Tektronix 2213 focus repair

I was checking out my Tek 2213 scope before selling it, and noticed that the focus was fuzzy. 

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A little Duck-duck-go-ing informed me that this is a fairly common problem in this series of Tek scopes.

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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.

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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. 

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Then you have to pull the pre-regulator board as shown above. 

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But don’t forget to take note of the color-coding on the leads that you must disconnect from the pre-regulator!

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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. 

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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.

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All ready for the new resistors.

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Everything reassembled. I had to touch up the astigmatism and auto-focus adjustments, following the procedure in the manual.

© Steve Byan 2011-2016