Tek 468, X-Y mode not working, spot hard left, requires beamfinder


I have a problem with X-Y mode not working on my 468 scope. The spot it hard left and beamfinder is required to show it. Other running traces are absolutely fine. I had another discussion open for the storage side of the scope which has not got rather long so it seemed sensible to open a separate discussion for this issue. I hope that is allowed?

While a trace is running there is a sweep ramp on both plates as might be expected. In X-Y mode there is 100VDC on one plate, zero volts on the other.

With U564 (4x transistor DIL package, horizontal amp section) removed, the spot is centered.

With pin 2 of U564 grounded the spot is centered.

With Q760 and Q772 removed and U564 in place, approximately 0.64V is present on U564-p2/Q760-c/Q772c. The spot is now hard right instead of left and beamfinder is required to locate it.

With Q760, Q772 and U564 present U564-p2 is at around 180mV. According to the diagram it should be 0V. I am not finding any faulty component so am not sure why the voltage is above 0V. At Q760 E there is 45mV but at Q772 there is 160mV. Swapping Q760 and Q772 around makes no difference. Same for Q781 and Q782. U690 and U789 have both been substitued to rule them out. Made no difference.

Clearly there is an imbalance somewhere but only affecting X-Y mode it seems. I have stared at the diagrams for a very long time today and have to admit that I am rather stumped at this point.

Any ideas would be appreciated.


This issue has now been addressed. Thanks to all who provided comments.

Details here: