TS-940S - Repair

2017 - 1 - 30
Document by David Lowrance AL5X

I was gifted an older Kenwood TS-940S by an Elmer. He said it was one of the best Ham Radios he has owned. One morning he turned it on and it was just dead.
First suspect is always the power supply, in this case.

Cover Off

DOA - Dead on arrival - Cover Removed
Transporting myself back in time, to the era when you fixed the circuit rather then simply changing out the broken board, you either enjoy the challenge of comprehension, or you run away and hide in depression, because looking under the hood reveals a system that appears so complex that no one could really keep it all straight.

A little research, a circuit diagram, and it became obvious that in the early models of this radio the power supplies had several design flaws. Many Ham radios traditionally had a separate power supply located outside the units. This one has one squeezed into it, and appears they were not very good at it.
Focus was quickly turned to rebuilding the AVR board and replacing opened transistors on the rear heat sync.

Lots of ideas and mods were found online for this Kenwood AVR board. Unfortunately I did not keep track of where they all came from, but I do wish to thank all the previous Hams who spent time designing them over the past couple decades. I have also added some of my own here that you will not find anywhere else, primarily moving the DC protection fuse location of F1.

Top View

AVR Power Regulator Board and Wiring Sockets

AVR Before Touching

A good idea to photograph all the plugs and wire colors for reassembly later. This Radio has high density packed components with little extra space anywhere. 4 screws removed with a magnetic Phillips, a whole lot of connectors that will pop right off, and the board lifts right out. Looks like it was designed with the ham in mind.

Initial Testing and Observations

The power regulator [AVR board] showed over voltages everywhere, 46.5 vdc on the 42.3 v circuit location, 35.86 vdc on a 30 volt location, 18.6 vdc at an 11.8v location, shown below in green. C12 a 16 vdc capacitor with 18.6 vdc across it! Tiny .5 watt zener's to regulate voltages, apparently sized too small and blown opened or shorted all through this board. It is my opinion that Engineering a circuit to stress components over the max of their ratings is just plain bad design. This will be quite a project since many of these transistors are no longer available new.

A power supply that starts with 46.5 vdc then provides an output of 28.5 regulated is burning up 18 vdc as heat or wasted power at something less then 15 amps. That is about a 39 percent waste of total energy. This is how it was done back in the 80's. Large heat sink with a cooling fan! Kenwood draftsmen also got the diagram wrong for a bridge rectifier, see D101 below.

Circuit Diagram - Clipped to show the Power Supply Section

Circuit Diagram

AVR Board Troubleshooting:

All testing was done with only the Supply voltage plugs connected and all load plugs removed. Socket 5 pin 1 to pin 3 was solder jumpered on the board to activate the 5 volt supply with plug 5 removed, shown above in blue. I left this jumper on as I have no interest in remote control of the radio and want the supply to maintain stable voltages if this plug is ever removed. Zener D9 had to be changed out as it was opened killing the 5v power as well. [1N4739A 1W 9.1V Zener Diode - Mouser]
I ordered a complete set of Zener diodes for all of them on this board of the 1 Watt variety. See references at the bottom.

28.5 vdc Adjustment
The 28.5 also goes through the power up circuitry [Q6] from the power on switch [connector 3] for the lower voltage supplies, so it must be working first.
I began with only plug 12 , 28A, FG, SG and CG connected. See the photo below. Be sure to lift the FB lead off to protect the finals until the 28.5 vdc is perfectly set to 28.5 vdc per the user manual maintenance section and is stable also on power down. When I got the 28.5 volt DC up and adjusted, I then started adding the other AC inputs, plugs 1,2, 6,7 AC3,4 to get the lower voltages up next. Each time I added a circuit it revealed a new set of burned out zener's and or transistors. Obviously the 28 volt supply had spiked upwards and stressed components all over the board without blowing the 15A fuse. It was then also noted that if the 15A fuse does blow it still does not remove the 46 vdc from the regulator circuit board, and continues to then stress Q1 to the point of burning opened if zener diode D3 shorts out, as D3 is far too small a power rating. D3 then shorts out rather then regulating correctly, causing Q1 to burn opened. Q1 is not heat synced either. As my line voltage is 120 vac and matches the print, the transformers are all putting out slightly higher voltages then the print indicates for the regulator input design.

