- 2024
- Dec
- 13
The Heathkit IO-21 Oscilloscope - Part 1: Observations
I’ve been wanting a small, AC powered oscilloscope for the bench, something that can do tube stuff without caring if you accidentally hit B+ for a second. Heathkit made several different styles that fit the bill, and I managed to run across this decent looking IO-21 3” scope for a reasonable amount.
It’s difficult to find one of these both in OK condition, and for a decent price. Like most items, people see “TOOOOB,” get some chatbot to write them garbage about how it’s a widely used accurate piece of equipment good for your toolbox, and slap a high price on it. It’s not, it’s something useful for a few of us, and the rest go and get one of those $60 Zotec Scopemeters for our toolbag.
Regardless, this is in ok shape. Most of the parts are there save for a terminal cap, which is no problem. I have many of those of the correct type from dead Heathkit devices of electronics past. It’s dirty, but that’s to be expected.
Note that many will label these as 10-21, but all scopes from Heathkit of this era were “IO” series.
The side
is a bit scruffy, and the back exhibits some broken plastic on the direct vertical inputs.
Those are just jacks, so they can be replaced if need be.
Time to take the back off…first impressions were this was built from a kit, and decently built.
But here’s a thing. The fuseholder was empty. That’s not a good sign, but here’s another fuseholder inside.
It’s not even really soldered, the one wire is just wrapped around the lower terminal of the mounted jack. Wut?
There’s a fuse in it, so I remove the extra fuseholder and put the fuse in the actual holder. Some have suggested that maybe someone was trying to parallel a fuse (maybe…) or didn’t have the correct physical size. Both are 3AG, so that’s not the case. I don’t know what was going on here, but hey - bonus fuseholder.
You’ll also note the large amount of insect debris here. That got a visit from Mister Shop Vac before proceeding.
The tube set is a mix of RCA made RCA branded, GE Made Sylvania Branded, and some washed tubes. This probably would have come with Mullard made Heath branded tubes, so this guy has seen a lot of use.
The ones in the back are 12AU7, but have nothing on them. No name, numbers, anything…chances are these came from a bulk reseller that would take used tubes, wash them of any marks, (maybe) test them and then (maybe) rebrand them as their own. That they work is amazing in itself.
Of note here is some capacitors used as filters for the CRT high voltage. 2x 1600V @ 0.01μF are prime candidates for being a Popping Patty, so they’ll need to go even if nothing else is wrong.
The bottom similarly reveals some capacitors that probably are going to need consideration, but overall it’s nicely built with just one add-in part that I assume was put there by someone other than the original builder.
Does it work? Some basic checks were made and I plugged it in. This device is a 6V parallel string for the tubes, and the 12AU7 are all folded over so they run on 6V. These tubes have their heaters split in the middle so you can operate on 12 or 6V, depending on your application. On almost all of them, only one side lit so a bit of wiggling later (dirty sockets) here comes a trace. It’s about as bright as it was when new, which is to say a bright room washes it out. But it’s there, and that’s the important thing - a dead CRT means a dead unit.
Stay tuned for the next part of this series, some cleanup and basic identification for parts that absolutely need to go.
Next part of this series: Coming soon.