In part 1, I established that this scope was just about working but there was some problem with the high-voltage (EHT) power supply to the tube which was making the trace very dim. The transformer in the power supply is not original, so I don’t know whether it’s correct or has ever worked properly.
The design of the EHT supply in this scope is quite clever. The tube needs about 4kV between cathode and anode to make the electrons fly fast enough to light the screen up. However, there’s lots of metalwork near the anode (especially the deflection plates) which need to be at roughly the same voltage as it. It would be inconvenient, not to mention dangerous, to have most of the scope sitting at 4kV, so instead they put the anode at about 300V and the cathode and grid at -3700V. The electrons don’t know the difference, and the electronics get simpler.
However, there is still a need to change the voltage of the grid relative to the cathode, in order to control the brightness and, for example, blank out the beam on its way back across the screen. The way it’s done in the 549 is to have two separate -3700V power supplies, one for the grid and one for the cathode. Then, by controlling the voltage at the ‘top’ of each power supply separately, we can control the voltage of the grid and cathode independently. The circuit diagram here shows the arrangement.
T801 is the EHT transformer. The bottom winding feeds the cathode and the top winding feeds the grid. Notice how the intensity control operates on the top end of the bottom winding: by varying the top between about zero and 100V, the bottom of the winding varies between about -3700 and -3800 V.
For this arrangement to work properly, it’s important that the two power supplies are near enough identical. If they’re not quite the same, then the grid and cathode will be too far apart in voltage, and the trace will either be very dim or uncontrollably bright. The good people of the TekScopes Yahoo! group confirmed this theory, and suggested checking whether the power supplies were the same.
A quick way of doing this without disturbing too much was to just swap the connections to the two windings on the transformer. If the transformer isn’t the cause of the problem, this should make no difference to the fault. If the transformer is the problem, the very dim trace should become bright. Switch on, with the intensity controls at minimum just in case there’s a risk of damaging the tube, and:
A trace! Success. So the transformer is the culprit. Whoever rewound it has not made the two secondary windings quite the same, so I’ll have to modify it. Though there’s now a trace, it’s impossible to extinguish it using the intensity controls, and the spot is still visible during flyback, so the scope isn’t quite usable yet.