Modifications for the Yaesu FT-736 Part 2
FT-736R poor EMC features


Hello dear Oms. Here are my experiences with the FT736R, the problems which I have encountered, and the cures that I have found.

  1. background noise given by the switching power unit. This problem occurs on 50 MHz and 144 MHz. By using a spectrum analyzer, I have seen that the power unit radiates a wide bandwidth of noise (can be compared to white noise) between 40 and 42 MHz. The level is high, more than the mean value encountered with a switching PSU built for computer using !

    --> the cure: no miracle, the simplest and efficient way is to use an external power supply instead of the built in one...
  2. overload of the receiver by the path of coaxial cable (between antenna and the TRx). Before introducing the problem, I have to tell that I live not far from a high power broadcast station (medium waves). The first time I have used the FT736R, it has been impossible to hear something on any bands (from 50 MHz to 432 MHz)! I have discovered that the problem comes from the DC sys- -tem used for the supply of the preamps on each band. The DC supply is ensured through the coaxial cable. With high values of RF fields the coaxial cable acts as an antenna. These spurious RF signals come through the DC line called PRE in the FT736R. Note that this problem can be met without any installed preamp or with the front panel push button (PREAMP) in "off" position.

    --> the cure: quite simple, you have to solder a 0,1 uF on the existing by-pass capacitor for EACH PRE line of all Rx module. That is to say: C32 for all the bands. The originals values for C32 are always 0,001 uF (1 nF).
    These by-pass capacitors are closed to each PA unit and can be easily found.
    By increasing the value of each capacitor, you decrease the cut-off frequency of the L11-C32 filter and avoid any problem.
  3. background noise given by the PTT lines (input PTT jack for data communi- -cation and output PTT STBY DIN jack for the remote of an external power amp- -lifier). My FT736R is put on a wood shelf and under is located a PA unit. I have also a TNC (HK232) which is also connected. At this time, I have remarked background noise mainly on 144 MHz, which is more noticeable when moving optic VFO encoder. These two PTT lines are sensitive to wide frequency signals given by "computer" signals coming mainly from the front panel (display) and uP unit (clocks, hardware bus).

    --> the cure: the only efficient solution is to put LC filter on each PTT line They can be installed outside of the TRx but rather closed to the rear panel of it. The filter uses the "pi" structure with two 0,1 uF capacitors and a small choke coil about 220-470 uH (not critical value). You have to try seve- -ral choke coils in order to avoid the QRM and also to have a normal PTT function (care of added resistance for DC PTT current given by the choke coil)
FT-736R Killing the beep


I bought a Yaesu FT736 about 6 months ago. It's a great radio but one thing annoyed me about it. When I used in on FO-20 or the microsats with my TAPR PSK modem, the PSK modem adjusts to the doppler frequency shift by sending pulses to the up/down buttons on the microphone. Only problem is, everytime it sent a pulse the radio went BEEP. This beep didn't come thru the speaker, instead it was emitted from a separate buzzer. As a result, even when using the headphomes, the beep would disturb the whole house. This was a particular problem on late night passes. Thanks to a suggestion from WB2IBO and the folks at Yaesu, I've discovered you can fix the problem.

If you remove the top cover you will find a circuit board right behind the front panel. As you are facing the front of the radio on the top left corner you will see capacitor C63 identified on the board with a red wire going into the circuit board next to it. Clip the red wire and no more beep. You won't actually see the beeper. To find the beeper you have to remove the top and bottom covers (see the manual) and then loosen the screws on either side of the front panel (see section 3.5.4 in the manual). At the bottom left of the circuit board is the black buzzer, about the size of a quarter. The red wire you need to clip starts from here. You don't actually have to remove the bottom cover if you clip it where it goes back into the circuit board on the top, however.

You could rig up a switch to turn the buzzer off and on, but I don't why anyone would ever want to turn it back on.
The FT-736R SAT switch can zap pre-amps.


