Vintage Audio: Heath IT-28 Capacitor Checker
This edition of Found in the Attic might better be called Found on the Workbench, because that is where this item lives. The Heathkit IT-28 capacitor checker is an incredibly useful device, particularly when working on vintage gear. Most modern test equipment, even inexpensive items, can run circles around older test and measurement devices. But there are a few items that have a timeless design that enables them to thrive in the digital age. The IT-28 is one of them; there are things you can do with it that can't be done with digital devices. Of course, the IT-28 is a vintage device itself, dating from the late 1960s.
One of the cool things about this capacitor checker is its ability to measure leakage at the device's working voltage, from 3 to 600 volts. The capacitor test function on most of today's DMM's simply can't do that.
Capacitors have a certain mystique to them. Beyond the basic specifications, some are known for long shelf life and reliability. Some types and brands are favored by audiophiles for the quality of the sound they produce in amplifiers. Others, like the 'black beauties' in vintage Tektronix equipment, spell trouble, and are usually replaced on sight. Most of us tend to hoard good, quality caps for future construction and restoration projects. Who knows how long they've been around?
Checking every cap for leakage before installation can seem tedious and time consuming, but it can save time in the long run. Some caps are OK at 200 volts, but totally fail leakage tests at their rated 600 volts. On most projects involving a couple dozen capacitors, testing with the IT-28 reveals at least one that is headed for the trash can. Leakage tests vary depending on the type of capacitor. The IT-28's front panel switch selects 'electrolytics,' 'miniature electrolytics,' or 'paper, mylar, etc.'
The Heath is also useful in reforming electrolytic capacitors. After sitting on the shelf for a long time, they may not react well to full working voltage. It's better to start with a low voltage, monitor the eye tube for leakage, and gradually sneak the voltage up to the rated value. This procedure may take a couple hours, or a couple days.
Of course, there are some things the newer DMM capacitor tests can do much better than the Heath, notably precision measurements of capacitance. For most consumer and broadcast electronics applications, capacitor tolerances of +/- 20 percent are fine. The Heath is reasonably accurate with the balance control around mid-scale, but notoriously inaccurate at the far ends of the scale.
Heath was not the only manufacturer of capacitor checkers with a magic eye tube. Hickok, Eico, Lafayette, Knight, and others offered similar devices, all with the same basic principles of operation.
The checker is designed around an AC bridge circuit, with a balance control being used to vary the resistance of two arms of the bridge. The third arm is the standard (internal or external), and the fourth is the device under test (DUT). At the opposite end of the bridge is the eye tube circuit. In the case of the Heath, there is a 6BN8 amplifier stage ahead of the eye tube to give a sharper null. Although its primary function is measuring capacitance, it can also measure resistance and inductance by substituting precision resistors or inductors on the EXT STD terminals. When the bridge circuit is perfectly balanced, the voltage will be at a minimum, and the eye tube will fully open.
The IT-28 has simple circuitry and is easy to work on. Bringing one back from the dead isn't too difficult. First off, most Heaths were assembled as kits, and if you were not the one who built it, assume nothing. If some of the solder joints look iffy, it may be worthwhile to reheat all of them. Hit all the switch contacts with Deoxit, and replace the power supply electrolytics. While you're at it, check the .02 and 2.0 uf standards caps.
The solder joints on the voltage switch can be particularly troublesome. Best practice is to check the 16 voltages from 3 to 600 volts first. This will shake out both bad solder connections and resistors that have gone out of tolerance.
Recalibration of the IT-28 is quite simple. Heath has two options for leakage calibration, one with and one without a milliammeter. The milliammeter method is more accurate, as you are actually setting the leakage current to the eye closed for each of the three types of capacitors.
Over the years, Heath produced a number of capacitor testers. One of the first was the IT-22 Capaci-tester, which is pictured with the IT-28. It was a simple device with a magic eye, and could test for shorts and opens. In 1961, they released the IT-11, a much more versatile device, in the usual battleship grey color scheme. It was replaced in 1968 by the IT-28. The two units had almost identical circuitry, most of the differences are cosmetic. The 28 had the beige styling and newer knobs characteristic of 1970s Heath equipment. It was produced for nine years, the last ones were sold in 1977.
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