HP 428B Clip-On DC Milliammeter (1970)


I bought this Hewlett Packard 428B DC milliammeter for one purpose: measuring the cathode current on the horizontal output tube of a vintage television.

I had restored a number of old TVs before I took the time to make that measurement. Particularly with a black and white set, if you replace the capacitors and get it working normally, chances are that the HOT cathode current is just fine.

For color televisions, however, like my RCA CT-100 or CTC-7, the cathode current is worth checking. Color TVs employ very high voltage and it's important to achieve the correct HV level, producing a good picture without frying the associated tubes or causing other mayhem.

The 428B is a specialized device and it has few controls. Here's a view of the spartan front panel.

Not much there, apart from a big honkin' meter, a range control, a zero-set control, and an on/off switch.

Using the meter is simplicity itself. After choosing the desired range, you slip the clip-on probe over the wire carrying the DC current that you want to measure, and then read the meter. In the following photo, I have opened the probe's jaws, as you would do before placing it on the tested wire.

Here's a photo of the 428B in use, measuring the HOT cathode current on my Philco Miss America television. I placed a mirror to show where the probe has been clipped, around the plate lead of the 6DQ6GTA horizontal output tube.

Set to its 300ma range, the meter shows that we're nicely within the 100ma-140ma range specified in the schematic. On this particular TV, the Width control directly affects the 6DQ6GTA's output, and thus, its current draw, so adjusting it couldn't be easier.

Essentially, you want to set the current as low as possible, consistent with adequate high-voltage output. Excessive current may cause overheating and catastrophic failure of the horizontal output tube and/or flyback transformer.

Flybacks are expensive to replace and some are almost impossible to find. By controlling the cathode current, you can keep these critical parts happy and long-lived.

Of course, with high voltage in the neighborhood, you want to make sure that the insulation on the HOT lead is in good condition, and avoid getting zapped through carelessness.

Since the current flow is directional, there is a right way and a wrong way to put the probe on a wire. The little red arrows on the probe head must point in the direction of current flow.

If you're curious about how this gizmo works, the manual is available at the hparchive.com website. In a nutshell, it measures the magnetic field around a wire carrying DC current, using a specially designed "second harmonic flux gate type of magnetometer" as its probe.

Fortunately, my 428B doesn't seem to need calibration. Before using your 428B, take a couple of minutes to degauss the probe as explained in the manual.

I paid $36 for my 428B on the used market. These were quite expensive when new, so expect to pay more if you buy from a dealer who guarantees that it is working and calibrated. Avoid buying one without the correct probe. These specialized probes are not something you can cobble together from junkbox parts.

I like the quality of Hewlett-Packard instruments. I own one other HP device, a model 200CD audio generator.

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