Monroe 650 Calculator

Manufacturer: Monroe
Model: 650
Year: 1971
Form: Desktop
Functions: Basic four, constant, rounding, square root, 2 memories
Number of Digits: 16
Display Type: NIX
Display:Hitachi CD81
Logic Technology: LSI-7
Diodes: 118
Transistors: 59
IC Complement:Texas Instruments TMC1761NC, Texas Instruments TMC1763NC, Texas Instruments TMC1764NC, Texas Instruments TMC1765NC, Texas Instruments TMC1766NC, Texas Instruments TMC1768NC, Texas Instruments TMC1793BNC
(7 total)
Tech. Data Level: 2
Tech. Data Source: RE
Tech. Data Pages: 5
Tech. Data:Schematic (pdf)

The higher-end model of the Monroe 600 series.

  • OEM is Canon.

Interior view.

The logic board.

Display module with bezel.

The 650 has a unique power cord connector, different even than that of the Monroe 610.

- Unit Log -

Serial Number: 840435
Year of Manufacture: 1971 (stamp in case of 46.12.6, 1925+Showa46=1971)
Date of Receipt: 2015 Jun
Source: JP.
State upon Receipt: Fair physical condition, no display, with special cordset.
Current State: Fully functional (2015 Jun).

Date: 2015 Jun
Procedure: Case cleaned. The four screws holding the display bezel had been overtightened and stripped threads. Two replaced and two left out.

Date: 2015 Jun
Symptoms: Nixies do not illuminate, other neon bulbs flash briefly at power on. Sometimes OK - intermittent.
Analysis: Clock signal from master clock astable absent. Voltage measurements while not oscillating suggest 4 Rs are good. Hot / cold application to 2 transistors and 2 caps show heat on the 60pF cap stops oscillation.
Solution: 60pF ceramic cap replaced with 62pF polystyrene.

  Monroe 650
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