1864

General Radio 1864

Megohmmeter

Available from 1970 to the present day

Manuals

Catalog History

Document	Year	Page
Catalog U	1970	98
Catalog73	1973	182

(All manuals in PDF format unless noted otherwise)

The GR Type 1864 Megohmmeter was introduced in Catalog U (1970). It is still available from IET Labs as of 2024.

Specifications

Test Voltage: 10 V to 109 V in 1 V steps, 100 V to 1090 V in 10 V steps; 5 mA limit
Resistance Range: 50 kΩ to 200 TΩ in 8 ranges
Power: 100-125 or 200-250 V_AC, 50-400 Hz

The Type 1863 Megohmmeter is similar but with a 20 TΩ top range, and a choice of only five test voltages (50, 100, 100, 250, or 500 V).

The 1864 is solid-state except for the test voltage supply pass element, a 6AB4 (EC92) vacuum tube.

Current through the unknown is converted to a voltage using decade reference resistors from 2 kΩ to 2 GΩ.

In the highest range (10 TΩ multiplier), feedback is used to multiply the effective reference resistance by a factor of 10.

A four-stage, FET-input amplifier drives an analog meter that is unusual in that its mechanical zero position is on the right end of the scale.