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Multimeter DT9205A problem

Discussion in 'Troubleshooting and Repair' started by Andrew Borg, Aug 10, 2014.

  1. Andrew Borg

    Andrew Borg

    36
    4
    Aug 10, 2014
    Hi guys I have this Multimeter with auto switch off. The problem is that I cannot switch it on.
    Once in a while it switch on but it had become rare now to switch on.
    Any help please What's wrong with this multimeter?
    Thanks
     
    zam likes this.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,154
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    Jan 21, 2010
    You've probably already tried, but have you replaced the battery?
     
  3. OLIVE2222

    OLIVE2222

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    Oct 2, 2011
  4. Andrew Borg

    Andrew Borg

    36
    4
    Aug 10, 2014
    Hi Steve and Olive2222
    thanks for response. Yes I've replaced the battery and even took off the fuse to reboot as I read somewhere but still same problem.
    I will have a look at the schematic and see
    Thanks guys
     
  5. Andrew Borg

    Andrew Borg

    36
    4
    Aug 10, 2014
    I've looked at the schematic but don't know what's wrong. Tried to clean switch but I think there's something with the chip.
    :(
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Thanks Olivier for finding the schematic!

    Here's the power-down circuit.

    power-off circuit.png

    It's controlled by the DPDT swtich at the left. When it's in the position shown, the OFF position, C19 is charged up to the battery voltage. When the switch is turned ON, C19 is connected to R53, and via D14, it is used to provide initial power to IC2A.

    IC2A compares the voltage on R53 with a fixed threshold of about 18% of the power supply voltage (set by R52 and R51). When the multimeter is first switched ON, the R53 voltage should be a lot higher. IC2A will drive its output high, turning ON Q2, which turns ON Q3 and feeds the battery voltage to the V+ rail.

    After a while, C19 discharges through R53 and its voltage drops below the 18% threshold. When this happens, IC2A's output goes low, Q2 and Q3 turn OFF, and the multimeter powers down.

    There are lots of things that could go wrong with this circuit. A good place to start would be to measure (with a different multimeter!) the voltage across C19 when the multimeter is switched OFF (should be equal to the battery voltage), then when the multimeter is first switched ON. It will drop somewhat, but should remain much higher than the 1.6V threshold for several minutes at least.

    If that's OK, measure the voltages on C18 (the V+ rail) and IC2 pin 1 (the comparator output) immediately after you have switched the multimeter ON. If they are both more than a few volts, measure the collector and base voltagres on Q2 and Q3.

    Post the results here and we can work from there.
     
    (*steve*) likes this.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,154
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    Jan 21, 2010
    For simplicity, follow Kris' instructions.

    My first test would be to check the voltage across pins 4 and 8 of the HA17358 (as a side note, it looked like pin infinity at first!).

    Does this voltage rise to 8V or so on power on, and stay there, or does it fall back to 0V immediately (perhaps you won't even see it going above 0V.

    If you see voltage across here (say 7.5V or more), then the rest of the multimeter is receiving power and the fault is not in the auto power off circuit.

    I would be hoping the fault IS in this circuit (and hence that you see the voltage across the op amp falling to zero quickly).

    This circuit, if it is at fault, should be easy to repair as noted by Kris.
     
  8. Andrew Borg

    Andrew Borg

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    4
    Aug 10, 2014
    Hi Kris thanks for this info.
    I checked the C19 it was without charge. I notice that when I shorted the bottom pins of the switch it switched on. When checking the cap after this it had charge in it as you've said.
    So what is the fault about not charging the cap?
    Thanks
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,154
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    Jan 21, 2010
    It sounds like the "upper" contacts (or at least one of them) isn't making contact.
     
  10. shrtrnd

    shrtrnd

    3,649
    450
    Jan 15, 2010
    Yep. Clean the on/off switch conacts.
     
  11. Andrew Borg

    Andrew Borg

    36
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    Aug 10, 2014
    Hi Steve and Shrtnd thanks for response. To clean on/off switch with spray solvit would be ok?
    Thanks guys
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    dunno what solvit is ?? google show a lot of different solvit's, none were cleaners

    use a contact cleaner or something like WD40

    Dave
     
  13. Andrew Borg

    Andrew Borg

    36
    4
    Aug 10, 2014
    Hi Dave I used WD40 but invain. The thing now is that I changed the switch; and while working I hit the board and not working now :(.
    I noticed that C14 is missing. What part has to be there?
    Thanks.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,154
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    Jan 21, 2010
    C14 is listed as "223" which means 22 * 10³ pF which is 22000pF, or more commonly these days 22nF
     
  15. Andrew Borg

    Andrew Borg

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    Aug 10, 2014
    Thanks Steve I'll will do one and see hoping that it will come alive again :)
     
  16. shrtrnd

    shrtrnd

    3,649
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    Jan 15, 2010
    Sometimes manufacturers make 'all-purpose' boards, but don't install some components in instruments that don't have certain 'extra' options.
    If a part is missing, it doesn't necessarily mean that one should be there for the model/revision/ or options in your instrument.
    You may have knocked something loose on the board when you hit the board, but it's more likely you just didn't reseat everything correctly when
    you reinstalled it. Displays are particularly touchy about improper re-seating. Try taking the board out again, and being particularly careful when reinstalling it
    that all contacts are properly made.
     
  17. AdamF

    AdamF

    1
    0
    Oct 22, 2014
    I just bought one of these and came with the exact same problem.

    Just checked the DPDT and one of the switches wasn't working; reason why C19 did not charge when OFF.

    Used a contact cleaner without even unscrewing the board, and now it's working like new... well, it is new. :)

    Thanks a lot!!!
     
  18. GayanDSilva

    GayanDSilva

    1
    1
    Oct 16, 2016
    As per the cct diagram i serviceable my unserviceable DT9205A DMM in very short time.
    In my case the problem was in the power switch (2P2D) and the capacitor (47uF/16V). I replaced both so found DMM serviceable. The logic is when power switch on charged cap (47uF/16V) has to discharged and give the voltage to non inverting pin of the op-amp and 10M resistor.
    when soldering the switch be careful to replace the switch correctly considering the pin configurations. (N/O & N/C positions must correct)[​IMG]
     
    davenn likes this.
  19. Andrew Borg

    Andrew Borg

    36
    4
    Aug 10, 2014
    Thanks Gayan for your reply but it's a bit late cause I already scrapped it off cause could not find the fault.
     
    davenn likes this.
  20. gribouilli

    gribouilli

    1
    0
    Jan 22, 2018
    hello, I have the same multimeter, I had the same problem, I started testing as notice in the post, and after a moment of lucidity, I did a test of my battery voltage, (9.37v ) then a test on the power cable of the battery, 0v.
    I "opened" the case of the pin of the battery te hop cable poorly welded XD
     

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