We’re used to low cost parts and a diversity of electronic functions to choose from in our projects, to the extent that our antecedents would be green with envy. Back when tubes were king, electronics was a much more expensive pursuit with new parts, so designers had to be much more clever in their work. [Thomas Scherrer OZ2CPU] has just such a design on his bench, it’s a Heathkit Capaci-Tester designed in 1959, and we love it for the clever tricks it uses.
It’s typical of Heathkits of this era, with a sturdy chassis and components mounted on tag strips. As the name suggests, it’s a capacitor tester, and it uses a magic eye tube as its display. It’s looking for short circuits, open circuits, and low equivalent resistance, and it achieves this by looking at the loading the device under test places on a 19 MHz oscillator. But here comes that economy of parts; there’s no rectifier so the circuit runs on an AC HT voltage from a transformer, and that magic eye tube performs the task of oscillator as well as display.
He finds it to be in good condition in the video below the break, though he removes a capacitor placed from one of the mains input lines to chassis. It runs, and confirms his test capacitor is still good. It can’t measure the capacitance, but we’re guessing the resourceful engineer would also have constructed a bridge for that.
[seeing the name on the instrument front] Now, it would be fantastic if there’s some real backstory with Heathkit and the (Star Trek) Daystrom Institute.
Clever Heathkit design for in-circuit cap testing. 60Hz or 19MHz. Uses 540VAC for the 1629 magic eye tube, and 55VAC for the test side. But no fuse and the mains death cap would make me not get cozy with it.
In the repair shop, Larrie used to charge caps up to 500VDC and toss them to someone. If you caught it, you got zapped. I just stepped aside but… it could be lethal if you used both hands to catch.
You did need to test electrolytic capacitors, old style cans at HV, for the leakage current and dielectric breakdown and the Heathkits delivered almost 600V like the Model C-3. You needed a brain to operate one and stay alive.
Dryer lint would also be unsuitable as a coolant for a nuclear reactor. Stupid combustible materials.
For the second time today, my reply was disowned by the intended parent.
For troubleshooting purposes, in case anyone actually cares, I had started a reply, cancelled the reply, then attempted again to reply to the same entry. When posted, my reply was not connected to the reply that I was replying to.
Test of that sequence on mobile chrome
(This should be a reply to a_do_z regarding the reply being in the wrong place)
Huh, it didn’t break for me I guess
You were replying to a bot, so no huge loss
But my reply was oh so witty.
There’s one for resistors too, it’s called an EICO model 950
The only problem is it has to be a really big resistor under test or it gets all hot and smoky :-)
for some reason magic eye tubes call for a Harry Potter, Made Eye Moody reference. LoL enjoy the writing as per usual
Nice! I have the EICO one of these. I also have the version that measures capacitance. Big dial on the front. You turn it until the eye closes/opens, and the value on the dial is the capacitance. Surprisingly accurate, plus it tests the capacitors at high(ish) voltage, which is better than your multimeter can do.