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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details. |
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13th Jan 2012, 3:28 pm | #1 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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Faulty Monroe 920 nixie tube calculator
I've got problems with a Monroe 920 nixie tube calculator.
Upon switching on, all twelve nixies light up and display 9 with a decimal point. The 'overflow' indicator also lights. Pressing the cancel button changes all the nixies to 0, again with the decimal points illuminated, but the 'overflow' indicator is off. Now if I press the 0 key nothing happens (still displays twelve 0s and twelve decimal points) but pressing any other, numeric, key sends the display to twelve 9s, twelve decimal points and the 'overflow' indicator on (exactly as it is when switched on) I've cleaned and reseated all the circuit board connections, but it hasn't helped. Has anybody got any ideas? TIA David |
13th Jan 2012, 4:21 pm | #2 |
Pentode
Join Date: Feb 2006
Location: East London, UK.
Posts: 231
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Re: Faulty Monroe 920 nixie tube calculator
Hi Dave,
I think the first thing I'd check is the power supply (as the thing's getting on a bit now), and make sure that voltages look OK, and also that the smoothing caps haven't dried up. Cheers, Alan . |
13th Jan 2012, 7:04 pm | #3 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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Re: Faulty Monroe 920 nixie tube calculator
I've checked the voltages, from the power supply and they are very close to what they should be.
David |
14th Jan 2012, 8:22 pm | #4 |
Heptode
Join Date: Jun 2006
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 931
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Re: Faulty Monroe 920 nixie tube calculator
Do you have a scope? Would be worth seeing if there's noise on any of the supply rails as that could send the logic circuitry into a complete fit of madness if there is. Smoothing caps would be my first prime suspect.
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21st Jan 2012, 4:38 pm | #5 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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Re: Faulty Monroe 920 nixie tube calculator
I've changed the electrolytics and re-soldered every joint. The calculator now works a little better, but it's totally unpredictable.
Anything can be displayed when switching on and the "overflow" is always on. Pressing the cancel button does bring up a line of zeros though . I've only got one decimal point now, but it can appear anywhere. It will add, multiply and divide, but it won't do all three during the same session! David |
24th Jan 2012, 3:38 pm | #6 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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Re: Faulty Monroe 920 nixie tube calculator
Good news! I've spend hours re-soldering connections that pass through both sides of the PCBs and I've been rewarded with a working calculator (I think). To be honest I'm not to sure how to work this calculator .
If I want to subtract 4 from 5 am I supposed to press 5, "plus equals key", 4 "minus equals key" to get the answer? If I press 6, "multiply key", 5, "plus equals key", I get 30, if I press the "plus equals key" again it adds the last number entered (5) and displays 35. Repeated pressing of the "plus equals key" does nothing to the total displayed. Does this sound right for a calculator of this vintage? Thanks David |
24th Jan 2012, 4:25 pm | #7 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Faulty Monroe 920 nixie tube calculator
These devices are basically just an electronic version of a mechanical adding machine. "+=" is equivalent to turning the crank forwards (adding to the accumulator), and "-=" is equivalent to turning it backwards (subtracting from the accumulator).
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