Modifications for the Yaesu FT-726
FT-726 AUDIO Modifications
Hello my name is Juan, this text talk about the mods for transceivers
of Ham Radio... I hope you interest this... If you need mods for others
tranceivers, ask me....
BE CAREFUL! Procede at your own risk.
Dont forget that transmitting outside the amateur band is not
allowed so please consult your local athorities for a special permit.
Subject: FT-726 AUDIO MODIFICATIONS
Further improvement in FT-726 audio quality can be achieved by a simple
modification to the product detector. The product detector seems to be
overloaded by the input IF signal, the 6dB attennuator described here
will improve the RX-quality
The circuit is proposed by LA8AK, and tested by LA7BI (Egil);
----------------- 1K --------------- to pin 5, Q13;uPC1037
the two resistors (1000 ohms) are easily mounted below the PCB
Break the pcb between the coil and IC pin 5, mount the resistor, and
connect the other resistor from the capacitor to IC pin 5
The audio quality should be much improved, and you should at least be
capable of understanding somebody who does not exactly speak the same
language as you. Englishmen should now be able to read foreigners who
speak english, although it is well known that foreigners from different
countries who use english understand each other, but the englishmen
often do not understand them.
TAPR PSK mods for the FT-726R
Throughput improvement for the FT726R - Tapr PSK modem
On the Lo-19 and Ao-16 digipeaters I was only averaging 1 in 5 UI frames
returned. Viewing my PSK signal on a scope at the FM varactor driver (Q08)
on the Sat Unit revealed a very distorted and jittery audio waveform.
Replacing the Tapr's obsolete 4030 (U21) with a 4070 gave me a steady but
still distorted wave. My throughput was now 1 in 3; better but not good.
Modifying Tapr's low pass output filter (as suggested by G3RUH for his PSK
modem) and a change in the Tapr's attenuator and alignment has given me
almost 100% throughput.
TAPR Attenuator mod:
Remove R5 (22K) by putting a jumper on JP7. Replace R6 (5K trim pot) with
a 100K trim pot. Be careful when unsoldering as those small trace pads come
off very easily.
TAPR filter mod:
Replace R7 (22K) with a 47K resistor and C8 (.047 mf) with a .01 mf disk
Find plug J04 on the Satellite Unit (front left corner). Buy an equivalent
plug and header pins and solder them together with short bare wire,
attaching the center conductor of RG174U coax to pin 2 and the shield to pin
1 of the adapter. Then plug the Yaesu's plug onto the adaptor's header
pins, and the adaptor's plug onto the Sat Unit board's male header pins.
You could solder the cable directly to the board or connector, but this way
the board's original condition is retained.
The waveform out of the TAPR (ungrounded end of JP8) will now be a somewhat
rounded sawtooth. Adjust the 100K pot (R6) to mid range. Set the FT726R's
Mic Gain to the first "tick" (8 O'clock).
Attach your scope to the RG174U coax and slowly increase the Tapr's output
(R6). You will see the PSK's distorted waveform smooth out and grow to
become more like a sine wave. Around .6 v pp it will be at it's best. As
you continue to increase the PSK output distortions will reappear as the FT-
726R's FM limiter clips the signal. Back off the audio until you get the
least distorted wave shape. It will be almost a sine wave.
When you set your transmit power lever, back off so your ALC is not active.
Assuming you have all your software switches and parameters set properly you
should now have greatly improved throughput. Let me know how this works for
you or if you improved your throughput in other ways so we can write it up.
Sensitivity for FT726, 50-54, 144-148,430-440 MHz
Remove the 2mtr module , remove the top cover of the module,,, It the PA module
end of the unit you will find Q01, it is a Mos-Fet a 3SK74L2X.
Replace this with a BF982,,, the leads are slightly thicker than the original,
so a 1MM drill is required to open the holes on the PCB.
The LONG LEAD of the BF982 should be fitted facing away from the PA Module.
When the Fet is fitted a very small length of wire should be fitted from the
Source lead (The lead facing into the center of the board) to the surrounding
This should be it, but if you really want to be a perfectionist tune up on T01,
T02 and T03 ONLY!!
Finally, Box up and enjoy the low noise, improved signal reception.
FT-726 mods for 9.6kbps FSK
RX data connection
RX signal is drawed from the junction R44-DO4(1S188) with 10K ohm resistor.
It must be connected to RX in of G3RUH modem
TX data injection
You must inject both TX unit and Satellite unit if you need full duplex
operation ( example for uosat4/5 ).
The TX data injection point is DO5(FC-53)-C27 in TX unit and
DO6(Fc-53)-C25 in SAT unit. You may be able to direct connect from
TXD out of G3RUH modem as DC decoupling is in the modem board.
You need selection SW for TXD injection to select sinplex/full duplex operation.
Mods for ft726r to do 9600
From James Miller G3RUH:
Inject your modem TXaudio directly to the transmitter varactor in the
SAT unit via J04 pin 2.
This is of course assuming you are using a G3RUH or K9NG 9600 baud FSK
modems. The receive side needs to come straight from the discriminator
which I haven't had a chance to locate yet. Perhaps if someone else
has the necessary hook up information for the receive side, they could
post it here.
Remove the main lid; remove the SAT unit lid.
Locate J04 which is a 3 wire connector in the front left-hand corner.
Unplug J04, and inject your audio on pin 2, ground on pin 1, using a
Hang AGC for FT-726R
Instructions: Remove R153, C144 from RX Module. Use different colors to made identification easier. These are to be connected to the Hang AGC circuit. Add the following five 10 in. lengths of #22 insulated wires off top of RX Module. +8 AGC SW - Connect to hole of removed C144 + side going to s/F pin 5 J06. +8 VDC Fixed - supply side of R155. GND - Anywhere available on RX Module. Det RF - from base of Q33 AGC Line - To Collector of Q33. Hang time now set to 480ms. Change the .22uf to .1uf for shorter, or change to .33uf for longer. The AGC-F switch acts normally in the Fast position, the Slow now has the Hang. Note: All resistors 1/8 to 1/4W. Capacitors > 12VDC.">
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9th February 2000
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