John Honniball; Electronics Whisperer

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.

John quickly realised that one of the resistors was not like the others:
John H was Very Suspicious of this resistor, and replaced it. Once replaced, the scope worked perfectly! Luckily for us, John was able to explain his abilities:

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.

Thanks John!