Modifications for the Kenwood TM-721
Modification from Kenwood on the TM-721A for adding cross-band
To operate the TM-721A in the repeater cross-band mode,
chip resistor R121 on the control unit (X53-3130-XX) (B/3),
must be removed. The following modification will detail how to
access the resistor.
Disconnect the power supply and antenna from the transceiver.
Remove the top and bottom covers (11 screws). Do not damage the
speaker wires when removing the top cover.
Remove the silver colored screws from the front panel chassis (2 on
each side, 1 on top, and 1 on the bottom).
Carefully pull the front panel away from the body of the
transceiver (do not disconnect any cables).
Locate chip resistor R121 on the Control unit inside the front
panel. Unsolder & remove the resistor.
If you wish to disable the built-in 3 minute time-out timer,
also remove resistor R122 on the Control unit inside the front
To place unit in repeater mode, press F key.
Press the A.B.C. key within 5 seconds of pressing the F key. Three
dots should be lit in the main display. Unit will now operate as
a cross-band repeater.
The TM721A provides a built-in 3 minute time-out timer. This automatic-
ally places the transceiver in the receive mode after 3 minutes of
continuous transmit. This function can be defeated by removing resistor
R122 on the control unit inside the front panel.
Tuning the 721 UHF for 460 MHz
Author: Edward Thomas, N2IHN
1. FIND L4 & L3 on bottom of set, turn 4 screws CCW @ 3 turns.
2. PEAK TC1, TC2,TC3 for signal strength.
3. May need to tune TC102 for transmit output and TC101 for recv.
New LCD Readout for Kenwood TM-721A
I just replaced the LCD readout on the TM-721A and thought you may be
interested in my experience. The replacement is easy and gives the desired
result: the readout is easy to read in high ambient light. The TM-621A can be
upgraded with the same unit.
I ordered the unit from Kenwood amateur equipment division parts
department which you can call at (213) 639-7140 (639-4200 and 639-9000 are
alternate phone numbers I have seen on the PBBS board) and order part number
B38033705, or just ask for the LCD conversion for the TM-721A. The price is
$37 plus California state tax and S&H, which came to $43.50. They will take
credit card charge over the phone.
What you get is the complete LCD display unit with its two IC drivers. On
the schematic that comes with your radio you can identify the subassembly as
the one labeled "LCD ASS'Y" which is connected to the control unit by an 8-
conductor ribbon cable, which you must unsolder to free to old unit and
resolder for the new one. The LCD subunit is attached to the control unit
assembly by bent tabs which are easily undone. The ribbon cable comes with a
small section of pressure-sensitive adhesive to help position it for
soldering. Complete step-by-step instructions come with the unit.
Installation requires a small soldering iron and is not difficult.
The results are quite good: there is no functional difference in the way
the radio operates. I suspect that the new LCD is electrically identical to
the old. If you have modified your radio to operate as a cross-band repeater,
you retain that function. The display consists of black digits on a milky-
white background. The incandescent lamps behind it operate as before, and I
find that even in the "dim" mode that the legibility is good in low light. In
bright sunlight, the back illumination is no longer relevant, and the
readability is very good. Since bright lamp operation is not really required,
they should last forever.
The old unit is not destroyed upon removal. You can return to the old
readout at any time (which is prettier as it is in color). Kenwood is to be
congratulated for recognizing the problem with a large installed base of TM-
721A's and making this field fix available.
While you have your radio open, you may want to activate the cross-band
repeater function. Lay the CPU board down so the two ribbon cables come off
to the north. A little northeast of the lithium cell, just east of the bare
copper landing at the top of the board is chip resistor R121. Carefully
remove it (you will probably break it). Now, you can put your radio in the
cross-band repeater mode by hitting "f" "ABC". The mode is indicated by three
decimal points in both main and subband frequency readouts and a signal heard
on either band is retransmitted on the other. A three minute timer is in
effect, which may be turned off by removing R122 (not tested). Return to
normal operation by again hitting "f" "ABC". During repeater operation, all
other functions are locked out. Of course, the TM-721A does not auto-
identify, so it must be under direct operator control.
John, W6BM @ W6PW, in Berkeley
IC-721E RX/TX mod 142 - 152 MHz mods
The following will allow the TM-721A to transmit on the VHF band from
142.000 to 151.995mhz. Specifications are guaranteed for the Amateur
Disconnect the antenna, mic, and power cord.
