Heathkit IT-1 Isolation Transformer Meter Repair

A vintage isolation transformer whose panel meter had quit, and an afternoon spent fitting a replacement.

A gray Heathkit IT-1 isolation transformer with a new round panel voltmeter, beside a Fluke multimeter reading 120.2 volts AC; the old square meter sits on the bench in front — The new meter mounted, tested and working: 120 volts AC on both the analog meter and the Fluke

I spent a bit of an afternoon in December 2025 replacing the voltage meter on an older Heathkit IT-1 Isolation Transformer with variable AC voltage. An isolation transformer separates the equipment on the bench from the wiring of the wall outlet, which makes it safer to work on old radios and test gear. This one also has a knob to raise or lower the output voltage, and a meter on the front to show what it is set to. Heathkit produced the IT-1 between 1953 and 1960.

1953–1960years Heathkit produced the IT-1

0–150 V ACrange of the old and the new meter

120.2 Von the Fluke, matching the new meter

100 Wrating (200 W intermittent)

1. A meter that always pegs

The meter had stopped responding. Whatever the output was, the needle would always peg to the right. In the first photo the actual output is 98.1 volts AC on the Fluke, but the old panel meter reads more than 150 volts.

The Heathkit IT-1 with its original square panel meter pegged past 150 volts, beside a Fluke multimeter reading 98.1 volts AC — The actual output is 98.1 V AC on the Fluke, but the old panel meter reads over 150 V

The front panel removed and lying face down, showing the transformer with its copper windings and the back of the old meter — Inside the unit, showing the actual transformer windings

2. Inside the old meter

This type of meter doesn't have any mechanical adjustment points. It is calibrated with an array of resistors. I tried a few different resistor values without luck: the needle still pegged right.

The old EMICO 0 to 150 volt AC panel meter taken apart on a bench mat: the dial and movement, the bezel and the glass — The old panel meter disassembled

The back of the old meter movement with two green resistors soldered across it — The panel meter's resistor array

The old meter movement seen from the side, standing next to its black case — Side profile of the old meter

3. A replacement that fits the hole

I found a suitable replacement meter online that would fit the cutout. It mounts differently, though, so I had to drill some holes around the opening to mount it.

The IT-1 front panel with an empty meter cutout and new holes drilled around it, a cordless drill behind it, the old square meter on the left and the new round meter on the right — Old meter on the left, new meter on the right, with the holes drilled around the opening to mount the new one

4. Tested and working

With the new meter mounted, I tested it against the Fluke. Both show 120 volts AC: the analog panel meter and the digital multimeter agree.

Close-up of the new round panel meter with its needle at 120 volts, next to the Fluke display reading 120.2 volts AC — Close-up of the two meter displays

About the IT-1

Heathkit introduced the IT-1 as a kit in 1953 and produced it through 1960. In the 1956 catalog it sold for $16.50. It is small, about 8 inches high, 4¼ inches wide and 4¾ inches deep, so it takes up very little room on the workbench. The Voltage knob adjusts the output between about 90 and 130 volts AC, and a 2 amp slow-blow fuse protects it.

To keep the price down, Heath used an inexpensive meter with no damping. Its scale is stretched out so that the useful range, 90 to 130 volts, is easy to read. You can see that on my old meter's dial, where 0 and 50 sit right next to each other. The replacement meter has an ordinary, evenly spaced scale.

What an isolation transformer like this is good for:

Working more safely on old AC/DC sets, such as the five-tube AM radios once found in most homes, where the metal chassis can be connected straight to the power line
Plugging in newly acquired or newly built equipment for the first time
Tracking down problems that only show up when the line voltage is low or high
A safe, adjustable source for checking AC voltmeters

The history and specifications here come from Heathkit of the Month #18: the IT-1 AC Isolation Transformer (PDF) by Bob Eckweiler, AF6C, first published in the November 2009 issue of RF, the newsletter of the Orange County Amateur Radio Club (W6ZE). It also explains in detail why an isolation transformer makes bench work safer.

Parts

Heathkit Model IT-1 isolation transformer, rated 100 watts (200 watts intermittent)
Original meter: EMICO 0–150 V AC panel meter (Electro-Mechanical Instrument Co.)
Replacement: CHHUA DH-52 round 0–150 V AC panel meter
Tools: a drill for the new mounting holes and a Fluke 233 multimeter to check the result