HP 456A AC Current Probe

I wanted a simple project to take a break from working on my RCA WV-98C VTVMs. I’ve had a Hewlett Packard 456A AC Current Probe sitting around for a while. 


The 456A is a clip-on AC current probe. It is comprised of a clip-on probe and a current-to-voltage amplifier and converter. It has a ±2% accuracy bandwidth of 100 Hz to 3 MHz and a 3 dB bandwidth of 25 Hz to 20 MHz. Its dynamic range is from “below 1/2 mA" up to 1 A RMS. The current-to-voltage conversion ratio is 1 mV per mA. It can be used with a scope or an AC voltmeter such as the -hp- 400D.


The 456A was introduced in the July/August 1960 issue of the HP Journal and first appeared in the 1960 HP catalog.


A scan of the 456A manual is available at the HP Archive.


The probe itself is similar if not identical to the current probe in the HP 428A DC Clip-on Milliammeter, which was introduced in the June/July 1958 issue of the HP Journal. HP later produced an AC version as a plug-in for the HP 150A oscilloscope. This was introduced in the May/June 1959 issue of the HP Journal.

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The 456A was powered by two Mallory TR 233R and one TR 234 mercury batteries. Mercury batteries are no longer available due to environmental concerns. They are a bit of a problem to replace, as they are an odd size and an odd voltage. The TR 234 battery supplies +5.33 volts to the circuit and the two TR 233R batteries supply -8.0 volts.

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An AC power supply was available as an option, but mine came with the battery supply. The AC supply raises the noise floor from less than 50 µA on batteries to 100 µA on AC power.


The circuit is pretty interesting. It uses two OC170 germanium transistors. These apparently suffer from the internal growth of tin whiskers that short out the transistor.

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I checked out the passive components, and all were fine except for C2, which was leaky, and C6, which had a high ESR, presumably because it had dried out. I replaced those two electrolytic capacitors and powered the circuit from my HP 6227B dual power supply. It functioned fine, so the transistors are still good. It’s just a matter of time, though, before they get tin-whisker disease. 

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To accommodate the optional AC supply, the battery holders are mounted on a removable plate, as is the AC power supply. 

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I removed the battery holders and designed a power supply using two modern 9 Volt batteries

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I used a single Linear Technology LT1120A micropower regulator in the supply. I built the circuit dead-bug style on a piece of PC board cut to the same size as the battery mounting plate. I soldered a couple of nickel-plated nuts to the board in lieu of real captive nuts.

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I measured the power draw at about 4.5 mA on both the +5.33 and -8 Volt rails. I didn’t measure the draw on the 0 Volt rail, but it can’t be more than a 100 µA or so. A post on the internet purporting to relay information from David DiGiacomo says it’s 20 µA. Rather than use two LT1120As, I took advantage of the data-sheet application information to use the comparator as an op-amp to generate the zero Volt rail.

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One trick I used was to rewire the power switch to switch the connection between the negative battery voltage and the -8 Volt circuit common for the regulator.


Well, the current probe head is reading a resistance of about 550 ohms. From the schematic, it looks like it should be less than 96 ohms. So I think I've got an open circuit in at least one of the paralleled 24 ohm resistors, or in the leads to the probe head coils. I doubt it is repairable, so I'm looking for a new probe head. 

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While the probe looks externally identical to that of the HP 428A and 428B, internally it is different, so I'll need a genuine HP 456A probe head.


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I managed to open up the probe head by driving out the split pin with a pin punch and a hexagonal anvil that I bought from Micro-Mark many years ago.

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The split pin (aka roll pin) drove out quite easily.

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Here’s one side of the PCB in the probe head. It has the two resistors for the lower jaw. The black wire is the common connection between the lower and upper jaws. Note the crack in the 24 Ohm resistor circled in red. This resistor and its paralleled coil read over 18 KΩ. Both the resistor and the coil are bad.

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This is a view of the other side of the PCB, with the other two resistors.

