Retro electronics (or “retronics”) projects are growing in popularity. Across the globe, professional engineers and DIYers alike are tweaking, updating, and hacking retro systems to create all sorts of innovative designs. Restoring and upgrading an old electronics tool, MCU-based design, or audio system can be a rewarding experience.

In the February 2012 issue of audioXpress magazine, Bill Reeve details how he restored a Hewlett-Packard 456A current probe (“Restoring the HP 456A Current Probe”). Here’s an abridged excerpt:

The restoration is finished and ready for cover installation (Soure: Bill Reeve AX 2/12)

The Hewlett-Packard 456A AC current probe is a treasure. It can be bought cheaply because many of the units sold were battery powered and all were designed with a now-out-of-date oscilloscope interface connector. However, when restored, the 456A is a fabulous addition to any test bench, matching the performance of more expensive modern instruments.

Released as a new product by the Hewlett-Packard Company in 1960, the 456A was HP’s first solid-state, stand-alone, clip-on current probe. Its elegantly designed amplifier uses two— then “state-of-the art”—PNP germanium transistors.

The Original Probe
In 1960, The Hewlett-Packard Journal (July-August, Vol. 11) proudly announced:

“This new probe measures current over the full range of the frequencies most commonly used in typical work—25~ to 20 megacycles—and over an amplitude range from below 0.5 mA to 1 A rms…The probe operates with an accompanying small amplifier…to convert the AC current being measured to a proportional voltage. This voltage can then be measured with a suitable oscilloscope or voltmeter. The current-to-voltage conversion factor is 1 mV/mA.”

The 456A operating and service manual is available at www.hparchive.com, but this scanned copy contains incorrectly annotated schematic values for R7 (should be 3300 Ω), R8 (should be 2700 Ω) and C5 (should be 0.01 μF).

Old battery-powered 456As are usually in excellent physical shape because when their batteries ran down these instruments were often shelved and forgotten. Another 456A advantage is that its probe head is wired directly to the amplifier, so they cannot be separated by surplus electronics dealers.

Restoration
Restoration of the 456A consists of three steps: replacing the old battery pack with DC power, restoring the amplifier electronics, and converting the obsolete oscilloscope banana plug interface to a BNC connector.

Step 1: Replace the old battery pack. Remove the two Phillips-head screws on the housing back to slide off the 456A’s cover. Re-thread the screws into the frame to keep them from getting lost. ….

Step 2: Restore the amplifier electronics. At this point, if you are happy with your current probe’s performance, you can skip the following upgrades, but these are five modifications you might need to perform to get your 456A working or improve its performance:

• Replace the electrolytic capacitors
• Replace the two germanium transistors
• Replace the 8-V breakdown diode (CR1)
• AC-couple the output
• Flow solder onto the printed circuit traces

Photo 6 is an annotated close-up of the amplifier’s single-sided printed circuit board. Following vacuum tube circuit convention, the +5 V is labeled “B+” and the –8 V is labeled “B–”. There are three electrolytic capacitors in the amplifier (see the horizontal silver cylinders in Photo 6), and their replacement is straightforward. ….

Photo 6: The amplifier's original printed circuit board (Source: Bill Reeve AX 2/12)

Step 3: Convert the oscilloscope interface to a BNC connector. This final modification can be performed one of three ways. Pomona electronics (visit the website pomonaelectronics.com) sells a female banana to male BNC adapter (Model 1296). You can cut the banana plug connector off the existing cable and attach a male BNC connector. This requires special tools.

You can replace the output cable with coax having one BNC end. This is a straightforward replacement. Photo 9 shows the new BNC output cable. …

Photo 9: BNC ouput cable installed (Source: Bill Reeve AX 2/12)

This restoration should make your 456A ready for another 50 years of service.

Note: The complete article appears in the February 2012 issue of audioXpress magazine. audioXpress magazine, like Circuit Cellar, is an Elektor group publication.

C. J. Abate (152 Posts)