Modifications for the Kenwood TS-940
TS-940S LCD Clock Display Erratic Operation
We have received a few reports concerning erratic operation of teh LCD
clock display when the Main dial is rotated, or the VBT or SLOPE TUNE
controls are operated. This appears as if you had pressed the SET, SCROLL,
or GRAPH switches.
This erratic operation may be caused by improper timing of the input pulses
applied to the auxiliary display microprocessor, IC1.
This symptom is easily corrected by replacing IC2 MBM2764-30T1 with
MBM2764-30T2, on Digital Unit A. Early models of the improved IC will
carry the old nomenclature but will be distinguished by a white dot on the
top of the chip.
After the IC has been changed reset the main microprocessor by
pressing and holding the A=B key, and then turning the power switch OFF and
then back ON. Then release the A=B key.
After this change is made the sub-display will not immediately track
the main display. A slight delay has been introduced int he new IC to
prevent any timing errors. It may take approximately 0.5 seconds for the
LCD to track the main display.
Please return the old IC2 to the Service Department along with the WSR
Perform this modification only if the symptom occurs, as a repair. This
change is not required otherwise.
Time required for this modification is 1 hour or less.
TS-940S SSB Talk Power Improvement
TS-940S's in the serial number range of 601XXXX through 708XXXX may be
limited n SSB peak power as compared to full CW output. The level may be
increased by changing the value of capacitors C182 and C183 in the ALC
circuit on the Control board. The recommended replacement value for each
capacitor is 0.0022 uF, 50v (CK45B1H222K).
Disconnect the power cord and antenna from the transceiver.
Remove the top and bottom covers (16 screws).
Locate the Control board on the bottom of the transceiver.
Locate transistor Q37 on the board (toward the back right hand side.)
The two capacitors are mounted on the foil side of the board underneath
To access the bottom of the board remove the 8 screws that mount the
board and heat sink to the chassis (See figure 1.)
Carefully lift the board/heat sink fomr the right hand side and pivot
it toward the front of the transceiver.
Replace C182 and C183 with 0.0022 uf, 50v capacitors.
Lower the board back into its original position (don't pinch any
wires!) and re-install the 8 mounting screws.
Put the covers back on the TS-940S and secure them with the 16 case
Connect the antenna and power cord then test the transceiver for
This is an optional change and may not be performed under warranty.
Time required for this change is 1 hour or less.
TS-940S erratic display
Intermittent control or display problems that are difficult
to correct with the reset procedure may be caused by a
faulty ROM socket. The socket should be checked to insure
that it makes good electrical contact with each pin of the
ROM. If it is found to be intermittent, remove the socket
and solder the ROM directly to the board. It should be
noted that units between serial numbers 701XXXX to 811XXXX
and from 903XXXX to 909XXXX are not likely to have this
failure. In addition, units with a serial number of 909XXXX
and above do not incorporate a socket.
Removing the socket requires good soldering skills. The ROM
is mounted on the Digital A unit and is designated as IC2.
The board, being double sided, has solder connections on
both component and foil sides. When the socket is removed,
it is very important to insure that each pin is completely
solder free. Having to pry up on the socket means that it
is not completely desoldered and will cause the circuit
foils to tear.
When performing any work on the Digital A board, CMOS
handling techniques must be observed. Such techniques
include using a grounded or isolated soldering tip, avoid
touching the pins of IC chips with your fingers, and ground
yourself with a wrist ground strap.
To remove the socket:
Disconnect the power cord and antenna coax.
Remove the top and bottom covers from the transceiver.
Remove the 2 flat head screws from each side of the
front panel chassis.
Loosen the round head screw on each side of the front
Carefully rotate the front panel forward. It will be
necessary to unplug the VS-1 cable from the transceiver.
Remove the 4 screws from the speaker mount.
Carefully pull up on the mount and rotate it toward the
front panel. Swing the mount toward the right side of
the transceiver and allow it to rest on the Digital B
Remove the 8 screws from the Digital A unit shield
Lift the plate and rotate it to the left side of the
While avoiding contact with the pins, remove the ROM
(IC2) and set it aside on anti-static foam.
Remove the 6 screws that mount the Digital A board.
Rotate the board toward the front panel to expose the
bottom side of the board.
Carefully desolder the socket and remove it from the
board. Do not pry up on the socket. If it does not
easily pull off the board, the top foils are still
soldered to the socket.
Install the ROM in the board and solder it in place.
Assemble the transceiver by reversing steps 1 - 11. Do
not pinch the power switch cables between the front
panel and the body of the transceiver.
Time required for this modification is 1.5 hrs or less.
On Kenwood TS-904's that do not have the optional Voice Synthesizer
module VS-1 installed, you may notice a hum in the RX audio. The
already present molex connectored cable for this module should be
dressed to the top of the shield case where the voice synthesizer
mounts. However, on some units they have been left hanging free
in the vicinity of the PLL and digital A units and are quite good
at picking up noise from these circuits.
The Fix, is simply to secure this cable to the top of the shield
case where the VS-1 would mount. Plastic tie straps, or even
simple electrical tape works very nicely.
Amtor switching time
Most stock TS-940S' require 30 to 50 mS to switch back from TX to
RX. This can be reduced to less than 20 mS by cutting the center
lead of Transistor Q68 on the Control Unit PCB.
After the modification, a slight "click" will be heard in the speaker
during switching. However, this is no big deal to the AMTOR nut
because of the improved switching time.
ALC time constant
This Mod will change the TS-940 time constant from approximately 1
second to .022 second. This means that the ALC will no longer
impose its own characteristics on your audio response; the ALC will
now follow your own sylabbic rate and emphasis. Usually the average
output will increase, which will drive a linear amplifier harder
(or at least those meters will swing higher!!). Use caution in this
mod and have another Ham-friend check around your TX frequency for
splattering while you are QRV.
The TS-940 has a 10uF cap (C31) and 100K resistor (R104) which make
up the time constant for the ALC. This tends to reduce the output
power for the duration of the ALC time constant (or till the circuit
charges up again). Then it starts all over again on the next word.
This procedure allows modification to the control PCB (X53-1420-11)
in the TS-940 WITHOUT having to remove the board. Remove the bottom
cover and locate the control PCB. Locate R137 and R104. These are
located in the upper right hand corner as the rig faces you upside
down (near VR-3). A service manual is helpfull for locating parts!!!
Using an Exacto knife, VERY CARFULLY scrape off some insulation
from the top of these two resistors. Now tack-solder a 1/8 watt
2.2k ohm resistor from the top of one of these resistors to the top
of the other. The time constant is now changed. Reassemble and
enjoy a superb rig.
TS-940 Mod for MARS and all band operations
Locate IC number 109.
Now find diode 130 and cut it for all-band transmit.
If you want just MARS coverage, locate IC 111 and 112, and snip diode 135 beside it.
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