A-B problem on HP 1741A

[qu1j0t3]

I have a 1741A whose A-B mode doesn't quite work (but other modes seem all fine).

If I switch a 2 channel signal to A-B, the sweep disappears far off the right edge of CRT. While BEAM FIND is pressed, it swoops back in (and looks correct).

I haven't opened it up to diagnose it per service manual but I figured I'd shoot off a lazyweb question first, just in case anyone had seen something like that before. I've tried all possible POSITION settings and can't bring the trace in. (I think POSITION is operating properly in other modes.)

I should add the Chan A and Chan B signals are both between 0/9V.

[qu1j0t3]

OK, it could be that this is just a biasing issue. I've never really used X-Y modes before so I wasn't sure if they expected a +/- range or + range. If I use AC coupling and adjust the channel V/div I can see the Chan B horiz sweep in positive quadrant (Chan A vertical is easily centred with POSITION). Pretty sure if I bias Chan B around 0V when DC coupled I can view the whole trace? Chime in if I've got any of this wrong...