
TX RF power-up delay (revised) and keying modification

Most of the VHF and UHF stations dedicated to weak signal propagation 
modes such as MS, EME, FAI, etc, run high output power and use big 
coaxial relays for T/R switching. Those relays are slowish and take some 
time (up to 1/4 s) before they have completed the switching from receive 
to transmit, while modern solid state transceivers and power amplifiers 
are capable to putting out full RF power within tens of milliseconds 
from keying. This has to be disabled, because switching with RF power 
applied causes arcing of the contacts, causing interference, ruining the 
coaxial relay and being a potential cause for preamplifier failure. 
There are numerous ways to solve this problem, some of them involve use 
of auxiliary coaxial relay contact wired from the tower to the shack to 
drive a dedicated T/R controller unit. Another more easy way is to use a 
"blind" fixed delay in the transmitter, which in it's most simplest 
from, consists of a method of feeding a DC pulse to driver stage 
blocking the RF power output, until all the relays can be assumed to be 
settled to transmit positions. 

This delay circuit was used successfully with IC-211E since the 1980's 
and now has been applied to FT-736R. ALC connection, whether the 
transceiver has such a connector or not, can not be used, because of the 
slow time constants, so some modification inside the transceiver is 
required. 
 

FT-736R
The earlier modification of FT-736R, where the delay pulse was connected 
to DRIVE controlling transistor, did not produce satisfactory results, 
so if you have it done according to the earlier modification file, you 
should consider changing the wiring a little. 
This circuit works on all modes, with PTT and VOX. It injects a short 
pulse of DC to the 13 MHz low power output FET's source on the TX unit 
in the FT-736R, blocking the FET in the RF amp chain and momentarily 
preventing 13 MHz drive power coming out of the TX unit. Incidentally, 
the same FET is also used for keying on CW mode. 

This new connection allows better, transient free rise of the RF output 
and much lower initial power level (-50 to -60 dB from full output 
power) and is not sensitive by any means to DRIVE setting. 
 

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TX power-up profile of a modified FT-736R. The lower image is a 10 x 
zoom of the rising slope of RF output. The 12 dBm reference level is 
equal to 16 W through a 30 dB attenuator. 

Between short dropouts to RX mode, the delay will be there, if the pause 
is longer than about 100 ms. Hopefully during shorter T-R-T trips the 
delay won't be needed. So the circuit is not perfect, but a lot better 
than it was before. Would need more work, and/or more components. 

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 
adjusting circuit (or the keying circuit), to know where to connect the 
delay pulse. 
 
 

INSTALLING

DANGER of ELECTRIC SHOCK:WARNING: 230 V AC VERY CLOSE to FT-736R's TX 
unit!!! PLEASE DISCONNECT AC CORD and BE CAREFUL! 
YOU HAVE BEEN WARNED!

NOTE:
You do these modifications at your own risk! I will not be liable to any 
damage you might cause to your equipment! 
 
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The delay pulse generation circuit. The 150 ohm series resistor is there 
to limit the current to prevent damage to other components. With other 
than FT-736 transceiver, you could consider using even slightly higher 
value, depending on what works there. The value of the 22 uF capasitor 
can be increased up to 100 uF if longer delay time is desired. (1 nF= 
1000 pF, ceramic or SMD) 


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Mid back corner of FT-736R's TX unit: the delay's PCB is installed 
vertically, the Q22 is lower left. Upper arrow: the MS-keying speed 
modification: replace C82 (4.7 uF) to 1 uF electrolytic capacitor. 

Install the components of the delay pulse generator on a small piece of 
PCB, 30 by 30 mm and mount it vertically to the rear between TX and 144 
MHz main unit (near connector J10), with a terminal lug soldered to this 
delay circuit PCB's ground and fasten the lug with the mounting screw at 
the corner of TX unit near AC input connector. Make sure the unit is 
firmly attached and can not become in contact with live AC connector 
terminals, or other live parts on the AC input PBC, like the fuse! 

Connect a short piece of wire from the p5 output of the delay circuit to 
the source of Q22 in the FT-736R's TX unit. The four leaded transistor 
(FET) is on the right side of the unit between the shielded RF 
transformers. If you wish, you can use a small 10 uH inductor to do it 
to make sure no stray RF can get to the source and if you remove the TX 
unit to change the C82, you can use the (correct) vacant drilling hole 
of TH01 that is not there, to solder the other end of the inductor. 

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 make it also work 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 included in the Technical Supplement, so I can not 
give you any pin numbers, but they are the leftmost (AC conn. side) 
feedthrus of the 5 caps. 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, are at +9V 
all the time, when the SAT mode is selected. You may verify the 
correctness of the pin with a voltmeter. Thanks to Mike, N1JEZ for the 
SAT modification and pre-testing. 

If you do not need this delay on one of the bands, do not connect the 
TX9V line from that unit to this circuit. Inputs from only two bands 
(p1, p2) have been drawn in the circuit diagram above. By adding 
parallel input diodes, all four bands can be included. 
 

Adjust the 10 kohm trimmer to give you an adequate delay. For the HF-400 
relay, a 250 ms (0.25 seconds) delay seems proper. Raise the value of 
the 22 uF capacitor or the trimmer to make the delay longer, if 
necessary. An oscilloscope (preferably a memory-), or a time domain 
sweeping spectrum analyzer connected to the 13 MHz RF-output connector 
at the back of the TX unit can be used to verify the delay and rise 
envelope of RF power. 

One way to set the delay on 2 m is to verify if keying TX on causes any 
noise lines to appear on a VHF band 3, (180 MHz) TV screen. Too short 
delay will cause some noise pulses on the TV screen even with 20 W 
output. Increase the delay setting and re-test. When the picture is 
unaffected, add a little more delay and it is set correctly. 

If you operate Packet Radio, set the TXDELAY in your TNC, or software 
long enough to wait, until RF power is up at full power and data may be 
sent. This reminds me some operators use brick RF amplifiers with RF-VOX 
relay switching on packet radio and this causes terrible noise pulses 
across the whole band. A delay circuit like this, would not be a bad 
idea to have in those systems too for many reasons. 

Naturally, don't expect the Digital Squelch system in FT-736R to work 
with the delay in use, because the code is sent out, before the TX power 
is up. 

If you did not understand fully what was explained here, it might be 
better not to open the cover of you radio, but let someone else do the 
modification for you, just to be on the safe side. 
 
 

HIGH SPEED KEYING AFTER C82 -> 1 uF

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FT-736R high speed CW keying waveform with time sweeping spectrum 
analyzer, "R" at 1500, 2000 and 2800 LPM after C82 was changed to 1 uF. 
The upper practical limit depends on what is acceptable and readable by 
the receiving operator, so no speed claims are in order here, but it 
might be well over 2000 LPM. 
 
 
(converted from HTML to text)
Rev. 1999-07-01 
 Ilkka Yrjl,1999 
 
