Now you're making me feel old. What you call a "display" is a "meter", which is short for "galvanometer", but I don't think anyone called them that after the dawn of radio. Permanent magnet and a coil of very fine wire. Current through the wire makes a magnetic field which moves the needle.
The GDO's meter has only a single scale, but meters used in VOMs and VTVMs often had several scales for various ranges. You had to be sure you were looking that the correct one. But we were used to that from negotiating the multiple scales of our slide rules... Some meters (such as my 1970-vintage Olson VOM) had a strip of mirror on the surface with the scales, so that you could line up the needle with its image and avoid parallax errors in your reading.
Regarding the "gigantic" capacitor - that one's actually pretty small (as implied by the "mighty midget" name) - only 15 uF and 150 volts. Capacitors in the TVs I took apart in my youth often had electrolytics with voltage ratings up to 450 or more. Often the "cans" were 1.5 to 2 inches in diamaeter and 3 or 4 inches long and contained three or four capacitors.
Note the capacitance is given "MF", using "M" for "micro" rather than the Greek letter mu. Small capacitors were specified in "mmf", or "micro microfards", rather than todays picofarads. Instead of nanofarads, you had values like "0.001 MF". Luckily, there was plenty of room on the parts to print this sort of thing.
Love these old Heathkit teardowns! One minor correction, the bottom slide switch is actually the diode/osc mode select; the power switch is on the sensitivity control. In diode mode the B+ to the tube plate (anode) is turned off, then the unit could be used as a wavemeter or AM demodulator into the headphones due to rectification between the grid and cathode.
I bought a used successor the GD-1B from a fellow ham while in high school. Think I only paid $15. Still have it, and will email you a photo of the plug-in coil set.
I had one of these years back. Very useful gadget. It's basically an RF oscillator with a grid current meter. So you can use it as a signal generator to test out radios and the like. And since it has a grid-current meter, you can put it next to a L/C circuit and you'll get a "Dip" in the grid current at resonance, so you can verify the working and frequency and approximate "Q" of a tuned circuit. very useful, as old radios had a lot of tuned circuits. If you dial back the knob until it just stops oscillating, it becomes a tuned RF voltmeter, very useful in debugging and tuning up live oscillator and transmitter stages. All of the frequency and voltage and stability aspects were very approximate and not going to put Boonton or HP out of business, but good enough for many purposes. Heath made a smaller, newer one years later using transistors and tunnel diodes, but they were not as good as this old tube-based one. You just could not get as good a "dip" with those low-impedance transistors and tunel diodes.
I too noticed all those "flying" components. That's the way most tube circuits we designed, I'd say until the 1960s perhaps. That's when you started seeing PCBs, even in some tubed electronics (miniature tubes mostly).
I bought a Knight Kit VOM way back Freshman year in college, and am still using it to this day. No tubes. Just passive components hanginmg out in mid air, as it were.
Pity you did not get the coils Caleb, I daresay it would work still.....
I have a friend who is a "ham" and he wanted me to help him make a GDO. I found a nice circuit with a FET that does from the 100's of KHz up to the mid 100's of MHz. If it works, maybe I should get into the kit business.....
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