Heathkit IO-12 Tube Oscilloscope

I'm happy to report that I got my Heathkit IO-12 vacuum tube oscilloscope working!  It probably needs some additional tweaking, but I learned what I needed to learn about reviving vacuum tube (valve) gear.

Why start with an oscilloscope? I was already familiar with how they work and they are sort of self-diagnosing, unlike audio gear. What displays on the screen gives pretty good clues to the problems.  So troubleshooting was much easier than it would have been on less familiar equipment.

SAFETY: before doing anything with this unit, I made darned sure I played it safe with the literally deadly voltages inside. Most of the supplies range from 100V to 370V, with the video filament driven by a 1500V supply!! Always make sure the machine is unplugged and capacitors drained before working on it. Use one hand behind your back if you must work on the machine when it is on. You don't want electricity going in one hand, through your heart, and out the other hand. Do your own research to be sure you are taking appropriate safety precautions.

The process I took was to follow the schematic an first replace all the old capacitors with new ones from JustRadios.com (they happened to have a big enough selection that I could order everything from them). I had to track down a bunch of tubes and got some from friends, and quite a few from eBay. When I finished replacing capacitors and installing tubes, I had some troubleshooting to do. Supply voltages were too high.

The schematic calls for 117V supply but AC here at the house is 125V. This scope like many vacuum tube machines use tube diodes and primitive resistor-capacitor networks to rectify and regulate voltage. So high mains voltage means high internal supply voltages which can burn up tubes and cause other trouble. (See the power supply circuit below; AA through GG are the supplies; 1V2, EZ81, and 6C4 are all vacuum tube rectifiers).

One can use a bucking transformer configuration or zener diodes of appropriate wattage to drop the AC input voltage to ensure the supply voltages are within spec. I happened to have bought some 5W, 10V zeners, and they dropped the voltage to an appropriate level. I then had to try a couple different tubes in the vertical section but pretty quickly got the trace above.

Left: a 6J6 and two 12AU7 tubes glow in the sync-sweep-blanking circuit;
Right: using my capacitor test harness as a signal source for the IO-12

There's little chance of this device replacing my solid state oscilloscopes, and there's not really any practical application of vacuum tube circuits to robotics... but it was a fun project and a good way for me to dip my toes into the wonderful world of tube circuits. The next tube project is a Heathkit frequency generator. After that, a couple of amplifiers.