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digital multimeter sinclair dm 450

Discussion in 'Troubleshooting and Repair' started by harrypicun, Mar 2, 2014.

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  1. harrypicun

    harrypicun

    4
    0
    Mar 2, 2014
    I am Trying to repair an old Sinclair dm 450 digital multimeter I'm searching for the circuit Can anybody help?
    Regards Harry
     
  2. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    Hi,

    Sinclair merged with Thurlby Electronics Limited to form TTI (Thurlby Thandar Instruments Limited). Your multimeter looks to be the same as the Thandar TM355.
    You can have a look in that direction for the schematic, also what's the faillure and do you already open the case ?
     
  3. harrypicun

    harrypicun

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    Mar 2, 2014
    Thank you, yes I opened the box
     
  4. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    So tell us more, what's the failure ? is there any visible sign of components failures(burns, leaks,..) .
     
  5. harrypicun

    harrypicun

    4
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    Mar 2, 2014
    is dead

    is dead, does not turn any segment, I'm trying to copy the circuit
     
  6. harrypicun

    harrypicun

    4
    0
    Mar 2, 2014
    no signs of damaged components

    no signs of damaged components
     
  7. ap2wf

    ap2wf

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    Mar 21, 2014
    Sinclair DM450 Repairing.

    Hi,
    I am also in the same boat and am repairing Sinclair DM450. The meter is not totally dead but shows 1 at the left most digit. Also, the next digit flickers if I touch somewhere under the digit driver and BCD to 7 segment decoder ICs. No effect if test probes are connected to any source of signal. Will appreciate if someone share advice. I am relatively better in electronics, etc.
     
    Last edited: Mar 21, 2014
  8. ap2wf

    ap2wf

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    0
    Mar 21, 2014
    Sinclair DM450 Repairing

    Can we exchange the photographs of the inside views, may be helpful for both of us?
    In the mean time we may get some senior's advice.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    One simple cause can be that the LCD has become fully or partially detached. This can be difficult to repair in older devices because the connections were often of a type that once removed could be almost impossible to reapply.

    Often you have a conductive rubber connecting the LCD connections (that may be transparent metal on glass) to plated connections on the board. This may be held in place using a combination of adhesive and pressure. The connections and the conductive parts of the rubber may be invisible to the naked eye (hence alignment is problematic). If the rubber has perished, you're in a very bad place.

    This is a good reason not to take apart old equipment using LCD displays. Newer equipment is more likely to have the LCD as part of a module (which again, you don't want to take apart) but the connections will be less troublesome.
     
  10. ap2wf

    ap2wf

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    0
    Mar 21, 2014
    Sinclair DM450 Repairing

    Thanks Steve for your response. I may correct that Sinclair DM450 Digital Multimeter uses 7 segment LED modules, not LCD. The DS75494N as LED Display driver, MM74C48J BCD to 7 Segment Decoder, ICL7103ACPI A/D Converter, and a had full of Op-Amps and chips like CD4066BE, LM3080N, CA3140S, LM555CN, TL061CP, TL062CP, TL072CP, TL080CP, BFR81 transistor and hp5082-7740. I am trying to trace the circuit but apparently 7 segment display section seems ok.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Interesting.

    Discrete means it's more repairable, but there will be many things to check.

    First off, if pressing the display makes it change then you have a bad solder joint (or joints) or similar. You should track them down first.

    The easiest way might be to resolder all the relevant connections and also to check the board under magnification for cracks
     
  12. ap2wf

    ap2wf

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    Mar 21, 2014
    I checked the display section as advised, but no change. Can I check by providing input signal to ICL7103 A/D converter? How and what signal?
     
  13. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    You can directly connect a few hundred mV source on the ICL7103 inputs (pins 10&11) but this should not help to solve the flickering display issue. Few pictures of both PCB's sides can help us to find what is all about. For instance are the IC's or the displays socketed ? Old HP displays pins tend to oxide when aging. How are the front display PCB connected to the main board?
    If it's intermittent you probably face bad contact or bad soldering joints problems.
    You can also check all supplies, on 7103 you have +5V on pin1, -15V on pin 6 and vref on pin7. More supplies are maybe to check, look on other IC's supplies Pins to be exhaustive.
    Olivier
     
  14. ap2wf

    ap2wf

    8
    0
    Mar 21, 2014
    Thanks Olivier,
    I'll check thru ICL7103 and all ICs supplies, for this is what I wanted to do. All the ICs are socketed while LED display is mounted on a sub-assembly/PCB and directly soldered. There is no oxidation or rust but looks to be handled carefully for proper soldering temperature because of aging factor. Incidentally, the display does not flicker due to intermittent contact but because of damping when touching the BCD to 7 Segment decoder/Display driver chips' pins. I shall send pictures of the board shortly. Hope Harrypican will also be benefitted. Thanks again Olivier.
     
  15. ap2wf

    ap2wf

    8
    0
    Mar 21, 2014
    Repairing Sinclair DM 450

    Hi...
    I have a few pictures of the Circuit Board. I took it out of the plastic box.
     

    Attached Files:

  16. Klaus Mewes

    Klaus Mewes

    1
    0
    Nov 14, 2014
    Hi,
    if you are still looking to repair this, and still checking this post, I have a suggestion. My unit had a bad tantalum capacitor in the power inverter board ( second removable pc board behind the 7103 chip. It got 'fried' when I used a battery 8.4V LIPO battery for power (don't use anything higher than 6V!!) It also burnt a trace on the top pc board layer leading to one of the pins, so look out for that also. Mine was looked 'dead', but came back to life when I repaired the trace (i.e. running a bypass wire) and replaced the 10 uF tantalum capacitor on the power board.
    Good luck.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,500
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    Jan 21, 2010
    Can you post some larger versions of these images. They're too small to make out any details. See here for more information. (if you're using a mobile phone, see here)
     
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