When I move channels 1 or 2 down the screen the trace is compressed at the bottom. As if the gain near the bottom the screen is about half that at the top. The effect is identical on both channels. Channels 3 and 4 are OK. Channel 2 output is OK. This looks horribly like U400 is in trouble and I've heard these hybrids are unobtanium.

I'd like to look at the differential outputs of U100 and U200 (and U300 for that matter, simply for comparison) where they feed into U400 in case there's a silly amount of common mode offset which might be causing the problem. But I can't see anywhere on the topside that I can probe. Getting at the underside looks tricky.

I've checked the power going to U100/U200. The +5V, +6.2V, -15V, -8V and -5V rails all seem to be within 2% of nominal.

I've looked at this article which isn't 100% relevant but illustrates an interesting fault and showing that U400 isn't always to blame: http://worldphaco.com/uploads/TEKTRONIX_2465B_OSCILLOSCOPE_MAIN_BOARD_INTER-TRACK_LEAKAGE..pdf

Your help, wisdom and advice would be very welcome. Even if it's only to say that U400 has definitely failed and the getting a replacement is well nigh impossible.