completely, preventing the rig from very momentarily transmitting into
an open circuit as the relay closes.  It solves what I beleive some folks
refer to as the "HOT SWITCHING" problem.


FT-990 BAND DATA                                              AL-1200 AMP
----------------                                              -----------

PTT >----------------------------------------------------------+
                                  1k*          1N4001          |
INH >-----------+-----------+---/\/\/\/\---+----->|-------+----+----> PTT
                |           |              |              |
                |           |      R       |              |
                |           |              |              |
               ---          +---/\/\/\/\---+---/\/\/\/\---+
             C --- 10uf            1k*           3.3k
GND >-----------+---------------------------------------------------> GND

*NOTE: Slight adjustment of these resistors may be necessary to 
       establish the exact timing you desire.  Remember: T=RC.

Another slight problem I encountered with the AL-1200 was the occasional
open circuit of the antenna when returning to RX after a transmission.
This was obvious from the absense of noise and signals suddenly after
making a transmission.  This, I found, was a slight misalignment of the
relay contacts which I beleive I have now fixed (it has not happenned
again for some time now).

Hopefully this information will help someone else.  If not, please excuse
the bandwidth...  I would welcome any constructive comments about this
simple project.

73 de Brian ke7gh@primenet.com

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