        TX POWER UP DELAY FOR FT-736R  by OH5IY 01-1995 (modif. 04-97)
        -----------------------------
(View this with DOS Edit)

+TX9V lines                                         QTY component

                                                     1  BC557 transistor
70c    p1 >Ŀ1N4148                           2...5 1N4148 diodes
"inputs"                                           2  1 nF cer. f.RF bypass
                                                 1  1 kohm resistor
2m     p2 >Ĵ1N4148     1nF      1k           1  10kohm trimmer
                                                  1  1.2 kohm resistor
                Ĵgnd    1  15uF 15V tantal cap.
                            1nF                  1  30mm*30mm PCB
      DELAY   10k             E                                   
 ADJUSTMENT trimm.<        /     BC557 or equal
                             /      PNP transistor
                   ĳ
                             B \      all resistors 1/4W
                       10k       \  C
                                                            
                   +                  1k2                       
                   15uF         >> p5  to Q32(E), TX-unit
                     tantal                                                    
                   15V          1N4148
                  gnd
                                             (no component values critical)

Most of the best V/UHF stations use high power and big coaxial relays for
T/R switching.
One must disable transmitter power from rising too soon too high,
before the relay has completed switching. This prevents arcing from ruining 
the relay contacts and reduces possibility of preamplifier failure.

The FT-736R has no (ALC) connector to connect any external sequencer.


This circuit works on all modes, with PTT and VOX. It injects a short 
DC pulse to the power control transistor in the FT-736R's TX unit, 
blocking the current of the PIN diode in the circuit and momentarily 
preventing drive power coming out of the unit. There is one thing to
remember, the delay disappears, if you turn the RF-power setting higher than
approximately one a clock (60%). 

This circuit is of general type and can be also used on most other
transceivers. You just have to find +TX voltage and understand
the power adjustment system, to know where to connect the output. 
ALC circuits have too slow time constants and are not applicable for 
this purpose.

Connect the terminals p1, p2 etc. to the TX9V lines entering each power
amplifier modules from their respective RF UNITs. 
The only way to make this and have it working also on SAT mode, is to 
connect the TX9V from the small PCBs that are soldered to the feedthrus
on the PA's shields. These small boards are NOT on the Technical supplement,
so I can not give you any pin numbers, but they are the leftmost (AC conn.
side) feedthrus of the 5 used. Inside the shield cage I can see an
orange wire connected to it. This is the correct spot to take the
TX9V out, since the other wires leading into this unmarked PCB on 432 MHz
PA, always have +9V, when the SAT mode is selected. You may verify the 
correctness of the pin with a voltmeter.

If you do not need this delay on one of the bands, do not connect the 
TX9V line from that unit to this circuit. By adding input diodes, all four 
bands can be included.
Only two inputs have been drawn in the circuit diagram above.


           ---- DANGER of ELECTRIC SHOCK ------

CAUTION: 230V VERY CLOSE!!! PLEASE DISCONNECT AC CORD and BE CAREFUL!!!

Place the small PCB required vertically to the rear between TX and 144 MHz
main unit (connector J10), with a terminal lug soldered to this delay circuit 
PCB's ground and fasten the lug with the mounting screw of the TX unit near 
the AC input connector. Make sure the unit can not become into contact 
with AC connector terminals, or other live parts on the AC input PBC, or
the fuse!

Connect a short piece of wire from the p5 output of the delay circuit
to the emitter of Q23 (or rather to R101) in the FT-736R's TX unit. 
The transistor is 15 mm to the left of connector J10, near where this 
new PCB is mounted.

Adjust the 10 kohm trimmer to give you adequate delay. For the HF-400 relay,
a 250 ms (0.25 seconds) delay seems proper. The delay can be adjusted 
up to about 500 ms. An oscilloscope (preferably a memory-) connected to
p5 is very useful in determining the delay the circuit produces.

One way to test this on 2 m is to set delay first a bit too short and use 
some 30 W of RF output on FM and key PTT while watching TV on ch E-5 
(around 175MHz, TV antennas some <10 m away from the ham antennas). 
If any arcing noise stripe flashes on the TV screen when keying the 
transmitter, increase the delay setting and re-test.

When it stops, add just a little more delay and it is set correctly.
If you operate Packet Radio, set the TXDELAY in your TNC, or software
long enought to wait, until RF power is up and data may follow.

The modification did not work on satellite mode in its earlier
form, but thanks to Mike, N1JEZ, it works now. Perhaps someone can find
out why the delay disappears at high RF power settings?
