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Fluke 123 Scopemeter Not Working

Discussion in 'Troubleshooting and Repair' started by Indiana Jones, Oct 1, 2016.

  1. Indiana Jones

    Indiana Jones

    Oct 1, 2016
    I have a Fluke 123 Scopemeter that I bought used off of eBay. It has worked great for months after my purchase and I was using it one day to help troubleshoot a problem on one of my vehicles. I powered it off after using it and put it on charge. Later that same day I came back to use it again and it didn't power up correctly.

    What I mean by this is with the battery only it would not turn on. When I plugged in the adapter it would power on, I would get a beep like normal and the LCD would be completely black and I could see the backlight was on as well. I pulled the battery to check it and tried powering it on with the adapter only and once again it would not power up at all. The only way to get it to power up was with both the battery and adapter plugged in. With the adapter plugged in I notice that the speaker does a rhythmic clicking. I've never heard it do that before.

    I downloaded the Fluke Service manual for it (thank you to Fluke for putting out such a very nice service manual and making it easily available) and I've been slowly working on troubleshooting it but I'm not getting very far. One reason is that while the service manual is very good it doesn't take into account all of the minor changes they've made to the circuits over the service life of the 123 and there are differences that make me have to dig even deeper to see if I've found a problem or just a revision that Fluke made. So my question is has anyone seen a Scopemeter act like the following and what was the issue if you have...

    1. Will not power on unless both the battery and the adapter are plugged in.
    2. When it powers on you get a normal beep, the screen is black and the backlight is on.
    3. With the unit off and the adapter plugged in you here the speaker clicking.

    Any suggestions would be greatly appreciated.
  2. dorke


    Jun 20, 2015
    Welcome to EP.

    I'm not familiar with the "Fluke 123 Scopemeter"
    Fluke has 3 different service manuals on their web site :

    123 Service Manual .
    123/124 Service Manual.
    123/124/125 Service Manual.
    Which one relates to your specific unit ?

    In general,
    operating units with external adapter with the internal rechargeable battery pack removed,
    may cause damage (that depends on the design).
    It may be the case here.

    What is the battery voltage you get?
    Can you adjust the display backlight brightness ?
    Those the unit "communicate" with a PC?
  3. Indiana Jones

    Indiana Jones

    Oct 1, 2016
    Thank you for the reply and for making me feel welcome by responding. I didn't realize there were three different manuals. I had only found the 123/124 manual. I'll look thru the other ones and perhaps they will give me the information I need to get this thing back up and working.

    When I started diagnosis my battery voltage was close to four volts. It is now down around three volts. I cannot adjust the display brightness. I've tried and it won't respond. I have not tried having the unit communicate with a PC because I'm not sure it will right now. Besides you have to initiate that communication from the unit and obviously I can't see to do that.

    I do understand that damage may occur when operating the unit without the battery installed. But if you look in the Fluke manuals they actually have you do that as part of the diagnostic steps. So trying to follow what the manufacturer said to do on that one.

    Have you ever heard of one clicking when the adapter was plugged in? I know it is coming from the speaker and it seems to click about twice per second.
  4. dorke


    Jun 20, 2015
    As I said I'm not familiar withe the Fluke 123.

    With 3V battery voltage, no wonder the unit doesn't work-it should be 4.8V nominal.
    Internal Battery Pack BP120 (Fluke 123): Battery Power Rechargeable Ni-Cd 4.8V(page 2-10)
    Is the charger charging the Battery?

    Please read paragraph 3.2.4 and on ( page 3-6 till page 3-12) ,in the 123-124 S.M.
    For the power- up circuit / sequence
  5. Indiana Jones

    Indiana Jones

    Oct 1, 2016
    Once again thank you for the reply. I'm well aware of what the battery voltage needs to be. After my last round of troubleshooting I left the battery pack on charge and it is now at 5.0 Vdc. I then tried firing it up off of the battery alone (with it at 5.0 Vdc) and got the same results I've been getting. It won't do anything on the battery alone, just as it won't do anything on the charger alone. When I have both plugged in it will fire up with the conditions I specified in my first post. I read the fire up sequence in the manual I had downloaded so I'm aware of what should happen. I did download the other manuals and I'm getting set to compare the unit I have to the diagrams in each of those manuals to see which is the closest match. That will tell me which book I should be using for my troubleshooting. Hopefully that will actually get me to a point where I can get this thing working once again.

