Modifications for the Kenwood TM-231

TM-231A MARS/CAP modification


  1. Disconnect power
  2. remove top cover
  3. Locate a green jumper wire sticking out of the front panel assembly (behind VFO switch)
  4. cut the jumper in half. Insulate the ends if bare wire is sticking out
  5. Assemble radio
  6. reset the microprocessor (hold mr key and turn on power)
this mod will allow the TM-231a to transmit from 142-151.995 MHz
N7LMJ @ KB7CFD.ID.USA.NA
TM-231A Out of band


05/20/90

This document describes the steps in enabling out of band transmit to the Kenwood 231A 2 meter transceiver. This modification will allow transmit throughout the 136.000 to 173.995 Mhz range. I do not condone, authorize or any way promote illegal out of band communication. More than likely, performing this modification will void the warranty, also no guarantees are made and you are at your own risk. In other words, don't blame me if you kill your radio! Please read all of text before deciding or continuing. Also performing either the MARS/CAP mod or the full extended transmit mod will erase the memories.

First of all you should be aware that this modification disables the automatic ARRL transmit offset feature. Second, if you do not feel comfortable taking your radio apart, soldering to a surface mount board, or working with very small items, please find someone who can help you.

Ok, now with all that out of the way, here we go. You will need: a VERY small soldering iron with a tip no larger than about a sixteenth of an inch, a phillips screwdriver, a pair of VERY small tweezers or forceps, some very thin solder, a 1N914, or similiar glass diode (NOTE: if you have a chip diode of similiar rating, all the better!), a small box to put all the tiny parts in. Also a small vise or circuit board holder and a pair of external snap ring pliers are helpful. At this time it would be a good idea to write down the memories on a piece of paper, as doing the mod erases the memories. Disconnect the microphone, power, and antenna.

MAKE SURE NO POWER IS CONNECTED TO THE RADIO!
  1. Take the radio apart --- Remove the 4 black phillips head screws from the top and bottom black metal shields, put them in the small box so as not to lose them. Remove the two metal covers and put them aside.

    Note:
    If you just want to mod the radio for MARS/CAP, just remove the top cover and clip the GREEN wire just above the VFO,MR,MHZ switches, and replace the top cover. This extends transmit about 3 Mhz on either side of the ham band.

    Place the radio on a pad or soft STATIC free work area. Looking at the front of the radio, you will see how the black face plate is held in place by 4 little plastic keepers that run over metal bumps in the sub-frame. By GENTLY inserting your finger nail and pushing each keeper, you will be able to remove the face plate, be careful, its plastic. Pull the volume, squelch and frequency selector knobs off, the freq. selector is a real bear, but a strong steady pull will remove it. Next, using a pair of pliers or a set of external snap ring pliers, remove the mic connector ring nut from the mic connector, now remove the nut that is on the freq. selector. Again put all the parts in the small box. Gently remove the LCD assembly from the radio. You will see a series of silver springs mounted horizontaly on the back of the LCD assembly, these mate with short silver pins on the cpu board. Put the LCD assembly aside in a safe STATIC free place. The next step is to remove the metal shield that covers the cpu board. Do this by removing 4 screws, two are located on the short sides and the other two are located on the top and bottom of the radio. Put the screws in the small box for safe keeping. Gently remove the metal plate from the radio and put it aside. Finally the cpu board must be removed. If your radio has the CTCSS encoder and/or the DRU voice recorder, you must remove these before continuing. Consult the manual and reverse the installation procedures to remove them. Be careful when pulling the small white multi pin connectors, forceps work the best. There are 3 screws that hold the cpu board on to the radio frame. Two are located to the right of the frequency selector and the third is located near the squelch control. Also there are two multi pin connectors located at the bottom of the backside of the cpu board that connect to the rf portion of the radio. Remove the 3 screws making sure not to drop them on the board. (Remember the battery backup!) Now using your thumbs at the bottom, and your fore fingers at the top of the board, gently disconnect and remove the cpu board. Be careful in handling the board as it contains many static sensitive components. Always hold the board by the sides as to not touch any traces or pin leads. If you have a small vise or circuit board holder, place the board in it, solder side up, so the row of switches (Call, F, Shift, Tone, Rev, Drs) are pointing toword you.

    What needs to be done, is to solder a diode to the pads located under the tone switch. (S205) This is D209 on the schematic. Note that Kenwood calls the radio with this diode installed a -21 version, for use in "other areas". You will probobly notice how small and how thinly spaced the pads are, and you will be wondering how that BIG diode is going to fit!...

  2. Making the mod --- If you have a chip diode that will fit, skip this part. Otherwise, like me, you will have to fabricate your own SMT diode. Do this by bending the leads of the diode under the body and clipping the remaining lead lengths so that space between the leads are equal to the spacing of pads on the board. THE DIODE MUST BE POSITIONED SO THAT THE CATHODE IS POINTING DOWN, TOWARD THE CPU CHIP. THIS IS VERY IMPORTANT.
    Try making about 5 of these guys, taking each one and visually fitting it to the pads. Look at the overall height compared to the height of the connectors. Also look at the pad spacing, a magnifiying glass works well unless you have eyes like a microscope. Once you've selected the best one, tin the leads of the diode so just a quick tap of the hot soldering iron will "plant" the part on the pads. Now, holding the diode in the proper orientation, tack down one lead, (hopefully you didn't drink too much coffee this morning!), then, using the tweezers orient the other lead and tack it down with the soldering iron. You shouldn't need any additional solder, just the solder on the pad and the tinning on the diode leads. Using the magnifiying glass, FULLY INSPECT THE CONNECTION, making sure there are no solder bridges and that the diode is positioned properly with the cathode pointing toward the cpu. Once you are satisfied proceed with step 3.

  3. Button it up --- Reverse the take down procedure to put the radio back together. Attach the cpu board by aligning the connectors on the back, press firmly to engage the connectors, insert the 3 screws and tighten, remember the radio frame is aluminum, so don't strip them out. Attach the metal sub frame using the 4 screws. Gently place the LCD sub assembly on top and hold it in place so the little springs make contact while you attach the microphone connector ring nut and the freq. selector nut. Tighten them down. Press the plastic face plate so that the little keepers "snap" over the bumps. Re-attach the CTCSS and/or DRU if applicable. Replace the top and bottom black covers with the four screws. Check the small box and make sure you don't have any "spare" parts.
That's it, just connect power, antenna and the microphone. Turning on the power will reset the radio erasing all memories. Oh well...
TM-231 Low Mic Audio Fix


A quick and dirty fix for the kenwood tm231a low mic output till (if) kenwood comes out with a fix is to bypass r 63 (3.3k). r 63 and r64 from a voltage divider at the input to the mic amp. r63 is on the bottom of the rig. with the bottom cover off and the front of the rig facing you, locate the mic amp board 3610 which stands vertically off the mother board. follow the 3rd pin from the left to r63. a jumper here gives the tx audio a nice boost. be sure to readjust the dtmf output in the mic (the only pot in the mic). 73 and good luck. greg.

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