Early
Electronic
Calculator		 Miida MC840 Calculator

Manufacturer: Miida
Model: MC840
Year: 1972
Form: Desktop
Functions: Basic four, constant
Number of Digits: 8
Display Type: VFM
Display:ISE DP82A
Logic Technology: LSI-3
Diodes: 10
Transistors: 5
IC Complement:OMRON / Hitachi HD32104
OMRON / Hitachi HD32105
OMRON / Hitachi HD32106
Toshiba TM4352 (3)
(6 total)
Tech. Data Level: 2
Tech. Data Source: RE
Tech. Data Pages: 1
Tech. Data:Schematic (pdf)

A straighforward little desktop in terms of functionality, but a sleek and stylish wedge-shape case design. The same case is shared by the MC1214.

Electronically, this model is very similar to a predecessor, the MC800. The chipsets are nearly identical and the keyboard construction is the same. The display tubes and display drivers are different.

Notes:
  • The keyboard of these calculators, as well as that of the MC800, is susceptible to problems with the keys binding. The source of the problem is the nylon plungers on which the keycaps sit, which split and expand and consequently bind against the cylinders in the keyboard base.

    The nylon of the plungers is difficult to work, but for some degrees of this problem, a light sanding or buffing of the plunger or the cylinder wall can improve the situation. To do so entails a fair amount of dismantling to separate the keyboard circuit board (to which the delicate reed switches are mounted) from the keyboard base.

    Removing the keycap from the plunger can be quite tough due to the wedge at the top. Depending upon access to the key it may be possible to pry the key off with two screwdrivers, or an IC remover or similar tool with two hooks might be used to pull on the keycap.

    If the key only binds near the bottom of it's travel, it may be possible to work the cylinder wall without having to remove the plunger.

    Note there is a slight lip at the bottom of the plunger which is the first area to bind.



Internal view.
Logic board removed, exposing the back of the display and keyboard.
The logic board with the Hitachi-produced versions of the Omron ALPHA chipset. The two blue IC sockets plug into the VF display module.

Logic board, solder side.
Many calculators of this era included a mini-user-manual on the bottom showing the keystrokes for basic operations.

- Unit Log -

Serial Number: 722638
Year of Manufacture: 1972 (ICs stamped with 2G,2H)
Date of Receipt: Jan 2000
Source: Haggerty warehouse.
State upon Receipt: Keys stick down.
Current State: Fully functional (Feb 2000).

Date: Feb 2000
Procedure: Cleaned.

Date: Feb 2000
Procedure: Black foam compression pad on top of the keyboard base had turned to goo. Keyboard dismantled completely, goo cleaned off, pad replaced with sheet rubber from inner tube.

Date: Feb 2000
Symptoms: Keys stick down due to plungers having cracked and expanded.
Solution: Some key plungers and cylinders sanded lightly to eliminate binding.

Date: Feb 2000
Symptoms: Display intermittently disappears. Powering off/on usually brings it back.
Solution: Solder connections around clock circuitry reflowed.

Date: Jan 2021
Symptoms: Display dims or flickers intermittently.
Procedure: V-9 observed to be sagging several volts in sync with Φ1 low state. This is consistent with PMOS transistors: negative gate pulse turning transistors on, increasing Vdd current. Several ohms of resistance around V-9 C and R solder connections, resoldered. Resistance gone, V-9 now stable. May not be solution though. Consider power switch contacts.

Date: Jan 2021
Symptoms: Divide key binding some.


  Miida MC840
Calculators | Integrated Circuits | Displays | Simulations
EEC 		bhilpert