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What is purpose of the CH button and CH switch position on the Omron Elite 8002 Taschen-Rechner?

(For those who don't know German, “Taschen-Rechner” just means “calculator”.)

The switch has three positions: ON, OFF and CH. Only in the ON position is the calculator display lit up; in the CH position the calculator apparently does not do anything.

See the linked website for a photo of the calculator. I will provide a better photo later.

I've tried these things to get information about the CH button:

  • Key in number 123, press CH. Nothing happens. Press again. The VFD shows 0..

  • Key in 10, press CH (VFD still shows 10.), key in 20, press =. Nothing happens.

  • Calculate 200+100 (VFD shows 300.), key in 1000, press CH. The result is 700..

According to the last try, it looks like it subtracts the previous value from the currently entered one. (More experiments confirm that it works in this way, as a reverse-subtract.)

So, the question is: What is the “CH” button for (maybe it is in German) and what is the “CH” position of the switch for?

I do not have any manual and I do not have any idea what these “CH” labels could mean. There is also a connector that looks like a 2-pin 3.5 mm jack on the bottom. What is its purpose? Could it be related to these buttons?

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  • 1
    It's not the power connector?
    – Tomas By
    Oct 20, 2021 at 16:26
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    On-Off-Ch sounds like "charge"?
    – Tomas By
    Oct 20, 2021 at 16:27
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    I agree with thomas about the "ch" switch position. The CH button likely is for "ch"ange as in money you have to return. The third example is: Customer purchases an item for 100 lira and another item for 200 lira. The payment is made using a 1000 lira bill, how much CHange to return?
    – Michael Karcher
    Oct 20, 2021 at 17:05
  • 6
    This calculator uses the "CH" switch position to charge batteries: vintage-technology.club/pages/calculators/decimo/…
    – snips-n-snails
    Oct 20, 2021 at 17:21
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    BTW, Taschen-Rechner or Taschenrechner is not a brand or model; it's the German translation of "pocket calculator."
    – shoover
    Oct 21, 2021 at 15:47

1 Answer 1

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This is most likely a position for charging in case a rechargeable battery pack is used.

Keep in mind, these are the late 70s. A automatic charging electronic would have been rather high effort for back then and especially a low end calculator like this. Kaufhof was a large German department store targeting low to average income customers and "Elite" their in house brand.

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  • The charge position most likely just connects the DC input jack straight to the battery. If you are lucky, there is a current limiting resistor, but designs relying in the infernal impedance of the matching unregulated supply were also common those days. The rechargeable batteries likely were early NiCd cells (like 300mAh in AA size). Some devices also had small lead-acid cells back then
    – Michael Karcher
    Oct 20, 2021 at 19:37
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    Recharging was very much a thing considering the high power consumption of VFDs and early NMOS/TTL logic, and the low capacity of zink carbon cells. It's today's luxury that we can run an LCD calculator on a single CR2032 cell for years.
    – Michael Karcher
    Oct 20, 2021 at 19:41
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    @MichaelKarcher - I like the idea of 'infernal' impedance...
    – Jon Custer
    Oct 20, 2021 at 22:41
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    @JonCuster nice catch. That typo was not intended. For NiCd charging, they built special transformers with a magnetic shunt that actively increases output impedance without (significantly) reducing efficiency. This means the internal impedance doesn't need to create infernal heat, as internal ohmic resistance would.
    – Michael Karcher
    Oct 20, 2021 at 23:47
  • @MichaelKarcher Might all be, the point here is simply that adding charge control like today would be way expensive in the mid 1970s and way above the price tag of a Kaufhof device.
    – Raffzahn
    Oct 21, 2021 at 8:10

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