Build by David McKinnon novak |at| gekkoscience.comI've always though old analog osciloscopes looked really nice but never had a chance to buy a good one cheaply. Well I finally got one, but although it was in excellent condition (externally), it wasn't entirely in working order, so I needed to fix it.
The scope I received is an Eico Model 460 Oscilloscope, and have been working on getting it back to working condition. When I received it, the only thing that seemed to be wrong with it was the lack of horizontal motion. It was not possible to adjust the horizontal location of the point, or to set turn on the horizontal sweep. It was however possible to vertically excite the scope, and exposing it to an AC current caused a vertical line to form, and the height could be controled by adjusting the gain.
Here is the inside of the O-Scope, as you can see it uses vacuum tubes and no PCBs- everything is point-to-point wired.
One thing that proved extremely helpful while fixing this was a scan of the original manual- available from The Boat Anchor Manual Archive. This helped me quickly identify a lot of the circuits. It even contained a diagnostic section for troubleshooting. It's worth noting that the inside of this scope is full of high voltage parts. A good way to discharge the voltage is to turn the scope on while unplugged- this should help empty out some of the sizable capacitors of high-voltage charge, though caution is still advised.
I began by replacing what I judged to be the most likely part to have failed- the capacitors in the horizontal amplifier, which were very old. I even replaced capacitors in circuits that seemed fine, just because old paper capacitors are probably not going to live much beyond the ~50 years that they already have.
Although they lack the necessary organic functions, these capacitors are old enough to be my parents.
Replacing the capacitors had no effect. I was having trouble getting all the tubes to light up so I ordered some replacements for the vacuum tubes- which were surprisingly only a few dollars on ebay. The new tubes did look much better, however, even these replacements were insufficient to get horizontal adjustment going. I began measuring voltages with the horizontal adjustment at either extreme. I checked the switches, all of which seemed to be working. I checked the outputs of the horizontal oscillator circuit into the CRT and at first there was nothing but that seemed to be an iffy socket, and once I made sure the horizontal amplifier tube was lit up, the voltage seemed to change as expected when the horizontal deflector was moved, but the spot still wasn't moving.
Blue wires are the horizontal amplifier input into the CRT.
That had me worried that the CRT might be bad, which is the one part which would be really difficult to get. However, I was still just checking everything with a simple multimeter. I somewhat doubted that there would be an issue in a single part of the CRT but that the rest of it would function flawlessly. I examined the high voltage supply circuit, and sure enough that tube was in a bad socket as well. Moving it just a bit caused the whole scope to start working. Basically the issue was that while the tube was still barely working to cause a high voltage boost, the filament wasn't lighting and it wasn't able to supply enough current to the CRT coils to deflect the dot.
It seems that a few different tubes were going out intermittently- whether because of bad sockets or just being old.
Here it is displaying a sine wave! Really nice looking.
Next on my list is to replace these bad sockets and I also got some tubes, just because the old ones were pretty worn. Most of them still said Eico, which implies that they are the original tubes. It's impressive that they work at all, but they're mostly dim and sooty looking. Better to go ahead and get them for a couple bucks than to have them fail intermittently in the future.
Questions, comments, corrections or humorous mockery? Email me at novak|at|gekkoscience.com.