We live in a time when having an oscilloscope is only a minor luxury. But for many decades, a good scope was a major expense, and almost no hobbyist had a brand new one unless it was of very poor quality. Scopes were big and heavy and, at the price most people were willing to pay, only had a single channel. Granted, having one channel is better than having nothing. But if the relative benefit of having a single channel scope is 10 points, the benefit of having two channels is easily at least 100 points. So what was a poor hacker to do when a dual-trace or higher scope cost too much? Why, hack, of course. There were many designs that would convert a single trace scope into a poor-quality multichannel scope. Heathkit made several of these over the years like the ID-22, the ID-101, and the ID-4101. They called them “electronic switches.” The S-2 and S-3 were even earlier models, but the idea wasn’t unique to Heathkit and had been around for some time.
There were two common approaches. With alternative or alt mode, you could trigger a sync pulse and draw one trace. Then trigger again and draw the second trace with a fixed voltage offset. If you do this fast enough, it looks like there are two traces on the screen at one time. The other way is to rapidly switch between voltages during the sweep and use the scope’s Z input to blank the trace when it is between signals. This requires a Z input, of course, and a fast switching clock. This is sometimes called “chopper mode” or, simply, chop. This wasn’t just the realm of adapters, though. Even “real” analog scopes that did dual channels used the same methods, although generally with the benefit of being integrated with the scope’s electronics.
Insides
The old ID-22 was tube-based and quite simple. The S-3’s internal design was almost exactly the same.
Heathkit wasn’t the only source of these electronic switches. We saw one from Sylvania in a [Mr. Carlson] teardown.
[Jeff] has a look at an old S-3 in another video.
Why Bother?
It is hard to remember just how unaffordable scopes were in the past. In 1969, for example, you could get a single channel 3″ screen scope with a 5 MHz bandwidth that you had to build — from Heathkit, of course, for about $80. Adjusted for inflation, that is over $650 today, and keep in mind, this is for a scope with crazy low specs. It didn’t even have a probe connector. The scope’s inputs were binding posts. You could, however, pay $8 a month for some period of time if you couldn’t find $80 all at one time.
A better scope — still single channel — but with a 5-inch screen and 8 MHz along with things we take for granted today like external sync inputs cost $259 as a kit ($399 ready-to-go; the equivalent of $2,000 and about $3,330, respectively). Keep in mind that the average salary in 1969 was about $500 a month. So a $250 scope was two weeks’ pay — more if you look at take-home pay.
If you wanted a real dual-trace scope from Heathkit, you had to wait for 1971. The EU-70A cost $565 and was all solid-state (except, of course, for the CRT). It had a relatively tiny screen (about 4″x3″) and a 15 MHz bandwidth. It supported both alt and chop modes. You didn’t even have to build it! In 1981, a 15 MHz dual-trace Tektronix T922 retailed for $1,090! Depending on the year, you could get a 10 MHz scope from a brand like B+K, Leader, Eico, or Conar for a little less.
In Operation
[KB9RLW] has a good video about the ID-4101 in use that you can see below. Granted, since his Dad’s old scope blew up, he hooked it to a modern scope, but you can still see how it worked.
DIY
There were plenty of designs for similar adapters published. [Leslie Solomon] of Popular Electronics fame used four standard CMOS ICs to display four traces simultaneously. The schematic below doesn’t show the trigger generation, which required a 4009 or a transistor.
When you think of a $60 scope these days, you might think it isn’t very good, but consider what $60 bought in 1969, and you’ll appreciate the cheap scope of today. If you want to practice your scope abilities, dust off an Arduino and check out this trainer from [Bald Engineer].
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