Turn the readout intensity fully anticlockwise, there should be a detent to turn the readout function off.
You need to turn R2135 slowly since if it does brighten up the missing trace you don't want to overdo it and damage the very rare 7834 storage CRT. It's probably clockwise to brighten the trace but if not try turning it slowly anti clockwise. You may see nothing at all or a trace with both X and Y signals or a dot or a line, it just depends on which of the X, Y and Z signals are missing by the time they get to the CRT. Make sure to return R2135 to its original position, you can do a final adjustment when everything is working. There are X, Y and Z signals to drive the readout, they clearly do get through to the CRT so the problem is to find the point where the main signals go missing.
Were both time bases working before the right one was removed?
Do you have a second scope for troubleshooting?