Repairing a Kenwood TS-940S
Manufactured in 1986 this transceiver was the top of the
line Kenwood radio until it was usurped by the TS950DX. This is a great radio
with receiver specs comparing well with the top 5 receivers made today.
Step 1; Identify all of the apparent problems with the Transceiver.
People have told me that this radio has experianced a few
problems over the years: Intermittent Transmit on all bands "
Intermittent receive on all bands; Internal antenna tuner not
working, LCD sub display not illuminated, memories not working
If you have recently purchased a second TS940S you will need to fully test the operation of the
radio, using all modes, all bands and all functions available
on the transceiver. Document all findings.
Step 2; RESEARCH the known problems
with the radio.
The internet is a great source of information for trouble
shooting many older radio transceivers. The TS-940S was happily not an exception to the rule; there is a lot of sites
returned when I did some basic searches using Google; (i.e. TS-940S
improvements, modifications, problems, circuits, etc.). Go to each
site in turn and print the details from the best sites.
These print outs went into a trouble-shooting file I created for this radio.
During my search I also located a complete copy of the Kenwood service
manual 28MB and the Kenwood service bulletins for the rig. These were downloaded and saved on a CD
for later review and use.
I also researched eham.com and reviewed the forum reviews
posted for Kenwood TS-940S radios. There were a significant number of postings
with lots of great comments regarding the performance of the transceiver, notes
regarding the known problems, and suggested fixes for the problems.
Step 3; Compare the known problems
identified in the Research phase with the problems confirmed during INSPECTION.
It was apparent to me that three of the published problems
matched problems that I identified during my inspection: Intermittent
operations (transmit, receive, and display) may be caused by poor plug
connections or bad solder joints. Sub-display may be caused by burned
out lights and poor connections.
Erratic operation of antenna tuner, transmit, receiving, and display functions may be
caused by bad sockets for main BIOS socket in controller board.
Step 4: Fixing the problems!
Using the manual as a guide; I took the top and bottom
covers off of the rig. This tranceiver was clean inside (no dust or critters;
sometimes a problem in some of the boat anchors I have restored). The prior
owner was not a smoker and took great care of the radio.
Using a finger as a probe, I wiggled every connector and
plug body that I could see on boards visible from the top and bottoms of the
radio. I plugged in the power cord and an antenna and turned on the rig. No
change. All of the same problems still existed. This was not a total surprise;
the prior owner used the same technique, and the prior owner before him. I
envisioned lots of people have been wiggling these connectors and lugs for
years; potentially the cause of some of the issues with the transceiver.
Next I detached and then re-attached each of the connectors
mounted to the transceiver printed circuit boards. Systematically I went over
each of the boards carefully; unplugging the connector, inspecting and then
reconnecting each one. This process went routinely until I got to the main
control board. On this board the fourth connector checked pulled completely out
of the board the male portion of the connector
completely separated from the board) leaving two very clean holes in the board.
I make a note of the faulty connector and continued checking plug connections.
The very next plug checked also pulled out of the board. My inspection of the
rest of the connectors did not yield any more problems quite so obvious.
I removed the board, inspecting the faulty connection
points, and re-soldered the plug bodies back into the board. This was quite a
process; First I had to remove all transistor heat sink connections from the
aluminum heat sink shield that surrounded the board, then determined how many
connectors I had to disconnect in order to fold the board out of the way
(allowing access to the bottom of the board) to rework the solder connections.
This did not take a lot of time; but before removing the connectors I sketched
a simple schematic and labeled it and the plug connectors. This enabled the
return of the connectors into the original configuration without doing a lot of
schematic wire tracing. I removed all remaining old solder from the original
plug bodies and solder connections, then re-inserted
the male plug bodies into the pcb board. I re-soldered these parts back into
the boards and while the board was accessible to the solder iron; I used a
jeweler's loupe and carefully inspected the solder points all over the board. I
pay special attention to the plug body pins for the numerous connectors on the
board. This process pays off big results! I find at least 8 other connectors on
this same board that are obvious cold solder connections. The pins were
obviously floating in the old solder and moved visibly when touched.
