HP 1741A oscilloscope - volts/div problem on channel A

[archimedespi]

I picked up a HP 1741A (and a Tek 475) for free. The HP performs great except for one problem - on channel A, the trace *disappears completely* on certain volts/div options. I mean *completely*, I can't find it *anywhere*, and if I hold "BEAM FIND" and rotate the A "POSN" knob, the beam-find trace doesn't move. I've done some testing - it seems to happen in the vertical preamp board (A3).
Does anyone have any idea what this could be, or how to fix it?

[lowimpedance]

Channel B still working okay?. If yes you have a reference for DC measurements to compare, which seems likely to be root cause of the trace stuck off screen.

Also check for mechanical issues with these vintage CROs (both TEK and HP have noisy switches and pots that occur with with age ).
Perhaps there is issues with the push selector switches, need a squirt of contact cleaner.

[archimedespi]

When the trace disappears, it's gone. It doesn't show up anywhere in the amp circuits.
Interesting thing though - I have the vertical preamp board completely apart, and in one of the gain-control stages connected to the v/div switch for channel A, I found this: (see attached)
It doesn't seem normal, apart from the scorch marks it looks like one of those resistors was replaced (badly) in the past.
I think that those symptoms could be caused by dumping too much voltage/current through on a certain v/div gain, which could have blown some of those passives.

[lowimpedance]

Okay so it does not matter which channel is selected, the disappearing trace is specific to the volts/div at certain settings whether using A or B??.
What about using Alternate or chop settings.
It is important to know if BOTH are affected the same or is it just channel A, (the description so far does not yet convey enough info.)

[archimedespi]

It only happens on channel A - channel B is perfectly good on all settings.
Turns out that burnt area of the board had a resistor in the signal path that *should* have been 50 ohms, but had failed open in the signal path.
I'm going to replace that and a toasted cap from that area later this week - hopefully that'll solve the problem.

[lowimpedance]

Okay sounds good, please leave an update as to the problem when fixed for reference of others.
Thanks