In part 2 I established that there was something wrong with one of the windings on the replacement EHT transformer, so the grid voltage was either too low, leading to a very dim trace, or too high, leading to a trace which was too bright. The only way to fix this problem was to modify the transformer. Knowing that it’s a tricky device which has to handle high voltages, it was with some trepidation that I took it out of the scope and dismantled it.
Whoever made the transformer knew what they were doing. They’ve taken care to put a gap in the core to prevent it saturating – notice the plastic washers used as shims between the two core halves. Looking at the circuit design, I believe that this power supply is running as a flyback converter, so a gap in the core is required. Everything is nicely taped up and varnished, and there is paper tape in between the layers of the windings, too, for good high voltage insulation. I was relieved to see all this: it makes me feel confident that a repair is worth doing.
It seemed clear that one winding, the one generating too large a voltage on the grid, had too many turns on it. I had to work out how many turns to remove. I’ve been designing some flyback converters for another project recently, so I could make an educated guess. I’d expect a transformer like this one to run at somewhere around 1 turn per volt, maybe more, maybe less. I want to reduce the grid winding’s output by about 50 volts, so I thought I’d start by removing 20 turns from it.
I put the transformer in the oven to soften the varnish so I could dismantle it (150 degrees C for about 15 minutes). Unwrapping the winding, removing 20 turns was easy – they were all on the first layer, so I only had to unpeel one layer of tape. I temporarily put the transformer back in the scope and tried it. Success! I could follow the instructions for setting the intensity in the manual: set to single sweep, so the trace should be blanked, turn the front panel intensity control to maximum and adjust the internal intensity range control so the spot is just visible. It was well within the range of adjustment of the control.
With the EHT working properly, I checked the voltages: the cathode is fixed at -3700V. For normal trace brightness the grid is at about -3750V, and about -3800V hides the trace entirely.
Incidentally, I was mistaken about how the intensity control works in my comments in part 2. Having studied it more carefully, the EHT supply regulates the cathode voltage to be -3700V. The intensity control varies the voltage on the ‘top end’ of the cathode winding between about +10 and +100V so the power supply has to work more or less hard to maintain -3700V at the cathode. That results in the voltage on the grid winding varying by about the same amount, so the intensity varies.
I also sorted out the horizontal sweep problem. I found a racing car in the relevant bit of the circuit diagram, but it seemed to be missing in the scope itself.
That wasn’t the problem, though. After checking the voltages around the horizontal amplifier, it became clear that R352, a 402k resistor which feeds a voltage from the normal/magnified registration preset control into the horizontal output stage, was open circuit. I replaced it and the trace looks good now, and can be adjusted properly.
Now to find out why the delayed timebase doesn’t work. Oh, and I slipped with my high voltage probe whilst checking it on the 500V connection on the storage PCB, and shorted it to another pin, destroying two transistors and two diodes in the ‘enhance’ circuit. Oops.