And here I thought I was one of the only ones with both a 201C and a 200B. I did not realize the 200B was that old. Mine still works fairly well, though it mainly sits on the shelf as decoration. I have had it about 25 years myself. The only thing I have had to do is replace the power cord. The insulation had become brittle and unsafe.
A large glowing blue or orange tube near the power supply section would almost certainly be a voltage regulator. These tubes usually had part numbers like OA2 or OZ3. Sometimes actual NE-2 neon lamps were used as voltage regulators. Better scopes tended to have at least one regulated voltage, but that didn't apply to the old scopes like the 425. Neon lamps were used as relaxation oscillators, but I can't recall a case of that in an oscilloscope circuit. They were also used as transient protection (much like a zener) on various input circuits.
I know of one case of a gas tube being used as the sweep oscillator, and this was the Heathkit model O-1 that came out just after the war, using largely military surpluus parts. This had an octal shielded tube in about the center of the chassis that was a gas tube. I can't recall the part number offhand, it was a fairly unusual tube.
Both Heath and Eico got their starts (for all practical purposes) selling low-cost test equipment shortly after WW II. There was a vast quantity of military surplus electronic equipment available, and this included parts like capacitors, resistors, switches, and tubes and sockets. The cost was pretty close to "dirt cheap" if you could afford to buy large lots. With this sort of very relible high-quality stock as a major component of any product, the companies could afford to make the unique parts like transformers, chassis, and rotary switches.
There were tens of thousands of men and women that had been trained in electronics during the war, so there was a ready source of customers for this sort of equipment. The TV repair shop was the 1950s equivalent of the PC repair shop of the last 20 years. Many a TV repair shop would be stocked with Heath and Eico test equipment. You would typically see a multimeter, a VTVM, a scope, and of course a tube tester.
Caleb's 425 was probably built in the 1960s. The blue-decorated faceplates were the original design from after the war. I no longer recall exactly when the design switched over; sometime in the very late 1950s, I think. Other than the cabinet change, and more reliance on commercial parts rather than ex-military, there were usually no changes to go along with the new case.
I don't know if Caleb really cares about whether these bits of antiquity work or not, but there is a chance that that scope is salvagable. As others have mentioned, there are lots of hams around that have equipment to test and repair it, if the main transformer or the display tube isn't blown. I'm hoping it was one of theose two fuses thet blew. Unfortunately they look from the pictures to be completely inappropriate 20 amp fuses, rather than the 1 or 2 amp fuses they should be.
Thanks for the REALLY interesting suggestion! Hadn't thought of that! Based on some things I've read, however, I still suspect the neon "lamp" was used in that ancient scope to generate a linear ramp voltage for x-axis deflection. I've never seen any other tube circuits that used a giant neon gas discharge tube as a regulator (but my experience is limited in this manner!).
Speaking of deflection... Sometime in the mid 1960s a friend of mine showed me an old (even then) oscilloscope that his dad had brought home from Cape Canaveral, where he worked (it was being thrown in the garbage). My friend doesn't even remember it any more, but I do. We were both in our early teens then. The scope looked something like this Eico scope.
But the reason I remember it is that we took the case off and looked inside and then powered it up. Of course the tubes looked cool (I love those orange and violet glows)! But there was also an orange glow from something that looked like a giant NE-2 neon lamp. I couldn't figure out why they would have this in there and it wasn't illuminating anything! My very first electronics project, a year or two earlier, had been to build an NE-2 neon lamp relaxation-oscillator "blinker" circuit.
I (much) later found out that neon relaxation oscillators were sometimes used in early oscilloscopes to generate the ramped voltage for the x-axis deflection plates.
January 2016 Cartoon Caption ContestBob's punishment for missing his deadline was to be tied to his chair tantalizingly close to a disconnected cable, with one hand superglued to his desk and another to his chin, while the pages from his wall calendar were slowly torn away.122 comments