Kenwood TS-940 Manual
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Kenwood TS-940 Manual

Kenwood TS-940 Manual

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Kenwood TS-940
Troubleshooting Hints and Updated Adjustment Procedures and Mods
Opinions, Notes, Corrections, Tricks and Hints from KB7JS
Revision 8.0.0
Many Thanks to all who provide critique, it helps make this document even better
.
If you are the proud owner of a Kenwood TS-940 you have one of the best performing desktop Amateur Radio
transceivers ever manufactured. But it is not perfect, it is not new, and it has seen many hours of band chasing, so what
do you do when it stops working? When it is time to take the covers off and go on the hunt for problems, you will need
to consult the Kenwood TS-940 Service Manual for guidance. I recommend you tread lightly with a good sense of
humor.
I have worked on these Kenwood models for several years now, and it has been both an enjoyable and painful
experience. I have suffered many frustrations in the process, most of them due to the less than spectacular
documentation.
The Kenwood TS-940 Service Manual Circuit Descriptions and Adjustment procedures have been a source of confusion
and frustration for owners and technicians for many years. The Circuit Description is laced with design errata but lacks
vital details and does not accurately describe how each circuit really works.
The Adjustments section is also seriously flawed. There are over 140 adjustments in the TS-940, but some of them are
not included in the individual Adjustment procedures, some procedures are inadequate to achieve the desired result,
and some necessary adjustment points are not even mentioned in the procedures. In addition, the procedures specify
the use of test equipment and methods generally not available to most owners and technicians. Worst of all, the most
critical problems with the procedures are several serious omissions and technical errors in the procedures. As a result,
attempts by many owners and technicians to use the Service Manual as a guide have rendered many good radios with
diminished performance or have outright failed in the attempt.
Kenwood attempted to improve on the content of the adjustment procedures by publishing three revisions of the stand-
alone document "TS-940S Adjustment Instructions" but they only further exasperated this problem by repeating the
inaccurate procedures while obfuscating or totally deleting some key steps.
Over the past few years, I have cluttered my shop with notes written in the margins, post-it's, and printed pages filed
away in the folds of many copies of Service Manuals, forever lost in the chaos of my shop. So, I decided it was time to
compile the notes and revisions that I have learned work with good results for me in my shop. This document will now
try to formalize these notes.
This document is a work in-progress living document, updated whenever I encounter additional vague or misleading
information while troubleshooting a customer's radio, so I will continue to update it in an on-going attempt to expand
on and clarify the circuit description and adjustment procedures necessary to make a Kenwood TS-940 transceiver
operate as it should. So far, I have not embarked on rewriting the entire TS-940 Service Manual, so the notes contained
Version 8.0.0.1
KB7JS
–
10/20/2024
This just keeps getting better

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  • Page 1 I will continue to update it in an on-going attempt to expand on and clarify the circuit description and adjustment procedures necessary to make a Kenwood TS-940 transceiver operate as it should. So far, I have not embarked on rewriting the entire TS-940 Service Manual, so the notes contained Version 8.0.0.1 KB7JS –...
  • Page 2 Hint – The PLL & VCO adjustment procedures in the Kenwood Service Manual are vague, inaccurate, and typically lead to a seriously broken radio). 2. ALC Adjustment ( Hint – The ALC adjustment procedure in the Kenwood Service Manual has a serious error that will render ALC inoperative 3. S-Meter Adjustment ( Hint –...

  • Page 3: Table Of Contents

    RAM Memory Content corruption ........................10 When to replace the RAM battery ........................10 The TS-940 PLL System - The Ugly Details – A Deep Dive .....................11 The Complicated Details of the PLL Unit ........................11 TS-940 PLL Circuit Description – How it Works......................11 PLL Reference Oscillator Input (OSC ) .........................11...
  • Page 4 Adjusting the PLL and CAR Units ..........................29 Beware of the Kenwood Procedures........................29 Adjusting for Optimum Results ..........................29 Revised Kenwood TS-940 PLL VCO, and CAR Unit Adjustments ..................30 Important Adjustment Notes ...........................30 Detail Description of PLL/VCO Adjustment Steps ......................30 PLL Unit Adjustment Procedures ..........................32 RF VCO Adjustment Procedures..........................33...
  • Page 5 Understanding AGC adjustment procedures ......................39 The S-Meter “SLAM” Problem ..........................41 TS- 940 RX S-Meter Adjustment – Calibrator Method ....................42 TS-940 RX S-Meter Adjustment – Signal Generator Method ..................43 Improved S-Meter Adjustment (By Jeff Gagnon) ....................44 Transmitter Adjustments ............................45 Tx Drive and IF Amp Adjustment Procedure (Tx Adjustment Steps 2 and 5) .............45 Error in the Transmitter Adjustment procedure step 5 ..................45...
  • Page 6 TS-940 Control Unit Test and Adjustment Points ......................61 TS-940 IF Unit Test and Adjustment Points ........................61 KB7JS Mods and Enhancements ............................62 Making the TS-940 “DIM” button work with LEDs in the Panel Meter ................62 Kenwood Service Bulletins ..............................66 SB-900 ..................................66 SB-908 ..................................67...

  • Page 7: My Opinions - What Were They Thinking

    The following rambling discussion is an attempt at providing helpful hints and procedures to correct problems in your TS-940. These radios have been soldiering on now for up to 35 years, and, beyond the well-known problems and failures of the original Kenwood power supply, with only a few exceptions, many TS-940’s have proven to be exceptionally reliable and are still in use today.

  • Page 8: About The Kenwood Ts-940 Service Manual & Adjustment Procedures

    Right off the top, I caution everyone that the TS-940 is a sophisticated and complicated design and a deep dive into RF theory is necessary to really understand how it all works to provide one of the most enjoyable operating experiences in Amateur Radio.

  • Page 9: Troubleshooting Common Ts-940 Problems

    The Main CPU chip is mounted in a socket, and several radios have suffered from intermittent problems due to poor solder flow on the pins of the socket, which need to bond to PC traces on both sides of the board. Kenwood issued a Service Bulletin (SB-951) recommending the removal of the socket and re-soldering of the ROM directly to the board.

  • Page 10: Compromised Wiring Connections

    WARNING: This will delete all stored frequency memories. If this RESET operation restores normal operation, it’s time to change the battery. There are actually two batteries in the TS-940, one on the Digital “A”, the other on the Sub-Display Module (a.k.a. Switch Unit “L”) that serves to retain the 24-hour clock.

  • Page 11: The Ts-940 Pll System - The Ugly Details - A Deep Dive

    VCO oscillator. Note that the precision of the TS-940 digital tuning dial step is 100 Hz (unless the 10Hz option is selected at the Aux Control Panel). If your radio is configured to display 100 Hz resolution, it potentially introduces a possible variance of +/- 50 Hz.

  • Page 12: 20 Mhz Frequency Standard Fstd

    Derived from 1 MHZ via IC4 Obviously, the calibration of the 20 MHz oscillator is critical to the operation of the TS-940. The Service Manual procedure uses a Signal Generator and O-Scope to achieve a heterodyne, then adjusting the oscillator for a stable wave form. While this may work, the percentage of errors hidden in the width of the trace lines on an O-Scope can exceed 5%, so the results are not as accurate as desired.

  • Page 13: Pll-3 (Pll Unit Ic17)

    Wow – that is a long complex chain of frequencies, but wait, there’s more. Note that PLL-2 is also complex in that it operates in two different frequency ranges. The PLL-2 loop controller (IC8) tracks the frequency range 46.2 ~ 51.1 MHz to drive the control voltage of Oscillator Q7, which operates in the frequency range of 35.5 ~ 40.5 MHz.

  • Page 14: Pll-4 (Car Unit Ic2)

    PLL-4 (CAR Unit IC2). PLL4 uses crystal oscillator Q1 and VCO Q4 combine to drive the 45.5 MHz. oscillator Q4. PLL-5 (CAR Unit IC2) PLL5 uses crystal oscillator Q6 and VCO Q9 combine to drive the 71.5 MHz. oscillator Q9. The two oscillators are then pre-scaled and/or mixed to produce the following outputs.

  • Page 15: Troubleshooting Pll Unlock Issues

    Yes, the PLL system in the TS-940 is a complicated design, and the attempt by Kenwood to explain it through mathematical formulae in the Service Manual is, well, interesting, in an academic way, but wholly inadequate for most troubleshooting efforts.

  • Page 16: Identifying The Cause Of Pll Unlock

    Although this problem is almost always associated with the PLL Unit, there are a total of six PLL’s in the TS-940, four of which are on the PLL Unit, and the additional two are located on the CAR Unit. The Unlock condition of all six loops is Or’ed into one signal, “UL”, the common inverse of all Lock Detect outputs, so any one or more of the PLL’s can cause...

  • Page 17: Broken Wires, Cold Solder Joints

    Physical damage to wiring due to intrusions by our collective fat fingers is the most common problem I encounter in Kenwood radios. The very small wires used are prone to breakage at the pin crimp inside the connector. I have even seen cases where the wire has separated from the pin and pulled back inside the insulation, leaving no external indication that the connection is open.

  • Page 18: The Myth That A Tweak Of The Het Oscillator (L34) Fixes It All

    Identifying which PLL is Unlocked PLL Unlock condition can be triggered by one or more individual PLL circuits. There is a total of six PLL’s in the TS-940, four of which are located on the PLL Unit, and two more on the CAR Unit.

  • Page 19: Unlock Signal

    The PLL Unit is now visible. Refer to the following diagram for the location of Connector 5. • Check voltages. Connector 5 Pin 3 should be 15V, and Pin 5 should be +5V. If either of these voltages is out of spec, determine why and correct the problem.

  • Page 20: Ok, What's Next

    reference used by PLL1 and PLL3, and a problem there can cascade to the PLL Unit. If Pin 4 of Connector J5 is low when connector 2 is unplugged, then the Unlock condition is being generated by one of the two PLL loops on the CAR Unit.

  • Page 21: Het Oscillator Pll Unlock (Ic19)

    HET Oscillator PLL Unlock (IC19) A frequent cause of PLL Unlock in the TS-940 is the HET Oscillator, PLL-0(IC19), which resonates at 36.22MHz. You will find many on-line posts that allude to this being the common PLL Unlock source, and that you can adjust this oscillator using a simple voltmeter to measure the PLL control voltage and “tweak”...

  • Page 22: Multiple Failures

    The TS-940 PLL loop system is interdependent, so it is also possible that a failure in one loop may result in more than one PLL becoming unlocked.

  • Page 23: Troubleshooting - Pll-3 (Ic17)

    Yep, it’s complicated, but note that a failure of any one of these reference frequencies will result in an Unlock condition of PLL-2 (IC8). Troubleshooting – PLL-3 (IC17) The Output of PLL-2 is also used by PLL-3. This creates an ugly chain on dependence, so it is a good idea to validate each point in the chain.
  • Page 24 Since PLL-3 is unlocked, this test will probably fail because the control voltage to the VCO is either incorrect or is ineffective. • Check the RF VCO CV reference voltage at J19 Pin 1 on the PLL Unit. This voltage should be @15.6V. If not, there is a problem in the RF VCO section on the RF Unit.

  • Page 25: After The Unlock Problem Is Fixed

    Digital control circuitry. NOTE: Early production units of the TS-940 had the ROM on the Digital “A” in a socket to facilitate ease up microcode update. The ROM socket was poor quality, and corrosion on its pins became a major problem over time.
  • Page 26 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 27: Ts-940 Adjustment Procedure Updates

    Kenwood Manual, then read the amended procedures I have provided here. The Kenwood Procedures also stipulate a rich assortment of test equipment that you may or may not have in your shop, so I have provided commentary in the “Technical Ramblings” section at the end of this document that may help rationalize what equipment you can use in instead to accomplish getting your radio operating as it should.

