Tek 2225 trace disappears when DC voltage present at input

[shane321]

Hello, hoping someone might be familiar with this sort of problem, I had a look around the internet, did find someone with same problem but no solution was presented.

Problem
Tek 2225 CRO Measuring AC ripple on 150 VDC supply, CRO is AC coupled.
Ch1: unable to locate trace, pressing 'beam find' shows half trace near top of display, position control has no effect.
Ch2: trace visible, shows AC ripple voltage just fine although trace wanders up a full division after 10-15 mins.

Problem was intermittent before becoming consistent, it would be that the trace would display just fine before slowly wandering upward until it could no longer be brought back by position control.  Upon shut down of the DC supply, as the DC voltage ramped down, it would return, slowing moving downward from 'off screen'.

Without DC voltage present at the input, it works just as it should.  I did replace the input coupling capacitor 0.022uF/630VDC but to no avail. 

Nice clean scope, worth trying to repair, any suggestions/help/advice is greatly appreciated.

Thank you
Shane

[bdunham7]

If you change the channel coupling to 'GND' with the DC supply still connected, does the trace come back?

Does the scope work completely normally on other signals?

[shane321]

Trace comes back with DC supply connected and input set to GND, yes.
Works fine with all AC signals so long as no DC present.

Thanks.

[bdunham7]

OK, then I think the thing to try is removing the attenuator board and cleaning the attenuator switch contacts with IPA.  You can check the 500K resistors R3 and R5 and make sure the voltage at the input to U10 stays at 0 volts with the input disconnected and set to AC or DC, but that isn't likely to be the issue.  The attenuator board is fairly easy to remove, but those switches are bit fiddly.

[shane321]

I'll give it shot, thank you.