Connect with us

Calculator Repair

Discussion in 'Electronic Repair' started by TObject, Jan 13, 2004.

Scroll to continue with content
  1. TObject

    TObject Guest

    I am trying to repair a calculator embedded in the Radio/Clock/Calculator/Mousepad combo.
    It works just fine, except the keys "1", "4", and "8". When you press any of the keys, nothing
    happens.

    I have a suspicion; it's a bad contact somewhere or maybe a short. When I went over
    the calculator board with my tester looking for bad contacts for the first time, I cleaned up
    a spot of solder splatter on the keys PCB board. I thought that what was causing the issue,
    as the keys started working after that.

    Shortly the calculator was returned to me again, with the same symptoms. I am having
    problem locating the bad spot. Any ideas?

    I could use help trying to find the schematics for this calculator. The chip is not marked.
    The main PCB board with the chip is marked "94HB-1 0307 01-008178-00 TY/210602"

    Here are a couple of pictures to help identifying the calculator:
    http://www.jeepbbs.net/tobject/pictures/problems/calc/PCB.jpg
    http://www.jeepbbs.net/tobject/pictures/problems/calc/Face.jpg

    Thank you!
     
  2. Jerry G.

    Jerry G. Guest

    I doubt very much that the manufacture will have any available replacement
    parts, and or schematics published that they will sell. Considering the low
    cost of these units, they would never have anyone spend the time to service
    these, therefore they would not invest in having service support for them.
    People don't realize the cost involved just to have stocking on the service
    manuals and replacement parts, distribution, and keeping track of the sales
    of them. It is even hard to get any service information on the medium to low
    end electronics consumer products these days, never mind a calculator.

    A common cause of keys failing is that the conduction coating on the bottoms
    of the rubber key-buttons wears out, and thus there is not enough conduction
    across the contacts. There are some remote control repair kits for these
    types of buttons, where you can repaint this conduction coating back on to
    the key-buttons. The kit to service one set of buttons probably costs more
    than several of these calculators.

    If you measure the contact resistance of the rubber key-button bottoms, you
    will see that the ohms reading must be very high. You can compare it to the
    other button readings that work, in order to have a reference to what it
    should be. If you put a short across the contacts, and if the rest of the
    calculator is working well, the contacts should be working.

    --

    Greetings,

    Jerry Greenberg GLG Technologies GLG
    =========================================
    WebPage http://www.zoom-one.com
    Electronics http://www.zoom-one.com/electron.htm
    =========================================


    I am trying to repair a calculator embedded in the
    Radio/Clock/Calculator/Mousepad combo.
    It works just fine, except the keys "1", "4", and "8". When you press any of
    the keys, nothing
    happens.

    I have a suspicion; it's a bad contact somewhere or maybe a short. When I
    went over
    the calculator board with my tester looking for bad contacts for the first
    time, I cleaned up
    a spot of solder splatter on the keys PCB board. I thought that what was
    causing the issue,
    as the keys started working after that.

    Shortly the calculator was returned to me again, with the same symptoms. I
    am having
    problem locating the bad spot. Any ideas?

    I could use help trying to find the schematics for this calculator. The chip
    is not marked.
    The main PCB board with the chip is marked "94HB-1 0307 01-008178-00
    TY/210602"

    Here are a couple of pictures to help identifying the calculator:
    http://www.jeepbbs.net/tobject/pictures/problems/calc/PCB.jpg
    http://www.jeepbbs.net/tobject/pictures/problems/calc/Face.jpg

    Thank you!
     
  3. It may also be that the 1, 4, and 8 keys share a common strobe line to the
    micro. So tracing back to the black blob may reveal if there is an open
    or short on that line.

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  4. TObject

    TObject Guest

    To everyone who answered, thanks for your help!

    I fixed the calculator, but as you have guessed it did not worth the time spent.
    The main problem with these cheap calculators is the quality of circuit boards.
    The buttons board on this particular calculator is double layered - one layer
    per side. Undoubdly in pursuit to make these things as cheap as possible,
    the whole circuitry on the side of the buttons made out of the button contact
    material.

    The button contact material introduces two things. First, it's conductivity is much
    worse (in comparison to copper). That makes it harder to locate the problem.
    You don't simply have contact or not have contact. The longer or thinner the run
    of that button contact material, the more resistance you get.

    Second bad thing - it is hard to make contact with that material. You can't just
    solder to it.

    I started by reverse engineering the schematics from the offending buttons.
    Then I checked resistance in the lines that go to the main chip. When I found
    the line where resistance was measured in Mega Ohms (where other lines were
    usually in single kiloOhms), I made a bridge over the offending part straight to
    the board with the main chip. The button "8" started working, but "1", and
    "4" were still not operational.

    Then I found that one of the interlayer connections on the buttons board was bad.
    It was a connection between your normal copper PCB layer, and that cost
    saving button contact material layer. I repaired the connection, by taking a piece
    of stranded speaker wire, unstranding it, taking one strand, tieing a knot in it,
    putting it through the board so the knot grabs the button contact material, and
    then soldering it on the other "regular" side.

    This project was not fun. It is definitely not worth time to fix the cheaply built
    electronics.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-