Early
Electronic
Calculator		 Monroe 925 Calculator

Manufacturer: Monroe
Model: 925
Year: 1970
Form: Desktop
Functions: Basic four, accumulator, constant
Number of Digits: 13
Display Type: NIX
Display:JRC B-5755
Logic Technology: SSI
Memory Technology: DELAY-MS
Diodes: 153
Transistors: 42
Principal ICs: SN3900/4500 Series
IC Complement:Texas Instruments SN3920 (7)
Texas Instruments SN3925 (26)
Texas Instruments SN3931 (11)
Texas Instruments SN4553 (4)
Texas Instruments SN4554 (21)
SN7400 (1)
(70 total)
Tech. Data Level: 2
Tech. Data Source: RE
Tech. Data Pages: 14
Tech. Data:list

Notes:

- Unit Log -

Serial Number: E017715
Year of Manufacture: 1970 (Acoustic Delay Line unit stamped with 08 Oct 1970)
Date of Receipt: 1970s
Source: Sears Service Department.
State upon Receipt: Good physical condition.

Date: Dec 1997
Analysis: Grounding of IC112 pin 7 intermittent due to poor solder on associated feedthru connecting the two sides of the board.
Solution: Resoldered.

Date: 31 Jan 2005
Symptoms: No display. Overflow lamp will light on powerup sometimes and can be cleared by C key but nixies are dark.
Analysis: Clock signals disappear at slice counter. Feedthru stub connecting 110-6 to trigger caps for 110-10,13 have bad solder connection on solder side.
Solution: Resoldered.

Date: 04 Feb 2005
Symptoms: Arithmetic operations intermittently produce incorrect results. Key sequence "1+" results in display of "1.88".
Analysis: When "1" is pressed, equivalent bits show up in YABC registers. Speculate that M pulses for X are being injected into delay line at wrong slice times, perhaps due to problem around diode gate feeding W1 during slices 123. Scope indicates intermittent problem with not-phase-S4 feeding diode gate. Confirmed on slice level simulation: forcing input TRUE (open) produces identical results.
Solution: Probably bad feedthru, not confirmed yet.


  Monroe 925
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EEC 		bhilpert