  • Page 28: Voltage Adjustment And Configuration

    Voltage Adjustment and Configuration. The “VOLTAGE ADJUSTMENT AND CONFIGURATION” section in the TS-940 Service Manual has errors and omissions. Notably, the adjustment for “RB” is at Connector J10 Pin 3 on the IF Unit, not Pin 5. Also, the procedure for setting the AGC Detector Gain bias was omitted from the Service Manual, and the S-Meter calibration procedure specified by Kenwood will erroneously adjust this control and never come back to make it right again.

  • Page 29: Adjusting The Pll And Car Units

    Beware of the Kenwood Procedures The Kenwood TS-940 Service Manual procedure for adjustment of the PLL Unit is seriously flawed and difficult to follow with reasonable outcome. The adjustment procedures in the Service Manual specify incorrect test points and values in several steps.

  • Page 30: Revised Kenwood Ts-940 Pll Vco, And Car Unit Adjustments

    Revised Kenwood TS-940 PLL VCO, and CAR Unit Adjustments Important Adjustment Notes The adjustment procedures below are amended from the original PLL adjustment steps in the Kenwood Service Manual. Note that the sequences of adjustment, test points, result, and pre-set conditions have been changed. This adjustment ensures the proper balance of the band pass filter and requires the use of an RF Sweep Generator.
  • Page 31 35.5 – 40.5 MHz for each 500 kHz on the main tuning dial. This test point is not even mentioned in the Kenwood Service Manual procedures but can be used to monitor the peak output level of PLL-2 when adjusting L7. I recommend adjusting after first adjusting L5-L6 for peak level, then adjust L7 to obtain the maximum level for the output at 40.5 MHz (Dial Frequency 13.999.99 MHz).

  • Page 32: Pll Unit Adjustment Procedures

    10.0000.0 importance of this level of accuracy. The Standard 0 MHz tuning increment of the TS-940 is 100 Hz, and Oscillator any error in this adjustment can result in an offset in tuning accuracy. For better accuracy, use the 10Hz accuracy setting at the Aux Control Panel.

  • Page 33: Rf Vco Adjustment Procedures

    Measurement Adjustment Test Specification/Remarks Item Condition Equipmen Termina Unit Part Method 7 – PLL-3 Display FREQ ADJ For @-30 dBm or greater @ 84.05 MHz : 13.999.99 Output: (C215) Max – (@ 250mV P-P) 83.55– 84.05 Display FREQ : 14.000.00 Check @-30 dBm or greater @ 83.55 MHz Adjust L17 ~ L24 for Max level to ensure...

  • Page 34: Car Unit Adjustment Procedures

    CAR Unit Adjustment Procedures Measurement Adjustment Step Condition Specification/Remarks Test Equipment Unit Terminal Unit Part Method 11 - CAR1 MODE: USB FREQ C ADJ to 453.50KHZ +/-50Hz J18 – Pin 453.50KHz NOTE: These are the “optimum” Adjustment CAR1 set points. Due to minute variance in filter passbands, CAR1 must be refined as described in the “SSB Frequency Response”...

  • Page 35: More Ts-940 Adjustments

    That works only if the entire Receiver IF chain is at least marginally functional. To get this process started, Kenwood chose to make several adjustments (L2, L31-L33, L40, L44) in the very first step of the Rx BPF stage.

  • Page 36: Step 2 - Monolythic Crystal Filter

    signal path after the Attenuator, L40 is a Trap Filter after the broad band Rx Amplifier (Q10), and L44 translates the balanced output of the 1 Rx Mixer (Q9) to a single-ended feed to the next stage. As a result, a potential problem here is that sub-step 1 assumes it is possible to observe change in the audio level at the other end of the Rx IF Chain.

