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Tektronix 2232 Oscilloscope - CRT won't focus



I have been using this scope for quite some time now and it has served me well. It is my go to for quick troubleshooting as it is quick and easy to setup compared to the scope functionality of my logic analyzer's scope. So you can imagine that I was a bit bummed out when I started having issues. Fortunately this one was a simple repair.

The CRT would not focus all the way. Basically, I would have trace and text if I turned it on, but it was blurry. Typically, it would be simple in the sense that you only have to adjust focus on with the adjustment knob. However, even turned all the way it wasn't quite working the way I needed it to.


The first thing I did was look at the schematics for the CRT section. Fortunately the service manuals for a lot of these old Tektronix scopes are available online for free. So finding the area of interest was super straight forward. The way it works is that the supply for focus is about 2000V, the focus adjustment basically just divides that voltage down just a little bit to tune the CRT. From there it seems that the rest is biased using several 510k resistors in series. 



With the knowledge of how focus voltage is attained, I tested all the CRT voltages. Everything seemed to be there, including the 2000V supply. Next thing I checked was that row of resistors. To my surprise, R888 (A1R888) was testing as open. That is an obvious problem so I went ahead and disassembled the rest of the scope and replaced the resistor. Unfortunately I didn't have 1/2W 510k resistors on hand, so I used a 1/2W 470k resistor since it would still have been in the 5% tolerance range anyway. That fixed it, I now can focus to a nice sharp image.




Crazy what one little resistor can do.

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