Remove the bottom cover from the transceiver (6 screws).
Locate resistor R57 on Control Unit (X53-3130-00) (A/3). The
resistor can be seen by looking through the cutout of the chassis
just behind the front panel. Please note that R57 is on the bottom
of the unit and should not be confused with R58 on the top of the unit.
Using a sharp pair of wire cutters, cut the resistor lead.
Reinstall the bottom cover.
Reset the microprocessor by following the procedure on page 22
of the instruction manual entitled MICROPROCESSOR INITIALIZATION.
Performing the previous mod will automatically reset
the processor. At least it did when I performed the mod.
You can determine that when you turn the unit on and
have lost all your memory settings.
Performing this mod does not give you the license to
transmit in these newly acquired frequencies unless you
have the permits.
TM-721A wide band mod
To modify the 721, remove the top & bottom covers.
Remove the 2 screws on each side that hold the front panel to the main unit.
[You can just loosen the bottom 2 screws, 1 on each side don't actually remove
Then remove the 2 screws in the top center & bottom center that hold the display
to the main unit (all silver screws, not the black ones).
Now you can swivel the panel out & down.
Look at the back of the display unit. To the left of the Microprocessor near
the lithium batttery you will see 3 chip resistors in a pattern like this:
H= chip resistor
X= blank space
--------------------> H X H H
add a jumper (or ideally a zero ohm chip resistor where the X is.
Hold the F key while turning the radio on.
The display will go from: 100 - 200 MHz on VHF
400 - 500 MHz on UHF
Remember to write all your frequencies down before you start because you
WILL lose them on both bands once you add the jumper even if you do not reset
with the F key.
The programming resistors do the following
Number 1 was easy, I just installed a jumper where there were a pair
of empty pads next to a row of four or five other 0 ohm (jumper) "resistors".
Select the European frequency bands (430 - 440)
Select MARS/CAP xmit freqs (141 - 150)
Select unrestricted out-of-band xmit (add a jumper)
Select cross-band repeater mod
Disable built-in 3 minute timeout timer for crossband repeater.
I found this quite useless and promptly removed it.
Number 2 is also very easy, but unnessary for 90% of us. If you need MARS/CAP
then you'll be able to get the details from Kenwood.
Number 3 is somewhat of a mystery as I've been told the jumper position
but have been unable to find it on the board. My interest was in
opening up the UHF receive to hear the public service channels.
Unfortuneatly, this mod opens up transmit as well, on both bands and
apparently without restriction (as the gossip goes).
Number 4 is real easy and involves removing the resistor marked R121 on the
Number 5 is also easy and just remove R122 to do it. (I haven't though)
What I'd like to know concerns the remote control capabilities that are
available through the microphone plug. From looking at the manual,
it's clear that when the RC-10 remote control handset is used, many of
the radio's front panel controls are made remotely available. Does
anybody know the manner in which the three data pins on the mike plug
are used for this. The three pins are PTT, UP and DOWN (in normal
mode). Presumably, there is a way to communicate with the onboard
microprocessor using these pins. If so, then somebody could
concievably use their PC to control the radio.
Modifying the Kenwood TM-721A for Extended UHF Coverage
Well I finally uncovered the specifics of the mod which will extend the
UHF coverage of the TM-721A. This mod changes the UHF receive from
it's factory setting of 438 - 449.995 mhz to approximately 420-460
MHZ. There are some peculiarities surrounding this mod so I'll
describe them first.
First of all, this mod will allow the UHF digital display to traverse
from 400.000 to 499.995. The radio will not, however, tune this entire
range due to a combination of a limited synthesizer lock range and the
input RF amplifier bandwidth. The apparent useful tuning range is
about 420-460, with sensitivity dropping off sharply at either end.
Naturally, the best performance is in the middle (the Ham Band :-).
Similarly, the VHF tuning range is extended in the same manner. After
the mod is made, the VHF side will apparently tune from between 100-199
mhz but as stated above, there is little or no response outside of the
published range (138-170 mhz), due to the same reasons as stated
As a final side effect, note that performing this mod will remove all
out-of-band transmit restrictions on the rig, for both VHF and UHF.
After the mod is in place, the radio will transmit on any frequency
that it can tune to.
The author of this report wishes to strongly remind those who perform
this mod that it is ILLEGAL to transmit on any frequencies outside of
the ham bands, EVEN IF YOU ARE OTHERWISE LICENSED TO DO SO. This is
because of the radio's lack of FCC type acceptance for these
frequencies. So beware! Unauthorized use of this feature could be
hazardous to your ticket. MARS/CAP users are probably OK with a permit.
