Oscilloscopes

(If you read my blah for the rants, you may want to skip this piece. There is little-to-nothing ranty in this one. This is actually not too far from a traditional blog post in content.)

Decades ago, as a kid (still living with my parents and all that), I dabbled in electronics. While I still do dabble in electronics sometimes, the reason I bring this up is that there is an oscilloscope which I used to use back then. Some time ago (years, but not too many of them), I got this 'scope with me here in Montreal. But, something like the third time I tried to use it, the main power transformer fried. (This is a fairly old 'scope, an all-vacuum-tube design with a big honkin' land anchor of a power transformer with something like eight windings.)

It would have been possible to have it rewound. However, I didn't know enough details to be sure I'd get it right, and it would have been expensive. (I did, and do, consider myself not competent to rewind it myself.) So I did what I often do when stymied: I shelved it. First, though, I did enough circuit tracing to work out the power-supply portions of the thing. (This design is not only pre-transistor, but pre-printed-circuit as well.)

I also have, picked up from I no longer recall where, a 3RP1, which is a 'scope CRT. It's substantially smaller than the one in the fried 'scope, but sometimes small is nice; in this case, it is a benefit because it makes it mechanically easier to work with. It also has the final anode connector on a base pin rather than a connector on the side of the tube (which also makes it easier to work with).

Today, I finally started doing something with this. My principal problem when working with CRTs is the voltage involved; most of my electronics dabbling has been low-voltage stuff, the kind of thing where 30V constitutes high voltage, but these CRTs expect a final anode voltage up in the multiple hundreds of volts to low kilovolts range.

The original circuit in my 'scope generated the high voltage with a high-voltage winding on the transformer, feeding two 2X2As (high-voltage low-current rectifier tubes). But the 3RP1 needs, according to its spec sheet, only about 1kV or so for its final anode voltage, something well within range of a voltage multiplier applied to mains voltage.

Today I put such a thing together. I dug out two transformers designed to go from 120/240V to 6.3/12.6V (primary and secondary each consist of two windings which can be wired in parallel or series). I wired them back-to-back as an isolation transformer, with the mains-voltage windings wired differently so they would together act as a 2:1 step-up transformer. Feeding the voltage multiplier with this, I was able to get up into the 800-900 volt range with only three diodes and capacitors. (I have a bunch of 1N4007s, 1kV PIV rectifier diodes, and found two .47F 630V caps and a .1F 1kV cap.) A third transformer gave me 6.3V for the CRT heater.

However, I found my collection of resistors in the 100K-10M range was somewhat deficient. One of my 3RP1 data sheets gives a sample circuit which uses a voltage divider totaling some 5 to 6 megs, including two pots (for focus and intensity). But I had neither the resistors nor the pots to build it, and on a Sunday evening there was no way I was likely to change that.

After fumbling around and never really getting anywhere (the data sheets indicate voltage ranges for the focus and intensity control pins, ranges I was having trouble hitting), I thought of using the high-voltage divider chain from the old 'scope. After a little experimentation, I got this working, giving me a nice spot on the screen. It's not quite right, probably because I'm not using the tube it was designed for; in particular, the focus adjustment can't reach the correct range to get good focus. But it comes close.

But just a spot is boring. The deflection electrodes are specified as having sensitivity between, approximately, 50 and 100 volts per inch of deflection per kV of final anode voltage, depending on which pair of electrodes and on the particular tube. Given the size of the tube face (some 2 inches across), this means that (North American) mains voltage, directly applied, is about right to get a decent trace. So I grabbed two more of those .47/630 caps and used them to couple mains voltage onto one pair of deflection electrodes (with 1M bias resistors so the DC level was equal to the final anode voltage) and got a very nice, if somewhat boring, trace.

As a result, I'm feeling rather satisfied. It's a long way from being a usable 'scope, but getting a usable 'scope out of this is a goal distinctly secondary to having fun playing with it (and in particular to the satisfaction of building my own).

Next steps: get the parts to build the high-voltage divider right, rather than hijacking the one from the old 'scope with clip leads and a few external resistors. Preferably, also find some caps rated up in the 2-3 kV range, so I can use a higher final anode voltage; this is supposed to improve spot brilliance and definition. Then figure out what to do for deflection amplifiers—I'm not sure whether I'd rather use high-voltage transistors or tubes for the final stage; I have the tubes from the old 'scope, of course, but in many respects I'd rather go semiconductor.

Oh, and try to find some more round tuits. Today just about used up my current supply.

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