  • Page 37: Step 4: Notch Filter

    In TS-940, the 1st Receive IF is 45.05 MHz. If a signal input from the antenna at that frequency were to pass through to the Receive chain, it would result in a heterodyne with the internal IF frequency. Not good. Step 5 of the Rx...
  • Page 38 The procedure step should specify 80 dBμ, and that is a very big difference. The injected level at 80dBμ equates to 5.0mV (5000 μV), a whopping big signal (S-9 is only 50μV), equivalent to -33 dBm, or S9 + 60. This is a very high level intended to overload the receiver front end and generate IMD.

  • Page 39: Understanding And Adjusting The Ts-940 S-Meter

    The procedure in the Kenwood TS-940 Service Manual for adjusting the S-Meter can be very confusing. Three controls are mentioned in the original Kenwood procedure, two of which share the same designation, VR1, one on the IF Unit, and another VR1 on the RF Unit. Adjusting the wrong one can cause serious problems.
  • Page 40 Before calibrating the S-Meter gain slope, the AGC threshold adjustment is set. The adjustment point for this in the TS- 930 is VR1 on the Signal Unit, in the TS-940, it is VR1 on the RF Unit. The TS-930 procedure is a bit convoluted but worked.

  • Page 41: The S-Meter "Slam" Problem

    This problem is due to a timing delay in the switching of the +15V supply to the AGC Amplifier in the IF Unit. To correct this problem, Kenwood Service Bulletin 919 modifies the circuit to insert a blocking diode and delay capacitor.

  • Page 42: Ts- 940 Rx S-Meter Adjustment - Calibrator Method

    TS- 940 RX S-Meter Adjustment – Calibrator Method The following procedure will produce the most consistent behavior if you do not have sufficient test equipment to generate accurate reference signals. This procedure uses the built-in Calibrator as a relative signal source, but remember it is not calibrated to any standard and will only get you somewhere in the ballpark.

  • Page 43: Ts-940 Rx S-Meter Adjustment - Signal Generator Method

    TS-940 RX S-Meter Adjustment – Signal Generator Method The second procedure will produce the factory accuracy in both AGC and S-Meter performance. Always check the calibration of the AGC Bias voltage. Measurement Adjustment Test Item Condition Specification/Remarks Equipme Termin Unit...

  • Page 44: Improved S-Meter Adjustment (By Jeff Gagnon)

    Improved S-Meter (By Jeff Gagnon) Adjustment Jeff Gagnon provided this very detailed amended procedure to adjust the S-Meter for better accuracy and linearity by adding additional steps that refine the adjustment of the AGC level and S-Meter Amplifier Gain slope to make it more accurate. Version 8.0.0.1 KB7JS –...

  • Page 45: Transmitter Adjustments

    Understanding and Adjusting the ALC Meter for accuracy The Kenwood TS-940 Service Manual procedure for adjustment of the ALC is wholly incorrect. As a result, it is often misaligned, which typically results in too much ALC action. This is easily identified by noting that the ALC meter deflection in SSB mode becomes highly active or even excessive with the MIC GAIN with control at or below the 12 O’clock position.
  • Page 46 Amplifier. The ALC Amplifier circuit in the TS-940 is specifically designed to turn on when detected forward power exceeds 100 Watts. Below that level, there will be no ALC action. The adjustment for the ALC Amplifier turn- on point is the Max Power control, VR2 on the Control Unit.
  • Page 47 (After ADJ, set STBY to REC) Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 48: Best Operating Practices Using The Alc Meter Indication

    When operating the TS-940 in SSB/AM/FM or FSK Modes, it is advisable to set the MIC GAIN(FM MIC GAIN) or the PROCESSOR OUT controls to get maximum power output with minimal ALC indication.

  • Page 49: General Ts-940 Technical Notes And Observations

    Specifically, an accurate Frequency Counter is a must for performing adjustment of the PLL circuitry. Some of the adjustment steps in the Kenwood Service Manual specify voltage measurements using an RF Voltmeter, a specialized type of voltmeter not common to most Amateur Radio work benches. Although RF voltage measurement is essentially the same as measuring RMS levels, most Digital Voltmeters that do measure RMS do not have sufficient bandwidth for accuracy when measuring AC signals above 1 Mhz.

