Repairing a Kenwood TS-940S

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Home : Amateur Radio Help

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 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 time.

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.

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