5000 Series Scope Deflection Plate Driver Transistors

Jim Adney

Many years ago, in a former life and former employer, I was using a 5112 Dual Beam Scope that was perfect for my project. The 2 MHz bandwidth was plenty for what we were doing and, while the dual beam wasn't really necessary, it gave us confidence that what we were shown on the screen was actually coincident in time. This scope, however, had a problem. It would occasionally stop showing one half of one of the traces. The EEs with me on this project would "fix" this by removing the affected plugin and SLAMMING it back into place. The shock often cured the problem, but this "solution" annoyed me.

I managed to get the scope manual and noticed that there were 6 identical output transistors which drove the 6 deflection plates, 4 vertical and 2 horizontal. This scope had dual beams, but only one time base. Reading thru the circuit description I could see that each transistor drove its beam over half the screen. So I figured out which transistor seemed like it might be the problem and swapped it to a different location. This was easy because they were in sockets. Sure enough, the next time the problem occurred, it was in that new location, and I found that I could make the problem come and go by tapping that transistor.

The manual gave a Tek part number for that transistor and also a suitable replacement from some Japanese manufacturer. I ordered 2 or 3 of the Jap replacements, so I would have spares. I replaced the bad transistor and taped the spares inside the scope case, in a zip lock bag. A couple months/years later, another of the OE transistors failed and then another. I ordered more spares, and eventually used all of them. As far as I know, the replacements never failed.

So the question is, was this a well known problem with these transistors and were these common to all the 5000 series 2 MHz scopes? The scope was probably 10-15 years old by the time I started replacing transistors, but I'd never run into streaks of failing parts like this before. Sorry, but I don't remember the Tek part number or the Japanese replacement; this was circa 1985-1990.

Other than those problem transistors, I really liked that scope.