
Restoring a Heathkit IT-11 Capacitor Checker for use at my local Makerspace.
One of our Makerspace members donated an old Heathkit capacitor checker that had belonged to his father for use at the makerspace. It was generally in good shape, but had bit of dust and grime buildup from many years of use.
The Heathkit IT-11 was built from 1961 to 1968 when it was replaced with the IT-28 which had a beige/tan front panel. This unit had several capacitors dated early 1963, so this unit is nearly 60 years old.
Capabilities include capacitance (4 ranges), resistance (3 ranges), capacitor leakage (3 to 600 VDC 16 steps), and using external standards, inductance, transformer turns ratio, additional capacitance and resistance ranges. There are three leakage settings for different types of capacitors including Electrolytic 2 mA, Min ‘lytic 15 uA, and Paper Mica Etc. of 2 uA.
The checker uses a “Magic Eye” electron-ray indicator tube for visualizing when the bridge circuit is balanced, and a reading can then read from the balance dial indicator.

The first part of the refurmishment was cleaning the case, front panel, line cord, and knobs. After disassembling the unit I used a mild citrus based cleaner and toothbrush on the case, knobs, and line cord with a final water rinse.

The front panel was cleaned using the same mild citrus based cleaner on a cloth towel to prevent any cleaner from getting into electronics or knob bushings, with a water dampened towel wipe to remove any remaining cleaner.
Next up was cleaning all the switch contacts and potentiometers with DeoxIt D5 contact cleaner. The vacuum tube pins and sockets were also cleaned with the DeoxIt D5 contact cleaner and then the glass envelopes were cleaned with isopropyl alcohol.
The 59 year old electrolytic and paper capacitors were at the end of their useful life so I replaced them with some Vishay, Nichicon, and Illinois Capacitor parts.
I used my ESI 250DE Impedance Bridge to select the replacement 200 pF, 0.02 uF, and 2 uF film capacitors used in the bridge circuit for best accuracy.

The original 600 volt DC filter consisted of two 40 uF 350 VDC capacitors in series, effectively making a 20 uF 700 VDC capacitor. Unfortunately Heathkit didn’t properly design this filter circuit in my opinion.
In a perfect theoretical world placing capacitors alone in series would be fine, but in practice the two electrolytic capacitor capacitance values will not match, even when they are brand new.
If one capacitors capacitance changes in the series string, so will the voltage across that capacitor. Another issue is unequal capacitor leakage currents, which will also cause voltage across the capacitors in the string to be unequal. Heat, age, humidity, electrical stress can also affect the capacitance value. Before you know it the voltage across the capacitor has exceeded it’s rating, causing premature aging or failure.
The solution is to utilize balancing resistors in parallel with each capacitor in the series string.
There are several different formulas for calculating the value of the balancing resistors, but my favorite is by VA1DER, when using high quality matched capacitors:
- N is the number of capacitors in series
- Vrate is the rated maximum voltage for any one capacitor
- Vbus is the bus voltage – the expected voltage across the whole series of capacitors
- IΔleak (in μA) = 0.0015 x C x Vbus
- C is the rated or average measured capacitance of one capacitor
- Rbalance in megohms
I usually half the resistance, to give myself a safety factor of two, as long as the balance resistor’s power dissipation is a reasonable amount.

I followed the Test and Calibration procedures from the Heathkit IT-11 Assembly Manual. The hardest part of the calibration was coming up with the proper resistances to generate the simulated leakage currents for each of the three ranges. There are two different procedures for calibrating leakage currents. The first involves using specific resistance values and voltages, and the second uses a meter that can measure milliamps and microamps. I ended up using the second, as it is more accurate. I didn’t have a 2 megaohm variable resistor, so ended up using a 1.5 megaohm fixed resistor and padding it to achieve 2 uA.

The Heathkit IT-11 capacitor checker is still a great tool for measuring capacitor leakage, but most new multimeters with capacitance modes and dedicated LCR meters easily outperform this checker by a wide margin.
It is still useful tool as long as the person using it understands it’s limitations and is willing to use it safely, due to the high voltage potential on it’s “Test” output jacks.

I’m still a little fuzzy on the math. The schematic that I have for the IT-11 shows 600V across those two capacitors. When I do the math, I get 2.36 Megaohms, or 23.6K Ohms. I don’t see how that translates to two 330K resistors. Can someone help me out? Am I making a basic calculation error or what?
The correct calculation for the capacitors I used in the refurbishment is 2.36 megaohms.
The 330K resistors in the picture were used for testing, and later replaced with Vishay HVR3700002324FR500
which are 2.32 Mohm, 0.5W, high voltage, safety metal film resistors.
Thanks!
It’s a great review. Thanks for sharing. I’ve have the same tester and it’s useful to check and renew capacitors. It’s sometime useful to check them with the real voltage that they’re supposed to withstand. New testers are more precise but surely don’t have higher voltage tests. Useful to anyone who restore older valve equipment.
Hi, I just restored one of these units. Can you tell me what is meant by “MIN’LITIC”? Are the tantalum capacitors?
Thank you
Hi Mirach,
The Type label “MIN.‘LYTIC” is short for Miniature Electrolytic.
Min Lytic capacitors are classified as High capacitance Low working voltage and small physical size.
At lower working voltage ratings most electrolytic capacitors have higher ESR values for the same capacitance values as units rated at higher working voltages.
Greg(Barbouri)
Thank you. Unfortunately my IT11 cannot be configured for 230V, it only has two black wires in the primary. strange because in the wiring diagram the input transformer has 4 wires. Perhaps this is a first version. I’ll have to use a external transformer..