Modifications for the Yaesu FT-840

Picture(s) of Yaesu - FT-840

19-07-1998 FT-840 General Coverage Transmit English language
26-07-2001 Yaesu FT-840 TCXO option English language
03-11-2001 FT-840 CAT/PTT interface English language
08-02-2002 SSB Power Modification for FT-840 English language

19-07-1998 add a comment
FT-840 General Coverage Transmit

  1. ON local unit, connect tp2003 to ground
  2. Switch radio on
  3. Hold SSB and AM down
  4. Switch off
  5. Switch on
  6. 02 OFF will appear on readout
  7. Rotate main dial until 02 ON appears
  8. Press AM NARROW--display will show 7.000 mhz lsb
  9. Press MEM DOWN
  10. Press MEM UP
  11. Switch off radio
  12. REmove earth at TP2003
  13. Switch radio on

This modification has been read 4790 times.

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26-07-2001 add a comment
Yaesu FT-840 TCXO option
Author: NB6Z

The 840 is a great radio but in digital use the rig will drift pretty much as the fan kicks on and off. A great and cheap fix is just as near as your cache of styrofoam.

Remove the upper and lower covers of the radio, making sure to disconnect the speaker from the top cover. Notice that the back screw on the cover is different from the other screws.

After removing the lower cover leave the rig upside down because that is where your mod will be. Locate the xtal board near the center of the radio. Cut a section of styrofoam close to the size of the xtal board, press down lightly over the xtal to mark the area, then trim away the styrofoam to fit over the crystal, trim pot, and connection points going only deep enough for a snug fit.

That's all there is to it. Why spend that $100 for the TCXO option? W5ZIP

This modification has been read 3892 times.

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03-11-2001 add a comment
FT-840 CAT/PTT interface
Author: Igor NN1I -

Computer RS-232 port requires voltages between +5 and +15 volts for logical 0, and voltages between -5 and -15 volts for logical 1. However, FT-840 CAT output voltages are TTL standard between 0 and +5 volts only. This radio requires FIF-232C interface that cost about $100. In addition, FIF-232C requires AC power.

You can make your own interace under $25, plus you can add computer controlled PTT option for PSK31, RTTY, SSTV and other digital modes sharing the same RS-232 port.

The heart of this interface is MAX232A. You can use any similar chip MAX203, MAX232, MAX233, but check documentation for proper connections. Power supply +5 volts is coming from voltage regulator 78L05 that takes +13.5 volts from the back of FT-840 radio (RCA connector). PTT controlled via optocoupler 4N33 by the RTS signal from RS-232. I used DB9 serial port. If you use DB25, then you have to change pin numbers according DB25 assignment. CAT plug is 6 pin Mini DIN.

PTT circuit is optional. You don't need it, if you just want to read radio frequencies and modes into you logging software. However, it is a good idea to share one serial port for CAT and PTT, if you use computer soundcard for digital modes. Make sure that you configured your software to share PTT and radio port, and set PTT control by RTS. Of cause, you will also need Rx audio cable to your soundcard, and Tx microphone cable with attenuator or isolation transformer. More detailed information can be found on the web (

This interface was tested with Logger 6.11 (freeware by K4CY, It did not work with Zakanaka 1.24 stand-alone, because this software had limited number of radio types in configuration, but it could read frequency from the Logger ( If your software does not have FT-840 listed in configutration of radio type, then try to use FT-900 settings. It may also work with FT-890 settings, but slow polling rate. Besides radio type, you must set baud rate 4800, 8 data bits, 2 stop bits, no flow control, no parity, echo off.

My design is FREE. If you are not completely satisfied, I will give you a full refund. No extra cost for having fun! If you have some questions, e-mail me at

73 Igor NN1I

This modification has been read 3284 times.

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08-02-2002 add a comment
SSB Power Modification for FT-840
Author: Phillip Long -

This modification increases the SSB output of your FT-840 by 30-50%.

  1. Remove the top cover.

  2. Locate the 50w/100w micro switch located toward the front and right on the circuit board.

  3. Flip the switch to the 100w position.

  4. Find the pot marked FWD located toward the front and to the right of the 50w/100w switch.

  5. Turn the pot full clockwise.

  6. Re-adjust your AM power if wanted by adjusting the pot labeled AM POWER if needed.

  7. Put top cover on.

Date: 16-03-2002 User comment From: n0vty
Subject: ft-840 power mod

The power mod for ssb I see here only affects how the fwd meter reads on transmit.

Date: 24-06-2002 User comment From: ALAN do not have call sign yet
Subject: make yaesu ft-840 from bassy to trebly

hi, i would like to make my yaesu ft-840 on the am side from bassy to trebly how can i do that please let me know thank you. i talk on am and ssb from 1-40 but i like to listen to the other bands to but i do not key up on them.please email me back at

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