A Homemade 2 inch Oscilloscope

Background

An article in the 2022-issue 6 of Nuts and Volts magazine features a construction article for a simple oscilloscope based on an August 1960 Popular Electronics article.

This reminded me that sometime during that same period, I had built an oscilloscope with parts from various surplus sources. It uses a 2AP1 scope tube and has all of the usual controls for horizontal and vertical inputs as well as centering and intensity.

I was probably around 17 or so when I built this project. It has survived through numerous moves and I found it tucked away on a high shelf in my garage.

Here are a few photos of it in it's classy handmade wooden case covered in period walnut contact paper complete with old fashioned Dymo type labels. I won't show it in operation as it is over 50 years old and would need a "bit" of restoration before it would be safe to power up.

This is a side view of the sweep board showing the original 884 thyratron "Shoulder tube" used as the sweep oscillator. The horizontal and vertical amplifier tubes are vintage 6SJ7 metal tubes with a 4-48 date code on them.

If you look carefully, you can see some original terminal boards similar to that used in the current NV article. Back then, there were tons of surplus materials still available for pennies. Just looking at all of those mismatched capacitors makes me cringe now. I used whatever I could find in the "junkbox" at the time that was the appropriate value.

This is the right side showing the 5U4GT rectifier, The power transformer, and the large rectangular cap for the high voltage supply. The 4 pin connector with the lamp cord goes to the power switch and supplies 6.3 volts to the pilot light on the front panel and filament voltage for the sweep tubes on the other board.
This is the back side of the power supply board showing the transformer, capacitor and rectifier socket. I even had the foresight to add a fuse holder to the AC input as seen on the left.

By removing the 5U4 rectifier tube and unplugging the 4 pin socket connector, I was able to power up the transformer and measure it's voltages. It has a 115 VAC input, a 6.3VCT filament winding, a 5 VAC winding for the rectifier, and a 450VCT winding for high voltage. I didn't do any testing beyond this point but it verifies that the transformer is still good so the project is probably restorable.

This is a side view of the wiring behind the sweep, horizontal, and vertical amplifiers.

Notice the two very early diode rectifiers. They were B+ rectifiers for the tubes while the 5U4GT on the other board was for the HV for the 2AP1. I don't remember what the slide switch did although it could have somehow been associated with the Z axis.

After over 50 years, it's hard for me to remember what I was thinking back then!

This is a back view showing some more of the construction. Note the pegboard used as chassis. When you are young and poor, you use what materials you have on hand. Also note the mu-metal tube on the scope and the RCA jack near it's socket. This coupled through a large capacitor to the cathode and is part of the "Z" axis input.

I vaguely remember seeing the schematic in an old ham magazine at the time but have been unable to find it in any searches. I have found numerous schematics on the Internet that use a similar tube complement. One example is a Dumont 164E "oscillograph."

I would have to trace out every connection and redraw the schematic at this point to even begin to restore this scope. I may just start again sometime in the future by reusing the tubes, sockets, and controls and make a proper chassis for it. I do have a lot of metalworking tools and could bend up a proper aluminum chassis easily.

It would be fun to see it in operation again. It also did not have a retrace blanking circuit so the trace always had that line returning to the beginning. As in the current NV article, I would probably add the retrace blanking feature.

Rev 04/24/2022 - GWC