This discovery was very encouraging, an obvious root cause of some of
the intermittent issues this rig has had in the past. I suspect that the loose
plugs and many of the cold solder joints were actually caused by the WIGGLE and
Plug/Unplug technique so heavily endorsed in earlier internet comments and
reports. The first time it probably had good results, over time this technique
actually increased the amount of transceiver issues.
I reheated the solder on the connector pads that are bad,
discovering that the old solder would not stick to the plug body pins. I used a
solder vacuum and solder wick to carefully remove the old solder from each of
the old pins that I know and even suspect are bad. This process is repeated for
any solder point that is suspect on the rest of the components on the board. As
you can imagine; this process takes some time. When I completed the control
board, it was re-installed into the rig, and the transistor heat sinks and
disconnected plug bodies were re-installed.
After completing the process noted above; I repeated the
process for each of the other remaining boards on the rig. There are 5 other
main pcb boards on the rig, not counting the little
specialized boards located on the back of the main panel. I went through each pcb with the same process; finding and correcting more bad
or suspect solder connections. In summary total; I corrected 2 completely
disconnected plug bodies, 12-14 visually obvious cold-solder connections and
another 30 or so suspected bad connections on various plugs and components.
Another operation was the inspection of the main eprom on the digi 1 board. This board controls most of the
front display functions of the rig and was mentioned on the internet as having
known problems involving the solder connections. My
rig was supposed to have a high enough serial number where this should not have
been a problem; but with so much work already completed; I felt that it was
prudent to remove and inspect the board for bad solder connections, and
especially to touch up the solder connections to this important chip.
As a final solder step, I removed the LCD sub-display unit,
removed the burned out mini bulbs and replaced them with some lower voltage
bulbs purchased from Radio Shack. These newer bulbs should last a very long
Step 5: Re-Assembly and Test
As I completed the process on each pcb
board. They were all re-mounted into the rig and reconnected to the connectors leading
to the other boards. As the last board was completed I carefully re-inspected
the entire transceiver for any connectors that were not plugged in, and made
sure that all screws had been tightened properly. While the fan assembly was
accessible; I used the opportunity to put a drop of oil on the fan shaft and
allowed it to flow back into the fan motor (this required picking the unit up;
face down). The top and bottom covers of the unit were re-installed, and the
rig was reconnected to the antenna, keyer, paddle, and power line cord.
I powered the unit up and to my great relief (never can account for Murphy completely) the rig came back
to life. Thorough testing of the rig confirmed that it was now receiving and
transmitting at full power and all ranges in between 0 and full power on all
bands. The antenna tuner now works as it should, and I confirmed its operation on
every band and at all power levels. The sub display now lights properly, and
all of the functions available to the sub display now work properly.
Summary and Recommendations
In my opinion; the main problem with most of the Kenwood
radios built during the late 80's involved poor solder technique and possible
sub-standard solder. A thorough re-work of the most accessible boards (boards
most suspect to prior owner tampering, poking, wiggling, and plug/unplug) may
yield the greatest results.
This is a super rig! It has impressive sensitivity and good
selectivity. Published QST test results compare well with the top rigs of the
last ten years. The TS940S compared well in side by side tests with my TS990S.
The addition of cascading 500hz Kenwood CW filters has further improved this radio,
and I have really been giving it a work-out on the bands.
The Kenwood variable band pass on cw works so well that
the casual cw operator may never need filters ( it
narrows the band pass to 600hz without any filters) , with the filters
installed the variable band pass reduces to around 160hz.
I suspect that many of these rigs are starting to show up more because of the
intermittent issues that are starting to occur as these rigs age. The cure for
these intermittent issues is the comprehensive and careful examination of the
multitude of plug connectors and their solder points used heavily through-out
the rigs. This requires modest electronic skills ( soldering,
and careful attention to detail) and can be completed by the most novice of ham
operators. A little small amount of effort will deliver an outstanding
bargain on a transceiver with very good performance specs.
>-Send me an email at: firstname.lastname@example.org with any comments.
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