HP4328A Milliohmmeter Overview & Probe Construction


This is a general overview of the HP4328A milliohmmeter.  This is an analog meter from the late-1970s which is used to accurately measure low resistances.  It uses the standard 4-wire Kelvin bridge method (separate voltage and current sense probes) to obtain the highest accuracy.

An extra wire connection in the bridge circuit is used for feedback to maintain the oscillator drive at a constant current.  This limits the power (heat) dissipation required by the device under test, making it possible to accurately measure heat-sensitive devices.

The HP4328A uses a 1 kHz (AC) signal and a phase detector to help avoid thermal differences, contact potential differences, electrolytic polarization measurement errors, and even DC offsets up to +/- 150 volts.  The device essentially synchronous demodulates the oscillator bias signal and determines the final resistance measurement as a phase-sensitive voltmeter.

Even expensive modern multimeters, like the Agilent 34401, still have problems measuring very-low resistances because they only perform DC resistive measurements.  The dissimilar metals in the device being tested generate a very small DC voltage which will effect the final resistance measurement.  The HP4328A is essentially immunne to this problem.

It should be noted that since the HP4328A uses a 1 kHz bias signal for measurement, the wire length of the contact probes comes into play during resistance measurements because of their inherent internal reactance.  The maximum probe length for the very low-milliohm settings is only around 18 inches or so.

The operating manual for the HP4328A has a little chart you should consulte for more information before doing such measurements.  But for just playing around, you shouldn't be too worried.

Pictures & Construction Notes

Input probe connector for the HP4328A milliohmmeter.

This connector is a Hirose RM12BRD-5S (female) and the mating connector is a Hirose RM12BPE-5PH (male).

The Hirose RM12BPE-5PH is available from Digi-Key (Part Number: HR1732-ND) for around $13.

The pinout is as follows:

Pin       Description                       HP4328A Internal Wire Color
1         Voltage at Current Source         Yellow
2         Control of Current Source         Green
3         Current Source                    Black
4         Voltage at Current Sink           Red
5         Current Sink / Shield             Shield

To measure a resistor under test, pins 1 & 3 should be connected to one side and pins 2, 4, & 5 should be connected to the other.

Soldering the new probe wires to the Hirose RM12BPE-5PH connector.

The wire shown above is Carol/General Cable C1352A 2-pair, #22 gauge, with a #24 gauge drain wire for the shield.  You don't have to use the exact wire, but having the shield is handy as that should be the Current Sink connection and will help shield the internal wires from external interference.

A few layers of heatshrink tubing were used to build up the diameter of the cable so the metal clamp on the Hirose connector would make better contact.

Testing the new Hirose RM12BPE-5PH connector on the HP4328A milliohmmeter.

The wiring for this homemade probe cable is as follows:

Pin       Description                     Carol/General Cable C1352A Wire Color
1         Voltage at Current Source       White
2         Control of Current Source       Black
3         Current Source                  Green
4         Voltage at Current Sink         Red
5         Current Sink / Shield           Drain Wire (Shield)

Internal overview of the HP4328A milliohmmeter with the top cover removed.

The green/yellow/red/black/shield wire bundle on the top-right is from the probe connector on the front panel.

You can use the shield wire for the common ground point when taking measurements for maintenance of the HP4328A milliohmmeter.

Internal overview of the HP4328A milliohmmeter with the left cover removed.

The case of transistor the arrow is pointing to can be used as a test point to check the oscillator bias.  It should be around +3.7 VDC.

Internal overview of the HP4328A milliohmmeter with the right cover removed.

The four potentiometers are adjusted to set the electrical zero adjustment (R228), full-scale adjustment (R229), recorder output adjustment (R233), and linearity adjustment (R232).

C208 is for the 1 milliohm setting phase adjustment and can be left alone.

Checking the bias on the oscillator output transistor.

With the probe leads shorted together, adjust R360 (oscillator bias adjustment) until you get +3.7 VDC at the collector of Q302.

Note that the "Maintenance" section of the service manual says +3.7 VDC while the "Schematics" section says +3.3 VDC.  +3.7 VDC appears to be the correct bias voltage.

Checking the oscillator output level.

Set the HP4328A to the 100 ohm range.

With the probe leads shorted together, adjust R306 (oscillator level adjustment) and R360, alternately, for a voltage of 200 mV peak-to-peak at the output of T301.  Check the level with the terminals marked "BLU" and "BLK."

Connect your oscilloscope probe ground to the BLK terminal and the probe tip to the BLU terminal.

The oscillator frequency should be around 1000 Hz, +/- 100 Hz.

Overview of the homebrew probe for the HP4328A milliohmmeter.

The four main connections are brought out to banana plugs to allow for connection of alligator clips or test probes.

The Control of Current Source wire is tied to the Current Sink wire at the banana plug.

Commercial HP probes for the HP4328A milliohmmeter are the HP16005A, HP16006A, and HP16007A/B, but are fairly difficult to track down.

GBPPR Vision #31: HP4328A Milliohmmeter Overview & Probe Construction

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