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Possible cure for the dreaded TS940 dot display problem

Discussion in 'General Ham Radio Discussion' started by Captain Kilowatt, Dec 31, 2017.

  1. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Apr 6, 2005
    Likes Received:
    First off this is NOT my solution. It was presented to another party via a private conversation on this forum and I was asked if I could move the posts to a thread. I cannot do that so I created this thread and have copy/pasted the relevant info here.

    Robalo posted the following:

    The TS940S along with other Kenwood transceivers are prone to a common fault - that of displaying a row of dots instead of a
    frequency readout. This is caused by one or more of the Phase Lock Loops (PLL) going out of lock.
    A number of suggestions have been put forward to cure the fault. The most common two being to remove all the wax from parts of the PLL
    board by heat or other methods and the other is to resolder all the joints on the underside of the board. Either method may cure the
    problem but (unless great care is taken) they could induce additional faults thereby adding to the problem and obscuring the original
    fault. An alternative method would be to narrow down the fault to a particular part of the board or boards as suggested below.
    In the case of the TS940 there are a total of 6 phase lock loops, any one of which could be out of lock and cause the 'DOT' problem. Only 4
    are on the PLL board - the other 2 are on the CAR board. At this point I suggest you click here and download the relevant pages of the
    TS940 service manual (4.7MB) which is in PDF format.
    Before making any tests on either board it is best to check that the supply voltages are correct. On the PLL board connector 5 pin 5
    should be +5V and pin 3, +15V.
    Each individual phase lock loop produces a permanent voltage of around +4.75V if it is in lock and just above 0V if it is out of
    lock. The signals from all 6 are gated together using diodes such that any one PLL producing a 0 voltage will 'override' the other
    signals and produce 0 volts at the final output designated UL.
    First we need to decide whether the fault is in the PLL board or the CAR board. With the rig powered up, connect a voltmeter (I use an
    oscilloscope) to pin 4 on connector 5 of the PLL board (be careful, pin 5 has +5V on it and pin 3 has 15V as previously mentioned). This
    is the final lock/unlock signal from all 6 phase lock loops. As we are out of lock, this voltage should read around 0 volts or a little
    above. Now unplug connector 2. If the voltage on pin 4 of connector 5 is still around 0 volts then the fault is on the PLL board. If the
    voltage is now around 5 volts then the fault is on the CAR board.
    For arguments sake let's assume the fault is on the PLL board (the most likely scenario) then we may now proceed to narrow down the
    fault to 1 of the 4 phase lock loops.
    We can leave connector 2 unplugged at this stage. Check the output of each phase lock loop as follows. With the rig powered up, check the
    voltage on IC8 pin 2, IC9 pin 2, IC19 pin 2 and IC17 pin 7 (yes pin 7). Whichever output is giving a low voltage of around 0V is the loop
    which is out of lock.
    Say for example we find that all levels are showing +4.5V except for the output of IC17 pin 7 which is showing around 0V. By looking at
    the circuit diagram it becomes obvious that we need to fault find around the components associated with IC15, IC16 IC17, Q22, and Q23.
    We have the option here of taking the board out and resoldering all the joints and components associated with those semiconductors or we
    can narrow it down further by checking the voltage levels and comparing them to those shown on the circuit diagram. The choice is
    If you have an oscilloscope available you can in addition check the RF signal on the base and emitter of Q22 and it's subsequent input to
    pin 9 in IC15. Check the output signal on pin 8 of IC15 and it's subsequent input to pin 1 on IC16.Check the output on pin 7 of IC16
    and it's subsequent arrival at pin 8 on IC17. Check also the RF input signal on the base of Q23 and it's output which goes to pin 1 on
    With the voltage readings and/or oscilloscope measurements it should be possible to narrow the fault down to a single component. Before
    changing it though, I would suggest that you resolder the component as most faults appear to be due to bad soldering on the board rather
    than to a component failure.


  2. Robalo

    Robalo Well-Known Member

    Aug 21, 2006
    Likes Received:
    Captain Kilowatt thank you for moving this conversation where people can see it, the reason is because few members from this forum and some guests have been asking how to fix it. Now, this information is not mine, I found it online a long time ago. I hope someone recognizes his work because it was great information. I don't know where and I just put it on practice and it came out ok, so I have another two ts-940 (mine) waiting to be fixed.

    I'm going to make another video where I can explain better with more tips so I know I can do it better, and I hope we can see more healthy 940's. I fixed a couple 940 for local friends and those radios rock on air they sound so good and I'm proud of that.

    If this post helped you out feel free to leave a like!
  3. W2EKG

    W2EKG 201 Mike Action

    Dec 30, 2015
    Likes Received:
    TS-940 back in the day "The UPS Special," shipping that thing back for warranty repair. Never by a low serial number.
    Rwb likes this.
  4. 338_MtRushmore

    338_MtRushmore Sr. Member

    Jun 17, 2012
    Likes Received:
    Rwb likes this.
  5. TruckerKevin

    TruckerKevin KD9NTD

    Nov 9, 2017
    Likes Received:
    Just an FYI for anyone in the future, the #2 plug needs to be connected to get accurate PLL readings. This article says to leave it unplugged which gave me high and low mixed results. However, when I reconnected the #2 plug, I got the proper 4.7 volts from all my PLLs except one. I found the original article, and it also says to plug the #2 back in as well.
    #5 TruckerKevin, Apr 18, 2020
    Last edited: Apr 19, 2020
    NZ8N, Robalo and Rwb like this.

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