Digital Test
Heathkit IM-1202

Manufacturer: Heathkit
Model: IM-1202
Year: 1973
Schematic: MF,RE
Functions: DCV, ACV, Ohms, DCA, ACA
Display: NIXIE
Digits: 2.5
Logic: discrete transistor, TTL

A basic low-budget digital multimeter from the early 1970s. The digital logic portion of this instrument uses standard TTL ICs but the A/D converter is built from discrete components.

No auto-polarity or auto-ranging.

Sold for CDN$ 89.95 in kit form in 1974.

- Unit Log -

Serial Number: 05406
Year of Manufacture: 1974 (Component date stamps 7312 to 7410).
Other Identification: Sticker inside for DC Cal. Voltage is 1.879 Volts.
Date of Receipt: 1974 Jun 13
State upon Receipt: Received new in kit form.

Date: 199?
Procedure: Darlington transistor in 5V supply regulator faulty. Replaced with 7805 IC regulator. Trace cut to make ground connection while leaving calibration circuit intact.

Date: 1997 Jul 14
Symptoms: Unit intermittently latches up with the display stuck at zero.
Analysis: Problem eventually correlated with the switching on of the "+" polarity neon indicator lamp. This lamp is located in close proximity to the constant-current FET of the A/D converter circuit. Apparently the FET has developed some sensitivity to the field(s) emanating from the lamp. The change of the field(s) during the switching of the lamp triggers the FET into some latch-up state.
Solution: 2N5458 FET replaced with a TIS58.

Date: 1997 Jul
Procedure: Input polarity switching rewired from original. Simplified to reduce the number of poles used on the polarity switch.

Date: 1997 Jul
Procedure: Additional wire connectors installed to permit easier disassembly.

Date: 1997 Jul
Procedure: Voltage jack and Resistance/Current jack connected together for user convenience.

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Early Digital Test Equipment