Zener Diode D3
This seems to be ground zero for the destruction of the power supplies semiconductors! When power is shut down, Q1 and Q2 cause the voltage to surge upwards due to the distributed capacitors in the regulator circuit at 46.5 vdc. To cure this they apparently added a 32 volt zener D3 that then fires and limits the surge to somewhere around 31 volts.
The purpose of zener diode D3 is voltage limiting via bias current bypass. It is not a fuse or a fusistor as some indicated was the design, that is merely what "sometimes" happens. It is a voltage regulating device, placed there to limit the bias on the output transistors, so they theoretically cannot go over about 31.5 vdc on the emitter or output side of the 2N2885's. The 32 volt zener should begin to conduct when the emitter voltage of the 2N5885's is about 31 volts due to the resistor in series dropping about .5v. This should also happen if you adjust VR1 to set a voltage higher then 30.5 - 31.5vdc on the power supply. It just so happens that D3 instead often chooses to short out, "saving the life of the finals" but destroying every other semiconductor in the power supply!
In rigs where the diode goes opened the finals may also be destroyed as the power down surge can then go much higher. The finals are rated at 30 vdc and feed from connector wire FB.

Photo of D3 Location

Resistor Mod Regulator Circuit
By placing a larger zener in the circuit [1 watt], we can still get the desired effect, and it is repeatable more times without destruction of the diode. However the two new resistors that were added 22k and 4.7k are to change the bias response at the control point on Q1 and Q2 to operate more closely to where they should have been and totally avoid the voltage surge to over 30.5 vdc on power shut down, which caused the zener to get a real workout and finally fail. This is a Kenwood mod, although they do not mention a larger zener diode which I also recommend.

Move 15A Fuse, Add Crowbar Circuit
With the addition of the "crowbar circuit" and moving the 15A fuse location, it may also shut off the power supply completely when you hit about 30.2 vdc threshold and you will be changing out a fuse instead of finals or D3, Q1,Q2,Q101,Q102 and probably the rest of the semiconductors on the heat sync for the lower voltage supplies. Take your time and ensure the 28.5 voltage is both stable and will not spike on power down, before you move on to reconnect the FB wire that powers the finals, or to work on the lower voltage supplies. A scope on spade tip FB is the best way to observe this but it is slow enough to see on a VM to.

AVR Board - Before Mods

AVR Pre Mod Photo

Heat Sync

The heat sync is unscrewed then lowered towards the back of the unit.
Replaced Q101- Q102 on the main heat sync panel with higher rating transistors as they were both opened, Q101-102  2N5885 60v  replace with 2N5886 80v [ Mouser ]
While the transistors are being replaced do the fuse relocation mod found below.

Heat Sync Dropped Photo

I ended up changing the other three semiconductors on this heat sync as well, shown on the left. I marked the cable plugs Outer, Middle, and Inner [Out - M - In] so all three could be removed at once. Left to right Q103, Q104, IC101.  2SD1266(P,Q) Ebay , 2SD1406(Y) Ebay, uPC7805(H)  Mouser.

Fuse Relocation Mod

AVR - Fuse

Move the 15A DC Protection Fuse off the AVR board to the location ahead of the power transistors for total voltage cut off when the fuse blows. This shown in blue above. A new in line fuse block is placed in the orange wire between the filter caps and the heat sync holding the two main power regulator transistors [2N5885(6)]. Also see photos below. It is a major design error not to protect the entire circuit with a main DC power fuse and since we expect approximately the same current on both sides of the output transistors this is a reasonable modification. This will then kill the 46.5 vdc to the regulator board as well, where we have regulating components often frying on power down. The output transistors, in emitter follower configuration, can easily handle 15 amps, but the emitter to base voltage difference cannot handle 32 volts! If there is a short on the power we need to kill both the collector and base to emitter voltages on the output transistors. It is like this circuit was designed to destroy all the transistors on failure, and it did!

New Fuse

This mod can be done most easily when Q101, and Q102 are being changed out on the rear heat sync because the Orange wire will be removed at that time. Fuse holder must be rated at or above 15Amps.