The 736R SAT switch has break-before-make contacts, which meanhat in between switch positions the circuit is the same as if the SAT switch is in the OFF po sition. If you have set up one of the modules to transmit for non-satellite op erations (i.e., w hen SAT switch is OFF), that module will be enabled between the RX-TX-NOR-REV p ositions of the SAT switch.f you are transmitting while you move the SAT swi tch between these positions you may accidently key up the unintended module and send a burst of RF down the antenna line into your pre-amp.

For example: VFO A happens to be set to operate on 2 meter FM. You don't give this a second thought, since you are working Mode B CW with the SAT switch ON.
You are listening on 2 meters and keying your 70 CM transmitter trying toro your downlink f requency. Changing the SAT switch positions under these conditions is perfectly normal.

However, if you happen to send a dot or dash during the open interval been switch positions it is not your 70 CM transmitter that gets keyed but your 2 meter FM transmitter. This sends a burst of RF down the 2 meter coax that you have configured for receiving. If you have in that coax a preamp without adequate RF-sensed pin-diode switching, then say goodbye to the GaAsFETs in the preamp.

If you are using the FT-736R preamp switch and the 12 VDC on the coax to operate coax relays at the preamp, you might be lucky -- if the relays drop out fastenough. I don't know if the control circuits operate fast enough to take care of this transient condition, but I doubt it.

This cost me two MGF-1402 GaAsFETs in my unprotected homebrew 2 meter preamp be fore I realized that they both departed this world under exactly the same circumstances of operation of the SAT switch while sending. Although I was on CW, the same thing could happen on SSB if you happened to be speaking and the VOX were to be operated during the switch rotation.

Thanks to John, KL7GRF, for clueing me to the real problemhe SAT switch while transmitting.
(2) Use adequate RF sensed mp switching.
A small modification to get A better audio-signal out of the TX from FT-736R


Hello FT 736 Fans,
The FT736R tranceiver is a well done rig, but the TX-signal is not very readable by the QSO-partner if the signal-stregth ist low. The FT736 prefer the low frequencies in the audio-signal an cut off all high frequencies in the speech-signal. In FM-Mode it sounds like a bass-man..hi . In SSB-mode the Power-Amplifier is working hard an only the lower speech-band is transmitted. There are a few little modifikation to reduce this problem:

  1. change the mikrophone in the handheld-case to a better one. (try several mikrofones at a resistance abuot 600 Ohms)
  2. open the FT736 (upper case) and locate the TX-Board (on the left upper left side if the front is showing to You). On this TX-UNIT cut the Capacitor C 14 (Value 0.001uf) . The Capacitor is located in the right-bottem corner of the board. Adjust for FM-Mode the Potentiometer VR 01 just a little tick clockwise ( deviation-limiter). If You can use a deviation-meter You can do this in a professional way. (about +- 5 Khz).
  3. If You are using a preamp-mikrophone, make sure to adjust the level from this mike so that the mike-gain-potentiometer at the frontpanel from the FT736 are minimum in the 11 o`clock position for a good drive of the powerstage. Otherwise You will overdrive the first mikeamplifier in the rig. This sounds like speaking in a train-station...
    When You are using a preamp-mike connect a small 600 Ohm resistor direct in the mike-plug between point 8 (middelplug) and point 7 (GND) So the rig always have the correct 600 Ohms resistance.
  4. Best results I got whit a mike, that produceses a strong signal in the higher audio-band (about 2000 Hz).

By the way, the PROC-Switch only works at SSB and only when You get down with the drive-gain... if the ALC-Meter is on the right scale, the PROC- switch has no affect !

ALLWAYS think of loosing warrenty if You modify something inside the FT736 !!

FO-20 Microsat & the Yaesu FT-736R


by James Miller G3RUH

If like me, one of the (many) features that attracted you to the FT736R was the special DATA SOCKET, then you were probably very pleased to read in the manual:

"This 3-contact mini stereo jack allows direct connection to the FM receiver demodulator and FM transmitter modulator for digital equipment such as a packet radio TNC. No pre-emphasis or de-emphasis is added to the signals at this jack" (page 14).

