Heathkit IT-28 Capacitor Checker
After building the Pico C-Super capacitor tester that was published in Elektor Magazine I decided to revive an old Heathkit IT-28 capacitor checker that had survived a storm that blew down the shed it was stored in. After cleaning many years of dust and crud the tester sprang to life as described in the article I wrote for Elektor Magazine's popular Retronics vintage electronics column.
The IT-28 was introduced in 1968 and in production through 1977. It replaced the IT-11 capacitor checker. In addition to measuring capacitance, it can also measure leakage and reform capacitors. And, by comparing to a known inductance, measure an unknown inductor. Though not as capable as modern capacitor testers in range and precision, it still is an useful piece of equipment and fun to use.
This German article first appeared in Elektor Magazine November 2013 issue.
The bridge circuit as shown in the assembly manual.The unknown component value is read from the position of R13 on the front panel scale.The bridge is balanced when reactances are balanced: Xunknown = Xstandard * R13A/R13B.
The magic eye circuit from the assembly manual.The 6BN8 triode section is an ac amplifier followed by a voltage doubler using the same tube’s two diode sections. A negative potential on the 6E5 magic eye control grid closes the eye.
A view of the component side of the chassis.The leftmost wafer is the Bridge/Discharge/Leakage switch.The middle wafer switch is the voltage ladder for selecting the working voltage.The three potentiometers in the upper right adjust the leakage threshold for each capacitor type.