Tektronix 2235, no triggering

[teus]

Hi all,
at work I adopted an old Tektronix 2235 that was supposed to be faulty. Plugged it in, worked fine. Took it home. It seemed to work okay, but struggled with triggering on high speed signals. I couldn't trigger on a 12mhz crystal. After a lot of fiddling, I noticed how I could set the volts/div a bit lower: the scope would trigger on the clock signal, but could only display the center part of the clock. Raising the volts/div to show the entire clock signal would cause the triggering to fail. It seemed a bit smeared out across the entire screen, like the entire display being dimly illuminated.
After some time, it stopped triggering. Turning the scope off/on would help for a little bit. I could calibrate the probe, but the signal would go away when trying to trigger on a clock signal.
I took the scope to work. Seemed to work just fine there. Damnit.

Took it back home. Scope doesn't trigger on any signal anymore.  Beam find still works. When I gently press the B/W limit button halfway, the signal appears on the display for a fraction of a second.
I'm digging through the service manual, but I'm nervous to ruin the scope. Looking at the troubleshooting flowcharts, I should  ""turn CRT bias adj r851 fully clockwise". I haven't found R851 yet, but I figured I would first ask here for some help. Maybe someone has an idea.

Find the service manual here: https://www.tek.com/manual/2235-service-manual

[teus]

this thread magically got bumped: https://www.eevblog.com/forum/repair/tektronix-2235-no-trace/

pressing P-P auto actually made the squarewave display, for calibrating a probe. it's flickering badly. I disabled it again, and managed to display the squarewave by carefully setting the level.
the scope seems to be working somehow again. a sound card producing a 20 000hz sine sometimes shows the signal twice: once in phase, once in counterphase, overlapping each other. the scope trace isn't flickering but is bouncing around quite a bit.
maybe this info helps.