Philips PM-2420 nixie DMM


A very nice Philips meter, probably made in 1968. It is a digital meter with nixies as display. A big contrast with my HP-410B, a huge vacuum tube equipped analog meter build in the same year. 

The meter only has 1 IC, a Philbrick-nexus opamp:

The SQ-10a from Philbrick

The SQ-10a is a general purpose opamp from Philbrick. 

From the 1972 Teledyne Philbrick catalog about their general purpose opamps

Philbrick had undergone several name changes. In 1968 it was Philbrick/nexus, in 1972 Teledyne Philbrick. 


The meter is a so called RAMP meter. There are two one-shot multivibrators that charge two caps. The ramps are compared against the test signal and earth. The result is a signal with variable duty cycle proportional to the scan time. This gives a signal ” modulated” with pulses. In the video you can see scope measurements from the ADC. A counter counts those pulses and shows them on the display. 


For instance a 10V signal gives a slope of 1V/ms, the clock is 390kHz, the counter counts 10ms and the result is 1000 pulses and the display showes 1000. The range switch sets the digital point.

The flip flops are special, they are made as modules:

A flip flop. You can see two transistors.

The counter with a bunch of FF modules

The meter is well build with very nice details and parts. Like the PCB lock construction on the next picture.

PCB lock pin

Even the screws are luxury with a tapered chrome washer and copper lockring.

Right side

Left side

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