The sweep gate rises at the base of U99B to redirect the current fromthe intensity control through the emitter of U99B to the collector.
The intensity is controlled by this current. If the -15 volt
connection to the intensity potentiometer is open, then the current
would *increase* as it is turned counterclockwise lowering the
resistance between the wiper and ground reversing its operation. The
intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted.
Though it looks like this, I think that the CRT will simply remain dark. With the wiper at 0 V the CRT should definitively be dark. An extra resistance to 0 V will decrease the current to U99B and reduce the intensity which was already at dark. IIRC there are other scopes in which the wiper actively pulls a voltage or current higher and lower (and the current to the wiper changes direction). In that case the effect of an open connection can be different.
So instead of checking to see if the intensity potentiometer isconnected backwards, check to see if the -15 volt supply is present at
the intensity potentiometer and that the wiper actually moves between
0 and -15 volts as the control is turned through its full travel
range. Note that -15 volts is suppose to produce the highest
intensity and 0 volts is suppose to produce the lowest intensity.
Sure that's the first thing to check anyway. I thought this would have been done already as part of checking for a cabling faults.