I was checking out my Tek 2213 scope before selling it, and noticed that the focus was fuzzy.
A little Duck-duck-go-ing informed me that this is a fairly common problem in this series of Tek scopes.
The focus voltage divider chain in the 2213 consists of six 1 MΩ 1/2 watt carbon composition resistors. Over time, these drift higher in resistance and eventually the focus control doesn’t have enough range to compensate for the changed voltage at the focus tap.
The repair is simple, just replace them with 1/2 watt or 1 watt metal film resistors, except that they are buried on the main board under the high voltage shield. So you first have to remove the HV shield.
Then you have to pull the pre-regulator board as shown above.
But don’t forget to take note of the color-coding on the leads that you must disconnect from the pre-regulator!
Four of the resistors are accessible once you remove the pre-regulator, but two more are buried under the auto-intensity control. So you have to desolder and remove the control, which is attached to a good-sized shield that is connected to the ground plane on the main board. Consequently you have to put a good bit of heat into the solder joint and suck up or wick up the solder thoroughly, so you don’t destroy the barrel in the plated-through hole when you remove the shield.
The four accessible resistors have already been removed. There were in the empty solder pads in the center-left of the photo above. The three pads arranged in a triangle in the center of the photo are the pads for the auto-intensity potentiometer. The pad to the lower right, on the ground plane, is the pad for the shield. The two pain-in-the-neck resistors are just above the pads for the pot. I’ve already lifted one leg of one of them.
All ready for the new resistors.
Everything reassembled. I had to touch up the astigmatism and auto-focus adjustments, following the procedure in the manual.