    I can assure you that the issue I'm having has nothing to do with the batteries (I have two) and it has nothing to do with the chargers (once again I have two). It is something that failed very suddenly in the unit and is keeping it from working the way it should. I just have to be able to figure out what that might be.

    Unfortunately it looks like I've been working from the wrong service manual and so I haven't been able to make any headway. Now that it looks like I've got the right documentation I should be able to figure out the problem fairly quickly and hopefully have it back up and running soon. My bet at this point is that a capacitor failed and that is what is causing my issue. I just have to be able to figure out which one.
  6. dorke


    Jun 20, 2015
    Yes, I agree you should use the correct S.M for the repair.
    When you do have it, use the power up circuit and sequence to find what went wrong.
    Also notice, there is a "rest to factory settings " procedure you may need to try.
    Let us know of your findings.
  7. Indiana Jones

    Indiana Jones

    Oct 1, 2016
    I will definitely let everyone know once I find out what happened. I've already figured out which manual is the right one for me to be using for my diagnostics. It's the early one that has 123 in the title only. So from there hopefully troubleshooting should be fairly straight forward. I cannot thank you enough for pointing out that there was more than one service manual out there for this series of meters. I've been scratching my head for weeks trying to figure out all the differences in the circuits and trying to figure out where the problem could be. Hopefully now it should be pretty smooth sailing to get the issue nailed down and resolved.
  8. dorke


    Jun 20, 2015
    You are most welcome,good luck with the repair.
    If more help is needed we are around;)
  9. artgineering


    Dec 19, 2016
    Hi Indiana Jones
    Did you already solve your Scopemeter Problem?
    I have nearly the same problem like you.
    The Scopemeter powers on, Beeps normaly but dont start.
    Backlight of Display isnt working.
    When unit is off and adapter plugged in, the speaker is clicking.

    Sorry for my bad english ;-)
    Greets from Switzerland
  10. chrihern


    Dec 10, 2018
    I have same problem OP describes, my scopemeter Fluke 123 display suddenly stoped working: Backlight turn on but lcd is black. Reading this post and others i see somethig: there are two totally different kind of problems related with display malfunction symphtoms.

    -One is many black lines horizontal or vertical in your Scopemeter : is caused in display itself having bad connections between PCB and LCD (a very common problem in Fluke 123). you can change display unit or try to repair connections.
    - Other very different situation is a dead display : is produced by a sudden change in your equipment (not related with slow decaying connections).

    Testing video signal easy: there are twenty something ribon display cable with test points in main PCB :
    My Fluke shows a reference voltage called REFPWM1 (fourth test point from top to down marked MS404 ) near 0.5V, manual says would be 3.3 V.
    Fluke 123 (early version) manual says ( ) "The LCD contrast is controlled by the CONTRAST voltage. This voltage is controlled by the D-ASIC, which supplies a PWM signal (pin 37 CONTR-D) to PWM filter R436/C436. The voltage REFPWM1 is used as bias voltage for the contrast adjustment circuit on the LCD unit."

    Clearly display wont work if bias contrast voltage is so low.

    I suspect two things:
    The good thigs is my display is in working order and can work in other scopemeter. The bad news is my Scopemeter main board is damaged.
    For a moment i had hope for find some discrete damaged component pulling low thar signal but...
    Tracing REFPWM1 i see origins from IC N301 ( Philips OQ 0257) in "Reference Gain" section (diagram Trigger circuit).
    Measuring others reference voltages from N301 (TP 301, 302 ,303) looks like that IC is dead.

    With my Scopemeter condemned to be an expensive yellow brick i can experimet puting an external 3.3 v signal in REFPWM1 display connection and see if display works.
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