Don't discharge the CRT before working on it. Its not good for the CRT unless its discharged by a high value resistor, typically 100 Meg Ohm in an EHT probe. The discharge advice really only applies if you turn the VDU off and want to quickly remove the CRT and carry it, because if you received a harmless zap from the anode cap, you might drop the tube on the floor. The stored energy in the bulb is less than 50mJ and is harmless. If you don't do under the EHT cap, it is impossible to come into contact with the charge stored in the CRT bulb.
Most repairs can be done without removing the board from the cabinet or removing the EHT cap. If I need to remove the cap I simply leave it overnight and the charge dissipates because of the non zero reverse leakage in the EHT rectifier.
Back to your problem. Your flyback transformer is working fine (Why everybody blames the flyback for every fault they see in VDU's is beyond me).
The bias on the CRT gun is off. The video amplifier appears to be working.
The video source drives the CRT's cathode. When that voltage is around 35V, the beam should be extinguished and when close to zero volts, the beam at full brightness , that is if the g1 Grid voltage is a suitably high negative voltage between around -15 to -30v. It is the relative grid to cathode voltage that determines the CRT bias.
In the case of failure in the brightness control system:
The most likely cause is a failure of the negative voltage source that supplies the brightness control has disappeared.
There are only a few parts in the circuit and it easy to test with a meter. There are two schematics variations of this VDU, I will go through both and you can check with your meter.
Schematic variant 1) The negative bias is derived from a secondary winding on the Flyback pin 4 and pin 1&5 ....check for ring cracks in the soldering around the pins..... Pin 4 passes to a 10 Ohm resistor (likely gone open circuit) then diode CR16 (check the diode) then to a 47uF filter cap C22...could be leaky then to the brightness pot...check pot not open circuit..... pot's output passes to a spark gap SG01 check not shorted and then via a 100R resistor to the CRT grid, check resistor is ok and connection to crt grid good.
Schematic variant 2) Winding on flyback....pin 8 ...again check for solder fractures, connects to D752.. check diode, C752 filter electrolytic then brightness pot and 0.01uF filter cap and CRT grid.
Notice that the filter electrolytics in these sets for the negative voltage source for the CRT's grid supply have their positive terminals connected to ground, If somebody in the past re-capped the set and in the customary manner with global recapping was not paying attention, then they might have fitted the cap in reverse. That would either open circuit the diode or the resistor in the circuit version that uses it. So carefully check the polarity of the cap that is there If your fault is the 10 Ohm resistor, if you have that version of the VDU, then you can replace that .
In the case that the brightness circuit is normal:
There are a couple of other issues that could cause what you are seeing, but the above is the first thing to check. If it turns out that the brightness control is providing a full range of negative voltage 0 to -40 V region to the CRT's grid, then we will have to move to the cathode circuit of the CRT.
In that case the output of the video amplifier would not be taking the CRT's cathode voltage positive enough, which could be a partial failure of the amplifier components or the the positive rail that powers it. In version 1, this about +90V power rail is protected by a 10 Ohm resistor also, and it is zener regulated down to +35 volts to supply the video output transistor. So check that. In the other schematic version, it is a direct +35V supply called E35, protected by a 2.2 Ohm resistor that supplies the video output transistor stage.
There is a rare condition that can cause this effect, but luckily not common at all, that is a heater to cathode short in the CRT, dragging the cathode voltage down that way, but hopefully that will not be the case.
In any case, make all the tests & report measurements and if you don't find the culprit , we should be able to figure out what has gone wrong from the test information you provide.