AVR Repair and Crowbar Protection

Regulator circuits

28 vdc Regulator Circuit
Replaced Q1, [2SB861 Ebay] also added heat sync,  and replace Q2, [2SC2458(Y) Ebay.]
Replace the .5 watt shorted D3 with 32v 1 watt zener diode [Mouser 771-BZX79-C30133], add resistor 22k across it.
Place 4.7k resistor across upper half of VR1 to limit the over-voltage on adjustment setting of VR1. This is a factory mod for the early issue radios. Zener diode should conduct for over-voltage protection and not fry or short in the process!
Added a crowbar circuit to blow the 15A fuse on 30 vdc over-voltage condition. BTB12 triac and a couple of 1 watt zener's that add up to 30 vdc. Shown on the right above, and second photo below mounted on the AVR board where the 15A Fuse used to be. This circuit was bench tested before adding it to the AVR board to ensure it could repeatedly blow 15A fuses if the voltage went above about 30.2 vdc. The purpose of this circuit is an absolute protection for the finals in the RF amplifier section rated at 30 vdc. The new output transistors can easily handle 15 amps without destruction so I do not expect them to fry opened again.

AVR Board Mods

Foil Side View
AVR Mods Foil Side Photo

Component Side Views

AVR Mods Compnent Side

I removed D3 from the top of the board and made a wire loop for a test point, mounting the new larger D3 zener underneath avoiding having to drill out the holes to a larger lead size for the new 32v 1 watt zener.

The crowbar circuit is now able to use two of the holes present from removing the 15A fuse F1 and only a new ground pin hole must be drilled through the board.
D9's holes had to be drilled to accept the larger 1 watt zener. This required a very tiny drill bit.

Capacitor C12 6800 uF 16vdc was changed out to a 5600 uF 25v capacitor [Mouser]
This was a Kenwood recommended mod. 


When troubleshooting the board, I ordered extra transistors for Q1 and Q2 as it is the main voltage control for the 28.5 vdc. I almost expected to blow some of them up during the process, so I decided to mount some make shift sockets under them, by cutting up some IC sockets which worked well. Turns out I did not need them, but decided to leave them in. It made scoping the pins a little easier too.

Q1[2SB861] opened, Q2[2SC2458(Y)] bad, replaced. Transistors were ordered from Ebay, as parts are out of production now. Q1 got a small heat sync from the junk box as well.

Side By Side Comparison

Side By Side Comparison Photo

Fan Mod

Fan Mod Circuit

Fan mod, replaces two resistors so the cooling fan will come on at lower temperatures. This was a Kenwood recommended mod.

Q6 Surge Protection Mod

Q6 Protection

A 47 ohm resistor is placed in series with the input supply voltage to transistor Q6. This mod was recommended by a ham online to shield Q6 from transients during power up and power down.

Q6 mod

As a final note, many have found the solder joints to be less then adequate on this particular Kenwood radio, with a high number of cold solder joints. If you are up to it and have good solder techniques, redo all of them when the board is out. All solder joints should shine nicely and not have a crystalline or opaque appearance.

Power Up

All voltages now reading correct, I put the load side connectors into the AVR board and turned on the radio. Power up revealed the green LCD display battery was dead. CR2032 with solder tabs on opposite sides. Clock memory cleared on each power down, then gave strange responses on each power up. The front panel is dropped, with the Radio setting on the edge of the bench, to access the 2 batteries soldered just behind the green LCD panel. The speech processor has a cable that must be disconnected to lower the panel, seen hanging over the front.

Replace Battery Photo

This radio has 3 of these, two on the front panel and one somewhere down in the guts of the inner boards that is apparently still working.

3v lithium photo

Amazon has these 3V Lithium Coin Batteries with solder tabs. They are used in something called Video Game Museum. For now I had some CR2032's and stacked them between pennies. You can solder tabs to the copper pennies then tape them together tightly with electrical tape and get it to work. One of those MacGyver things you do, while thinking you will fix it later, then never do because it works, then you forget about it.