Don't believe a word of it! A cursory inspection of the circuit diagram shows that the TX side is simply merged with the microphone audio just after the Mic Gain control, and is then murdered by several subsequent filters. On the RX side the data audio output is similarly indirect.

My initial day with FO-20 was disastrous. (Yours too?)

So I looked at the FM signal on a calibrated monitor receiver when transmitting FO-20/Microsat "Manchester" uplink signals. The waveform was appallingly distorted.

The problem is that the TX LF response cuts off at 800 Hz. But the data has substantial energy at 600 Hz and below. Indeed, when transmitting a 600 Hz square wave it was clear from the droop distortion that FO-20 or a Microsat would almost certainly not decode the uplink reliably.

The cure is simple; modulate the FM varactor directly. Refer to the circuit diagram; inject your TXaudio at the junction of R32/C29 on the TX Unit. The signal level at this point should be 800 mV peak-peak, and will give +/- 3 kHz deviation. DO NOT EXCEED THIS LEVEL. Set Mic Gain to min.

The implementation is simple too.

  1. Disconnect FT-736R from the mains electricity. (Safety).
  2. Remove top cover only.
  3. TX Unit is the module flat on the left (not the one tucked down the side vertically).
  4. R32 is just to the left of the rectangular shielded enclosure. The resistor is "on end". Scrape the paint off the free leg.
  5. Your TXaudio lead should be a fine screened cable; connect the inner to R32, and the outer braid to the adjacent enclosure.
  6. Route the cable out though any convenient aperture in the case.
[ TXAudio of 800 mV pk-pk can be obtained from the G3RUH PSK modem by adjusting the components C9= 1uf, R3=47k, R5=infinity (i.e. remove). C10 stays at 10nf (0.01uf).]

Modulating the FM transmitter this way you get an LF response down to 18 Hz (at which point the associated synthesiser PLL begins to track the modulation), and an HF response which is flat to some 10 kHz.

*** FO-20/Microsat uplink modulation is now absolutely perfect. ***
FT736 & 9600 Baud Operation


by James Miller G3RUH


These notes tell you where to get FM RX audio direct from the discriminator, and where to modulate the FM TX varactor directly. These mods are non-destructive and take no more than a few minutes. The signal bypass the "DATA SOCKET" for high grade FM operations.

The RX mod is suitable for:
The TX mod is suitable for:
FT736 - FM Direct from Discriminator

Detected FM direct from the receiver discriminator is available from the RX UNIT at the junction of R91 and C83. These components are shown in the top right-hand corner of the schematic.

Proceed thus:
  1. Disconnect FT736 from the mains electricity. (Safety).
  2. Remove top cover only.
  3. RX Unit is the vertical module on the left.
  4. Locate R91 which is about 25mm from the top, 50mm from the radio rear. the resistor is "on-end", and near a couple of glass diodes.
  5. Scrape any paint off R91's free end and wet with solder.
  6. Your RXaudio lead should be a fine screened cable; connect the inner to R91, and the outer braid to a ground point (e.g. can of TO09)
  7. Route the cable out though any convenient aperture in the case.
  8. The discriminator sensitivity (FM Normal) as about 6 kHz/volt.
Important note on 9600 Baud Use

Some FT736 receivers are fitted with an LFH12-S IF filter for FM. (CF01 at the top front of the RX Unit). This is a 12 kHz bandwidth filter which is a little too narrow for 9600 bps FSK operation. It is recommended you change this to 15 kHz or better still for UOSAT-D use, 20 kHz bandwidth which will allow more tolerance for doppler shift, and give a far better "eye". Suitable filters are: LFH-15S or CFW455E, and LFH-20S or CFW455D.

FT736 DIRECT VARACTOR FM MODULATION

Refer to the circuit diagram; inject your TXaudio at the junction of R32/C29 on the TX Unit. The signal level at this point should be 800 mV peak-peak, and will give +/- 3 kHz deviation. DO NOT EXCEED THIS LEVEL. Set Mic Gain to min.