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In this photo, I’ve separated the PCB and the insert for the Microdot connector from the outer shell.

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This is the center pin and insulator of the Microdot connector.

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The photo is a bit out of focus, but you can see there’s a big divot out of the ferrite core of the upper jaw.

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There's a smaller defect in the other jaw.

The HP manual warns you to insulate the wire that’s being measured, if it is bare. One side of the probe coil is grounded to the cable shield, which in turn is grounded to whatever instrument the probe is plugged in to (remember, it’s battery powered, so there’s no power line ground). The probe coils also have a grounded copper shield around them.

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My guess is that someone neglected to follow the advice in the manual, and hooked the probe up to a bare wire carrying a significant amount of energy. The subsequent sparks then blasted a divot in the ferrite, fractured the resistor, and burned out the grounded end of the coil.

I replaced the resistor with a 22 Ω resistor. The leads on my precision 23.7 Ω resistors were too big fto fit in the PCB holes, so I had to compromise. I lifted the defective probe coil away from the jaw and took a look, but it is encapsulated, so there’s no way to replace the burnt out coil. I thought I’d give it a try even with only three out of the four coils working.

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I made up a current transformer and 10 ohm load per the directions in figure 4-2 in the manual.

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I made the transformer from ten turns of 30 gauge wire-wrap wire. I threaded the wire through four small sections of 1/16” heatshrink tubing (I should have used 3/32” instead, the 1/16” was a tight fit). I then shrunk the tubing with my heat-gun and fixed the windings in place with some thin cyanoacrylate glue. Finally, I mounted the coil on a small plastic box from my junk box, using thick cyanoacrylate glue to fix it in place. 

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I connected the coil to two banana jacks at one end of the box.

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For the 10 Ω load, I used a precision 10 Ω 0.1% CMF55 metal film resistor mounted on a Pomona double banana plug with solder turrets, model 1330-ST-0.

I calibrated the 456A using my HP 652A Test Oscillator and my HP 3456A Digital Voltmeter. I cranked the gain in the 456A amplifier up all the way, but it still reads about 10% low. I may try fiddling with the value of R12 to get the gain a bit higher.

Even with the calibration being a bit off, the 456A will still be a useful instrument.


Raising the gain turned out to be quite easy. R9, the 100 Ω gain calibration pot is paralleled by R10, a 100 Ω carbon composition resistor. I lifted one end of R10, and was able to bring the 456A into calibration at 1 KHz. I've yet to check the frequency response, but I expect the upper end will be short of spec due to the extra gain.


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The frequency response is 3 dB down at 7.2 MHz rather than at the specified “greater than 20 MHz”. So while the repaired probe is usable, it would be nice to obtain a replacement.

The measurement was a little tricky because the inductance of the 10 turn coil starts to become significant. I had to keep an eye on the voltage across the 10 Ω load and keep raising the output from the signal generator as I went higher in frequency, to keep the current through the coil at a constant 10 mA.

I haven’t measured the self-resonant frequency of the 10 turn coil, but I suspect it may be low enough that part of the response droop is due to some of the current flowing between ends of the coil via the parasitic capacitance of the coil. So my measured results may be somewhat pessimistic.


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I measured the self-resonant frequency to be 21.35 MHz using my Measurements, Inc. Model 59 Megacycle Meter. (Thanks Peter!) The Model 59 is basically a high-quality grid-dip meter. 

I verified the measurement using my Heathkit HD-1250 Solid-State Dip Meter. It’s frequency calibration is nowhere near as good as the Measurements, but it did show a dip at about 21 MHz.

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Next, I used my DE-5000 LCR meter to measure the inductance of the current-sense coil at 1 KHz ...

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… and at 100 KHz.

Given the self-resonant frequency of 21.35 MHz and the measured inductance of 6.247 µH, I calculated the effective parallel capacitance as 8.90 pF.

© Steve Byan 2011-2019