That looks like there is a fault related to the vertical channel switch, U214 (schematic 3 - Vertical Interface). There is an input to U214 labelled 'X-Y shutdown', this disconnects the vertical signal from the plugins while the readout is active, there is a similar input to the horizontal channel switch which appears to be functioning OK so the most likely culprit is U214 itself. There is an identical IC (Tek part number 155-0022-00) used in the trigger circuit, U324. You should be able to swap U324 and U214 and check that the readout works OK but the scope may not trigger correctly. If you are lucky it may just be dirty contacts on the IC and all will be well with a little cleaning.
I believe you will need to partially remove the power supply from the rear of the 7613 in order to get access to the interface board. If there is a detachable bottom cover, removing that may also help.