Locate the reset switch before you close up the front panel.
It is on the back right side of the board just behind and to the right of the batteries you just changed out.
Put the front up but leave the covers off. Do a microprocessor initialize on the radio. Turn on the power with the A=B switch pressed. If the display fails press the reset switch.

Battery Stack Photo

Final  Tests
Unit was powered up and loaded into a 50 ohm dummy load at 100 watts output. The fan came on just fine after transmitting for a time. Function generator was also used to verify receiving on several bands. The transceiver appears to be fully functional.

Ts-940S-Power UJp Photo

Station Setup
An original Kenwood MC-60A microphone was purchased from DX Engineering, whom just happened to have brand new ones still in stock!
The MFJ keyer to the right is about 25 years old and still working with the nice dual paddle.
An old stereo speaker modified with a 3.5 mm audio plug, and we have a nice sounding HF station.

TS-940S Station Setup Photo


TS-940S Operations Manual.pdf

TS-940S Service Manual.pdf

TS-940S Circuit Diagram.jpg

TS-940S PWBA.jpg

Parts and Suppliers
[Power supply rebuild project - AVR PC board and Heat Sink power semiconductors]

Replace components [some bad]
C12    16v 6800 uF - Change out to a 5600 uF at 25v per release notes, this cap has 18 v on it. [Mouser] Done
D3    32v Zenner      BZ-320         shorted              [Mouser 771-BZX79-C30133 BZX79-C30,133] Done
D8     Zenner                MTZ18JC or MTZ20JZ            [Mouser MTZJ18C] not done, good
D9     Zener                 MTZ9.1JA  [1N4739A 1W  9.1V]  [Mouser 2 ea] Done
Q1     PNP Transistor     2SB861 (C)    opened        [Ordered Ebay 2 ea  1/19/2017] Done
Q1      PNP Transistor     2SB861                               [BD708  Better Replacement Ebay] Not Done
Q2,3,4      NPN    2SC2458Y  Q2 done          [BC639   Better Replacement Ebay] not done
22K   1/8w Resistor   (Added Option)                     [Mouser] Done
Q101 x    NPN Power Regulator     2N5885 60v    Replace with 2N5886 80v      [Mouser] done
Q102 x    NPN Power Regulator     2N5885 60v    Replace with 2N5886 80v      [Mouser] done

Critical AVR mods -
28.5v  Over Voltage when power shut down - Crowbar Circuit - Move Fuse Location 15A
 Do mods for 22k and 4.7k voltage regulator on AVR - add resistors [Done]

Fan starts on cooler temp
Do mod for fan 470 ohm and 4.7k - change out resistors 370 and 10k [Done]
Fan works.
Parts ID
Transistors --------------------------------------------
Q2,Q3,Q4x                         2SC2458 (Y)                       [EBAY 5ea]
Q5x                                     2SC1959 (Y)                        [EBAY 1 ea]
Q6x                                     2SC2235 (O)                       [EBAY  20 ea]
Power Regulators  on Heat Sink --------------------
Q101,Q102                     2N5885                              [Mouser 2N5886]
Q103x                                2SD1266 (P,Q)                  EBAY  4 ea
Q104x                                2SD1406 (Y)                      EBAY 1 ea
IC101                                uPC7805 (H)                     Mouser
Diodes -------------------------------------------------
[D4SC6M Dual Rectifier Diode - Replacement for D1,2,10-13 Rectifiers]
D1,2,10~13   [U05B  2.5a 100v diode  NTE5801]
D5,6,7                                1SS133                               [Mouser 5]
Zener Diodes ---------------------------------------
D4 x 12 v        MTZ12JC       [1N4742A 1W 12V][1N5349B 5W 12V] Mouser 2 ea
D8 x 18 v        MTZ18JC       [1N4746A]  Mouser 2 ea 
D14 x 22 v      MTZ22JD      [1N4748A 1W 22V]  Mouser 2 ea
D9 x  9.1 v      MTZ9.1JA     [1N4739A 1W  9.1V]  Mouser 2 ea
Fan Mod   ( increase cooling at lower heat level, and speed up fan)
R11  470 ohm                 Mouser
R17  4.7 K Ohm              Mouser

David Lowrance AL5X

Authors Photo