Take a long plastic tube, at least 1 foot.? You could use cardboard, but plumbing material is fine.? Stuff a microphone down one end and seal it in with tape.? Run the microphone to an amplifier.? You now have a sensitive directional sound probe. Putting the end on the suspected part should tell you a lot.
You can also use a stethiscope? with a long piece of tubing (plastic!) to do the same thing.
Harvey
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On 1/5/2025 10:56 AM, James wrote:
Thanks Zen, much appreciated.
I checked the main caps fairly cursorily for capacitance, which was textbook value, but didn't check them at proper operating voltage for issues. I will certainly do so when I pull the PSU apart. I forgot to mention that the crackling continues (decaying) for a minute or two after switch off, maybe that helps narrow the culprit down?
Thanks for the info about the resistances, understood!
Re the neon, my poor description, it flickers dimly (like most neons do) when the psu is switched on and very quickly stops after it is switched off. It doesn't blink though if that matters. I note that the reservoir caps don't hold charge for any time after switch off. The switch, like all Tek switches, looks built to last forever. It functions fine thankfully.
My understanding is that the PSU in the 7904A is rather different but have no experience to back that up.