Now for the nuts and bolts.....
Turn the radio on and write down all of your memory channel
frequency assignments, PL codes and anything else you have in memory -
It will all have to be reprogrammed after the mod. After writing down
all of your data, turn off the unit and disconnect it from the antenna
and power supply.
Next, remove the bottom cover on the unit. Note: this radio
contains a number of CMOS parts which could be damaged by static
discharge. Take all of the regular precautions to make sure that you
and your tools are properly grounded for anti-static work. Note that
there is a hole in the rear of the subchassis which supports the
controller circuit board in which you can see a small, black, 1/4 watt
resistor which is labeled on the board as R57. Using a small pair of
sharp cutters, clip the lead on the end of the resistor. It is not
necessary to remove the part, just clip one end and bend it slightly
out of the way.
Next, remove the top cover and then open the front panel assembly
into the service position. To do this, remove the four silver screws,
one on the top, one on the bottom, and one from each side. Two of the
screws are in slotted holes. Loosen the screws in the slotted holes
slightly so as to allow the front panel to be pulled out and swung down
in a hinge-like fashion. Once open, set the radio on a table with the
hinged front panel hanging over the edge of your workbench.
Familiarize yourself with the inside of this compartment. The most
notable features are a silver, button type lithium battery to the left
of center and a large, multi-pin microprocessor chip on the right. DO
NOT DISCONNECT ANY OF THE RIBBON CABLES.
Locate the lithium battery and to it's left you will notice a row of
five programming resistor positions, with the middle position vacant.
The arrangement looks something like this:
| ____ ______
| I I:II (Bat-) |MPU |
| (tery) | |
| ---- ------
| I <- R121 (remove for cross-band repeater operation)
VVV front of rig VVV
The (:) above indicates where a jumper is to be placed. Use care in
soldering since everything is quite small.
Reassemble the unit in the reverse order as described above. When
you power the rig up, the display should show 440.000 and 144.000 on
the displays. It is now ready for operation. If necessary, perform
the microprocessor reset function by holding down the F button while
turning on the power. Reprogram the unit with the data you saved in
There is nothing special about operating the rig once the modification
is made. All functions operate exactly as before except that the range
on each band is extended as described above. You will probably want to
program the band scan limiting channels (A and B) on both bands since
otherwise your vfo scanning will be too broad and will spend a lot of
time scanning in areas which it cannot receive.
In addition, my rig also has the cross-band repeater mod. I do not
know if having this is a prerequisite to performing the mod mentioned
above. If it is, then R121 will also have to be removed. The cross-
band repeater mod has already been discussed on the net so I won't go
into it at this time.
Enjoy this mod but don't endanger your license! Don't transmit out of
band and don't allow others to, even if they're licensed for those
TM-721A Low/No VHF Transmit Power
Some early model TM-721A transceivers may develop low or no output power on
the VHF band. This may be caused by coil L6 or L9 on the final unit. The
coils should be inspected for burn marks on their insulation. If any flaw
is detected in the insulation, replace the coild with a wider spaced coil.
Coil, 9.5T L6,L9 Part Number L34-1238-05
This modification may be performed under warranty.
Disconnect the power supply and antenna from the transceiver.
Remove the top and bottom covers (12 screws). Do not damage the
speaker wires when removing the top cover.
Remove the VHF Final unit shield (4 screws) from the top of the TM-
Unsolder the Final module (Q1), antenna coax, and terminal pin W4 from
the Final board.
Unplug the wires at connectors CN2, CN3, and CN4 on the VHF TX-RX unit
(top side of the transceiver.)
Unplug the wires at connector CN10 on the UHF TX-RX unit (bottom side
of the transceiver.)
Remove the 7 screws from the VHF Final unit. Remove the Final unit
fromt he transceiver.
Unsolder an remove the coil (L6 or L9). Install and solder the new
Assemble the transceiver by reversing steps 1 - 7.
Time required for this change is 1 hour or less.
TM-721A for 9600 baud
The mods to do 9600 baud packet using a K9NG or G3RUH modem are fairly simple
to do with the Kenwood TM-721/621 radios. Look at your schematic.
Receive data is taken from the dicriminator output. Look for the 3.3K resistor comming off pin 9 of the last IF stage. Connect a 1K series resistor from the modem RX input to the junction of the .001 and 3.3K.