Modulating the FM transmitter this way you get an LF response down to 18 Hz (at which point the associated synthesiser PLL begins to track the modulation), and an HF response which is flat to some 10 kHz.

Proceed thus:
  1. Disconnect FT736 from the mains electricity. (Safety).
  2. Remove top cover only.
  3. TX Unit is the module flat on the left (not the one tucked down the side vertically).
  4. R32 is just to the left of the rectangular shielded enclosure. The resistor is "on end". Scrape any paint off the free leg.
  5. Your TXaudio lead should be a fine screened cable; connect the inner to R32, and the outer braid to the adjacent enclosure.
  6. Route the cable 0 BAUD FSK MODEM: Adjust TXAudio level with VR1
Notes compiled by G3RUH @ GB7DDX 1990 Mar 16
Yaesu FT-736R TAPR PSK Modem freq shift prob

I bought a Yaesu FT736 about 6 months ago. It's a great radio but one thing annoyed me about it. When I used in on FO-20 or the microsats with my TAPR PSK modem, the PSK modem adjusts to the doppler frequency shift by sending pulses to the up/down buttons on the microphone. Only problem is, everytime it sent a pulse the radio went BEEP. This beep didn't come thru the speaker, instead it was emitted from a separate buzzer. As a result, even when using the headphomes, the beep would disturb the whole house. This was a particular problem on late night passes. Thanks to a suggestion from WB2IBO and the folks at Yaesu, I've discovered you can fix the problem.

If you remove the top cover you will find a circuit board right behind the front panel. As you are facing the front of the radio on the top left corner you will see capacitor C63 identified on the board with a red wire going into the circuit board next to it. Clip the red wire and no more beep. You won't actually see the beeper. To find the beeper you have to remove the top and bottom covers (see the manual) and then loosen the screws on either side of the front panel (see section 3.5.4 in the manual). At the bottom left of the circuit board is the black buzzer, about the size of a quarter. The red wire you need to clip starts from here. You don't actually have to remove the bottom cover if you clip it where it goes back into the circuit board on the top, however.

You could rig up a switch to turn the buzzer off and on, but I don't why anyone would ever want to turn it back on. 73 de WA0PTV @WA0PTV

(Message originated at NS1N from KC1HO reqMOD Server v2.0)

TX-PLL Modification
YAESU FT-736R 9600 Mod

Modification to the Yaesu FT-736R. G4WFQ 12/1/92.

This modification was given to me by Zeno Wahl, G0NJC/VE3LMX (U.O.S)

The modification lowers the frequency response to 3 HZ, and gives a far better "eye" by reducing L.F flutter.

Proceed thus.

Locate "TXPLL UNIT" (Vertical board on Tx unit).

Locate R01 (Scrape any paint off. Wet component with FINE solder.

Solder 560ohm Resistor on R01 (end nearest to pll board) Solder 47micro fara tantalum in series with 560R. Take (-) negative leg of Cap to Gnd, eg case of Txpll unit.

73 Dave G4WFQ.

=============================================================

Better UO-22 Decoding

Dave - you are right. UO-22 is less than optimum. The problem starts in the satellite which does not have a transmit spectrum extending to DC, nor even to the desirable 30 Hz. In fact it is 3db down at 100 Hz. The effect of this is to cause "droop" on short runs of 1s or 0s. It can clearly be seen on a scope. Display the eye diagram, and slow the sweep speed down so that a dozen or so bits is visible. Looked at another way, the poor LF performance introduces wobble on the trace, and this blurrs the eye. So if the receive system was so-so (say with UO-14) then it may well be very error prone from UO-22.

The cure is to make the receive system have as good an HF performance as possible, and a good LF performance. Having a good HF response ensures a good eye, and thus a better margin to cope with the LF wobble. And having a good LF response minimises and additional self noise from the RX/modem interface.

On the modem increase C25 to 1 uf. This is the RXAudio input coupling capacitor.