  • Page 50: Rf And Af Voltmeter

    Measuring RF Voltages Most of the RF signals you will need to observe while adjusting the TS-940 will have very low levels, microvolts. An RF Voltmeter is useful for this work as it allows accurate level display of these signals. It also allows comparative measurements in decibels, useful when obtaining maximum gain of tuned circuits.

  • Page 51: Obtaining The Correct Signal Levels For Adjustments

    So, they meant dBµV, but there is still a serious problem with the levels specified in the Service Manual Procedures. The odd levels used by Kenwood result in confusion of what the proper level should be in adjustments. For example, Step 6 of the Receiver Adjustment procedures in the TS-940 Service Manual specifies an SSG output level of 8dB to produce an indicated signal level of S-1, and 40dB to produce S-9.

  • Page 52: Kenwood Signal Level Notations For Signal Tracing

    Kenwood Signal Level notations for Signal Tracing The charts on Page 82 of the Kenwood Service Manual illustrate expected signal levels at different stages through the RF path for both Transmit and Receive. In the Transmit Section, Kenwood chose to reflect the signal levels as RF Voltage, essentially VRMS.

  • Page 53: Spectrum Analyzer

    Spectrum Analyzer The RF Spectrum Analyzer is an essential tool for anyone who works with RF circuits. While an Oscilloscope is an excellent tool for observations of the varying levels of a signal over time, measuring frequency is difficult, and almost impossible with modulated RF waveforms.

  • Page 54: Rf Sweep Generator

    Band Pass filters, which uses Germanium Didoes. This is an example of an Old School RF detector and is the exact same circuit used in your TS-940 4 IF stage. You can still find 1N60 Germanium diodes, but you can also get excellent results using any good Schottky diode in place of the 1N60 Germanium diodes with only a minor reduction in sensitivity.
  • Page 55 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 56: Ts-940 Adjustment Point Diagrams

    TS-940 Adjustment Point Diagrams TS-940 Top Chassis Adjustment Points Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 57: Pll Unit Test And Adjustment Points

    PLL Unit Test and Adjustment Points This Excellent Detailed reference for PLL Unit testing is provided Courtesy of Jeff Hilliard Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 58: Ts-940 Car Digital B Unit Test And Adjustment Points

    TS-940 CAR Digital B Unit Test and Adjustment Points Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 59: Ts-940 Bottom Chassis Adjustment Points

    TS-940 Bottom Chassis Adjustment Points Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 60: Ts-940 Rf Unit Test And Adjustment Points

    TS-940 RF Unit Test and Adjustment Points Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 61: Ts-940 Control Unit Test And Adjustment Points

    TS-940 Control Unit Test and Adjustment Points TS-940 IF Unit Test and Adjustment Points Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 62: Kb7Js Mods And Enhancements

    LED backlighting in your radio. The procedure for making the DIM feature work with the K6IOK LED Upgrade is very similar, but in the TS-940, the meter lamp power supply is 15V instead of the 28V used in the TS-930.
  • Page 63 With the Power/Timer switch assembly set aside, now look at the Switch Unit “A” from above, you can see the two Lamp bias resistors, R55 and R56 on the edge of the circuit board side facing the front panel, highlighted on the photo below.
  • Page 64 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...
  • Page 65 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 66: Kenwood Service Bulletins

    Kenwood Service Bulletins SB-900 SB-900 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...
  • Page 67 SB-908 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

  • Page 68: Service Bulletin

    SB-919 SERVICE BULLETIN TS-940 S-Meter Needle Slams to full scale during Power On/Off 09/19/86 Reports of the S-Meter needle slamming to full scale position on Power On/Off. This is due to the delayed switching of the +15V DC supply to the AGC Amplifier.
  • Page 69 SB-951 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...
  • Page 70 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...
  • Page 71 Version 8.0.0.1 KB7JS – 10/20/2024 This just keeps getting better...

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