The Heathkit IO-21 Oscilloscope - Part 2: Recapicide!
Monday, August 31, 2026 at 08:10:13
Some time ago, I picked up a moderate condition Heathkit IO-21 Oscilloscope. I was planning on doing some work to it, but in-between then and now, I was able to find a much better unit.
Other than a few minor dings, it’s in great shape.
Of course, it’s full of old capacitors. While these seemed to be ok-ish, the ones in the HV circuit were not.
Including this one that wasn’t soldered. I wonder how many problems this caused for the original builder.
First thing to do is install the new HV capacitors. That’s easy enough, they’re right on top, and the resistor that connects them gets replaced as well in this build.
These were old ceramic encased…somethings.
I’m not sure what type these are, but this stuff is gooey wet.
Many of the capacitors in this unit were ceramic disc, and they’re fine…for now. The rest were encased paper, so get replaced with new films, and a new electrolytic where appropriate.
The main filter didn’t seem to be in that bad of condition. A little tired, but not bad. I’m going to leave it alone for now.
So…does it work?
Sure does.
It works pretty good, and is about what I would expect for a scope this age. I’m pleased with the result, and I now have a vacuum input scope for the bench.
Capacitors
So, what was removed from this thing?
None of these tested bad, but they’re probably bad in the leakage department. I would suspect that the HV caps, with their leaking liquid, are probably not long for this work if you were to use them for any length of time.
I did a quick check on what each one read, it’s about what you’d expect:
As far as I’m concerned, this device is finished.
Wrapup
This one only needed a little bit of work, so it doesn’t get an entire series. I will probably do the filter at some point, but not now.
In all, the only two surprises waiting here was the no-solder, and the gooey HV caps. Not sure what’s going on here. I did take the time to clean the controls and give the switch a coating of cleaner, for good measure. Everything seems to be working, and no other issues presented themselves.
I have two other examples of this unit - I’ll probably keep one, and sell the other one off for whatever I can get from it. If you’re interested in one of these and you live in the Columbus-Pittsburgh corridor, let me know on Mastodon!
Previous part of this series: https://wereboar.com … part-1-observations/
References
Coming soon.