PTT is taken off pin 2 on the microphone connector. TX data is fed through a 1K series resistor to the top of VR3 (the deviation conltrol).
This method has been tested and works quite well on 440 MHz and 220 MHz. The mods haven't fully been tested on the 2 meter portion of the radio.
The TM-731 radio is different enough to cause problems. What needs to be done is to add another RX Mute transistor. (Look at the schematic and see how the TX deviation is shorted to ground during receive.) Use a plain NPN and a pair of 10K resistors (copy the one used on VR3 in the schematic). Take the TX data and feed it to the collector through a series 1K resistor, and then feed that through another 1K resistor to the wiper of the deviation pot. This was a
result of Kenwood changing the deviation pot value from 10K to 100K. (It wasn't broken, but they fixed it anyway.)
As with any modifications, take reasonable care, and I won't be responsible if you trash your radio. BE CAREFUL; it is full of surface mount
This MOD file was written by Jeff, WA6FWI @ WB6YMH.#SOCA.CA.USA.NA Schematics and layouts are available by request through the author.
TM-721 cross-band repeater (another)
Chip resistor R121 on the Control Unit must be removed. Here is how.
- Disconnect po and antenna
- Remove top and bottom covers 2 screws)
- Remove the silver coloured screws from the front panel chassis.
(2 on each side, 1 on p, 1 on bottom)
- Pulfront panel away from the dio but DO NOT disconnect cables.
- Locate chip resistor R121 on the Control Unit and desolder. When looking at the Control U, you will see the lithiuattery located to left of centre and the power switch sub-assembly located on the bottom right. Look at the centre of lithium battery a you will see R121 locatedown and to the left of the battery close to the bottom edge of the board about 3/4" left of centre of the battery. Remove ts chip resistor.
- re is also a 3 minute timut timer that will place the radio into the receive mode after 3 minutes of continuous transmit. I removed mine and I willescribe the location of R1 which defeats the timer. If you look again at the lithium battery, look at the bottom edge of e battery and then over tohe right about 1 inch from the bottom centre of the battery and you will come across an upper set of 2 resistors and below that another set of 5 resistors (chip type). Remove the first resistor (left one) in the upper set.
Repeater cross-band operation allows the 721 to receive on one band and retransmit the signal on the other band. An incoming signal will automatically be swithched to the subband. The 721 will then re transmit the signal from the main band. Each band may contain offset and subaudible tone information with the tsu-6 tone board.
The encode and decode tones for a single band must be the same. Each band may contain a separate sub-audible tone frequency.
ENABLING CROSS BAND REPEATER
- Select the first frequency in the main band and select a repeater offset if desired.
- Select TONE if needed.
- Press the BAND key to transfer contents of MAIN into SUB.
- Select the second frequency and if desired a CTCSS tone.
- Adjust the main and sub band squelches to the threshold.
The repeater will always transmit out of the main band. I will give you an example of accessing a 70cm repeater with a 2m handheld on a simplex freq.
- Press the F (orange function) key
- Press the A.B.C. key within 5 seconds of step 1 Three dots will light in the decimal place locations in the MAIN band.
Desired repeater RX 444.8 TX 449.8 2m 145.54
In main band enter 145.54 and press BAND key to transfer to sub display. Enter 444.8 with + offset. Press F + A.B.C.
Key up simplex handheld and you will see the sub-band receive your frequency and at the same time you will see the main band key up and transmit on 449.8 Mhz.
Release the simplex PTT on the handheld and the radio will quickly flip 145.54 into the MAIN band and 444.8 into the sub-band. The 70cm repeater will be transmitting to your radio in the sub-band at 444.8 and the 721 will be transmitting out of the MAIN band on 154.54.
Neat stuff !!!! Warning, disconnect your mic or be quiet and DOUBLE WARNING DO NOT PUT A 2M repeater in the MAIN and a 70CM in the sub or they will continually key up each other back and forththrough the 721. Also notthat when the 721 is transtting on the MAIN band ouo your 2M simplex radio and receiving a signal from the 70cm repeater, you will not be able to gain
control of the transmitter of your 721 until the 70 cm tail drops as the radio does not sample the incoming audio for a carrier while transmitting.
The mod is neat and if you disable the tail timer on your 70 cm repeater, this works great in situations where a hand held ( 1 watt) cannot access the 2M repeater.
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