On an FT736R:
  1. Use a CFW455B (or C or D) IF filter in the RX UNIT.
  2. On the RX UNIT, remove C82. This is a little ceramic capacitor tucked in close to the grey cube marked "455D". Bend it back and forth until the legs snap off. You can reach it by removing the radio lid only. DO THIS!

When you have done these changes, TX selection 10 transmitting to an FT736R gives a virtually perfect eye. Since UO-22 also transmits selection 10, you can see the extent of the LF aberration as a blurring at the "eye" convergence point. However you should now have reliable decoding.
Other radios seem not to be as reluctant as the FT736R, probably because they have a better basic HF response. However, changing modem C25 should help.

I am evaluating the feasibilty of implementing LF equaliser to rectify the UO-22 LF problem. The perfect project for all you DSP freaks. I'm on holiday for two weeks. I expect one of you lot to have done it by the time I get back.
No kidding.

UO-14 RX frequency tracking for TS-790/FT-736


The doppler shift range in overhead pass reaches to 20kHz. It is essential to tune-in RX frequency for good through put.
I assembled auto freq. tracking for UO-14 FSK signal reception and achieved good result.
[Block diagram]
RX discri.out<-- voltage comparater--INV--NOR--switching TR---> Down control
                      (Q1)           (Q2) (Q2)      (Q3)
                                           |
   G3RUH board  DCD -----------------------

[Circuit connection table.]

                GND         Q1:LM324 or TL084  Q2:74HC02  Q3:2SC945 etc.
                 |
              (C:0.01)
                 |    ----      ----      ----              -----
   *a<-(R:1M)--------3 Q1 1----6 Q2  4----3 Q2 1---(R:470)---B Q3 C---> DWN *c
     +12V--(R:8k)--- 2    |   -5     |   -2    |            -E    |
                 |    ----    | -----    | ----             | ----
                 |            |          |                  |
              (VR:10K)       GND         |                 GND
                 |                       |
                GND                      |          GND   +12V  +5V
    *b<-- G3RUH board DCD line------------      Q1   11     4
                                                Q2    7          14

*a: to RX discri out (=G3RUH board RX IN)
*b: to G3RUH board DCD line (U10 pin 13)
*c: to down control pin of mic terminal (TS-790 mic pin 3)

setting of VR:10k
        The discrimiator IC output level is 5.8V+-2V.
        Whitout RX signal, set the VR to Q1 pin 1 level turn HIGH->LOW.

In my case, I assembled the circuit on a small universal PCB and enclosed into
TINY-2 with NB96 board.

In the case of FT-736, DL signal polarity is different from TS-790 so
change as follow
     ------
     3 Q2 1------|>------> DL signal to mic terminal
     2    |    Diode (1S1588 etc)
     -----
73s sueo asato JA6FTL
FT736R - VHF Attenuator Mod


A little trick to built in an RX-Attenuator in the FT736R (VHF Band)

Hello FT736 fans,

Using a preamp in the VHF Band causes sometimes trouble with noise on this band. The S-Meter readout 2-4 without any signal. The AGC is working only to reduce the noise.

A simple trick decrease the sensivity for about 6 dBm.

Look in the schematic-diagramm 144 MHz MAIN-UNIT. The first RX-Stage is at the left side of the sheet. Locate the transistor Q01 3SK122L and look at the source-resistor R02 (47 Ohm). A second resistor with the value 1.2 KOhm in series with R02 decrease the sensivity for 6 dB.
Using the preamp-switch (this switch has a free row) You can easyly switch between attunation and normal use by making a bridge over the new 1.2 KOhm Resistor with this contacts.

I think, this trick is not a excellent way of a RX-Attunator, but it works ufb here in my rig.

By the way, remember of loosing warrenty bĀ modify Your rig ! I am not responsable for failures after modification !

FT-736 - improving the 2mtr RX


Just read an article out of OSCAR NEWS about improving the 2mtr RX.
This is how it is done.......

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 groundplane.

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.


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