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When good, a typical value would be in the 200 to 600 VDC at the CRT. The screen (it may also be called master brightness, bias, or background) control should vary this voltage. However, it may be difficult to measure as the resistors in the voltage divider network may be quite large - hundreds of M ohms. If your unit has an external screen control (less likely these days) and it has no effect, trace out the circuitry in the immediate vicinity and check the resistors and potentiometer for opens, look for bad connections, etc. If it is built into the flyback transformer and is sealed, the entire flyback will need to be replaced unless the actual problem turns out to be a bad connection or bad component external to the flyback.
However, in some cases, it only shows up when operating and one must deduce the presense and location of the short from its affect on voltages and bias levels.
See the section: Rescuing a shorted CRT and other related topics.
First, check for bad connections/cold solder joints by gently prodding with an insulating stick. Check voltages and bias levels.