HP-3400A RMS Voltmeter

Elektronik, Messen
Page content

HP-3400A is a high quality AC voltmeter that operates in range 10Hz..10Mhz. It calculates RMS values by use of a cross thermocouple element. Using this mechanism, signals can be arbitrary and not only simple sinus signals.

AC input can be in range 10mV .. 300 Volts, or -60dB .. +50dB.

Serial number of this device starts with 1207G, manufactured in Germany. From date codes inside, I suppose meter is from 1974 or 1975. From serial number, the design date is 1972 (12=years since 1960), week 07.

A second one, from around 1965, is described here. This description includes also a repair/replacement of the chopper amplifier.

Images

Meter showing 775mV, which is 0dB.

Details from inside

Bottom vew:

The PCB with the Nuvistor tube 7586 is not fitted by screws. To keep the Nuvistor save, some foam is used. The foam has become dust. I replaced it with some new foam.

With new foam pieces:

PCB with Nuvistor tube 7586:

Close view of Nuvistor:

The black capacitor is already a replacement from some repair in the past.

The thermocoupler are visible at the left, TC401,TC402

Repair

The foam that holds the Nuvistor PCB was replaced. The neon bulb was close to not light emitting anymore. Someone already had fiddled around the bulb and the connections were also not soldered anymore.

I replaced that with a solution with a red LED together a 100K resistor and a diode 1N4007. The result is close to original, and I reused the lamp fitting for that. The original neon bulb together with its 33K resistor was kept in a safe place inside the device, if someone I intend to revert that modernization later.

Checking capacitors showed that on capacitor, C710, has 85 instead of 100µF. Because the meter still seems to work fine, I did not replace the cap.

Performance tests

I did some testing with the device.

Input from a Rigol DG811 signal generator, sinus, 400 Hz.

Output DG811 [mV] Real output Ausgang DG811 [mV] Value read at HP3400 Range [dB]
3.5V 7V 6.85V +20
3V 6V 5.9V
1.5V 3V 2.95V +10
500 1V 0.95V
150 300 290 0
150 300 295 -10
50 100 95
50 100 100 -20
25 50 50
15 30 30
15 30 30 -30
10 20 20
5 10 10
5 10 10 -40
4 8 8
3 6 6
2 4 4
1 2 2
1 2 2 -50
500µV 1000µV 1120µV
400µV 800µV 960µV

For smallest ranges (-50dB and -60dB), measurement setup is crucial. In another test, I used 50 Ohms resistor directly at HP3400A and got some stable measurements in -60dB range. These are off about 2.5dB, so even that setup is not the best way.

Referenzen