Tek 2215 vertical gain


 

Hi,
I recently picked up a well used Tektronix 2215 late serial number scope (with A18 pre-regulator board) which has an issue with vertical gain. Same occurs on Ch1 and Ch2 so must be in a common circuit. The symptom is when displaying square wave (off the front panel), the height of the square wave varies wildly as the trace is moved up and down using the Ch1 or Ch2 vertical shift control. When moved to the bottom the height is around 2.5 squares, when moved to the top the height is around 4 squares.

The only check I have made so far is to R378,R379,R388,R389 which are meant to be 340 ohm but all measure very closely at 315 ohm each (in circuit). I don't think these would be the cause as they are the same in + and - sides of the push-pull output circuit.

Any suggestions on what to check please? Hopefully a pot tweak will help balance the vertical.
Thanks.
John

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