Richard brought in a couple of dead oscilloscopes in case they could be fixed. The slightly newer one (last PAT tested in ’96) couldn’t be focused.
I saw that it had been replaced, yes. But I also saw that the ‘new’ part was a carbon composition resistor (they’re a slightly different shape), and therefore an old-stock part. Carbon resistors don’t work well in high-voltage circuits, and this focus circuit is certainly at hundreds if not thousands of volts. All that made me suspicious:
1. In a high-voltage circuit.
2. In the focus circuit, and that’s not working.
3. Been replaced before.
4. Been replaced with unsuitable replacement part.
And then I cheated, and measured it with the Ohm-meter. Should have been 2.2M Ohm, was open-circuit. That clinched it. We didn’t have any 2.2M Ohms, so I fitted two 1.2M Ohm in series. Didnt unsolder old part because it was under the CRT, and open anyway.