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Stuck with fixing SZBJ DT9205a multimeter selector mechanism

Discussion in 'Troubleshooting and Repair' started by Erastro, Dec 2, 2017.

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  1. Erastro


    Dec 1, 2017
    Hi, it's my first post here!
    I have this cheap multimeter:

    The on/off switch got dust in it so I opened it to clean it and I messed up the switcher conductors order:
    And also there's these two things fall that I have no idea where to place:
  2. kellys_eye


    Jun 25, 2010
    They hold the LCD in place.
    Erastro likes this.
  3. Erastro


    Dec 1, 2017
    Still wondering how to place the conductors in the switcher.
  4. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Erastro . . . . .

    I now ask you HOW---HOW--- HOW ? could you not have realized the IMPORTANCE of placing an indexing / reference mark, with a SHARPIE, between the rotary knob and the instrument case . . . bet you will take note in the future.

    Should you ever be field stripping an AK-47 . . . . I believe that you could be left stymied and thereby defenseless and thereafter . . . . . dead .

    First part is already covered by the Esteemed Kelley and his eye.
    This 2905 series instrument is RE-manufactured by scads of manufacturers, its success and real cost saving manufacturing feature is the etching of the complex switch stepping on the PCB and then just sequencing the rotary action of the five pressure contacts onto the PCB for all of the necessary switching actions.

    I am giving you a visual reference of what looks like a much CHEAPER / QUALITY flash molded case and different shapes of your metal clamps, that are used for building up the pressure being applied between the connection strips of the PCB . . .(which your photo cropped off .)
    There is the (RED RECTANGLE OUTLINED) dual silicone rubber membrane strip / block that has hundreds of individually INSULATED conductive carbon fibers that are molded in side by side and then run the short distance, from the front to back in the strip.

    When the VERY delicate (read as multi microns THIN) of vacuum deposition flashed on conductive coating pattern on the displays glass . . . is pressed to one end of the above fiberstrip . . . . . .it then presses into and then electrically interconnects to the specifically related connections at the PCB foil patterns side.
    Voila . . . . . . an electro mechanical interface between PCB and glass LCD display.

    DVM 2905 Meter Case.png

    Now on my illustrative case above, I have shown how the metal strips would enhance the rigidity and pressure between the case and display, are being installed on THAT particular case.
    On your unit , you are going to have to do a screw count to see if they allocated two screws to each clamp or only one , and I am sure that you can see that the curved end goes down toward the area to be compressed.
    Then you just select out the proper plastic studs that stick up and mate with your straps hole placements.

    Now . . . do you KNOW . . . how to put that type of mounting screw back in ?
    Well . . . . . .first you place the screw tip upon its mounting studs hole and engage the screwdriver to the slot of the screw and then you put slight pressure downwards and start rotating the screw COUNTER CLOCKWISE / (BACKWARDS).
    Within less than 1 1/2 turns of CCW rotation, you should have heard two slight clicks, which were being the original threadings within the plastic stud, now, just AFTER hearing the next one, reverse your screwdrivers directiion and that will reinstall your screw within the original threading cut into the plastic stud. Creating DOUBLE threadings usually leads to a fracturing of the brittle stud and then having a real mess on your hands.
    (Metal sleeve the outside of the stud, pour in epoxy -hard cure- reface the end of the stud-drill -tap and you can then start over where you otherwise should have been. )

    Now your knobs detents / defined stops action is accomplished by one or a set of single ball bearing that use compressive springs to keep a constant pressure upon them. Then they drop down into depressions in the plastic for each marked range / function on the face of the meter . . . . . which looks to be 32' ish or so.
    I just guessed at their mechanical placement areas by the GREEN circles .
    I also used a YELLOW HIGHLIGHTING of each possible positioning of the bifurcated tensioned contacts.
    Thereby . . . letting you see the 5 used and 5 unused positions of the contacts.
    Also I used minimally visually invasive fine PINK arrows to note the notched out area of the knob. One might expect the instrument power off position . . . . . to have that slot being at the very top . . . or . . . at the bottom of the unit.
    On initial test after reassembly of the unit, place the front knob (to the closest to the power pushbutton) . . .DIODE test and beeper position.
    Power up and short the test leads and expect a beep shortly thereafter. No Beep ? Then move one position at a time on the knob and seek out your beep position. Then take apart and reset the knobs position to correct the error, then reassemble and test, THEN the next NON DESTRUCTIVE, confirmational testing that could be done, is then being the OHMS ranges .

    Last caveat . . . . .since you initially blamed erratic contact action . . . . . you might as well have degreased the contact pads on the PCB to be able to then use an eraser on them to safely get any oxide from them and then leaving them cleaned and shiny, then smear on a thin coat of silicone grease on their surfaces. Plumbing supplies carry silicone grease . . . in case you are not already familiar, in having access to it in either the chemical or scientific community.
    ALSO . . . run a coat of it around the case area that the indexing /stop positions that the ball bearing(s) will be rotating within and popping in and out of.

    Thaaaaaasssit . . . . .

    73's de Edd

    Attached Files:

    Last edited: Dec 4, 2017
    DaveKZN and Erastro like this.
  5. Erastro


    Dec 1, 2017
    I really thank you Mr Edd!

    Promise won't make that mistake again!
    Let's just hope I won't be in such a position.

    Pretty informative, never knew how this delicate thing worked before!

    I didn't manage to get the right order of them.
    However I reached out the manufacturer, they might give us the order?
    I bought a better one with an LED anyways!

    Thanks a lot Edd & kellys_eye
  6. Bluejets


    Oct 5, 2014
    Deja vous....$10 meter.
  7. DaveKZN


    Aug 4, 2020
    That was fun. And interesting how many different versions there are of the DT9205A. Mine has 7 contacts, so the contacts will go in different places. Repairing this thing was never on the designer's mind - they could have made it so the contacts can only go in one way, but no, they had to make it more of a challenge than that.

    It served me well for 10 years, so time for a new one.
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