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Uncle Bulgaria · 14th Jul 2021, 2:42 pm

I'd say it's best to measure DC voltages in situ first, before testing at low voltage. The capacitor testers can be useful, but there's no substitute for seeing how the circuit is behaving when in use, and the fault (the hum) is present.

A particular capacitor may measure fine, but if tested at its working voltage of a few hundred volts, it may have great leakage making it unusable.

If you have an oscilloscope, you can probe the PSU capacitor connections and see if 50Hz ripple is coming through, indicating the the capacitor is not doing its job properly. I recently had motorboating on my 4A, and was able to see the failure on my oscilloscope at the junction of the choke and C24. Replacing this dual can (from Cricklewood) cured the fault.

If you have 'The Manual of the Ferrograph' you can use it to diagnose the problems by reference to the DC voltages referenced in the text, then go from there.

The danger with taking things out and testing them individually is that you can introduce a raft of new faults with replacement components without knowing if the originals were part of the existing fault.

14th Jul 2021, 2:42 pm	#2
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,339	Re: Ferrogaph 2A ampllifier hum. I'd say it's best to measure DC voltages in situ first, before testing at low voltage. The capacitor testers can be useful, but there's no substitute for seeing how the circuit is behaving when in use, and the fault (the hum) is present. A particular capacitor may measure fine, but if tested at its working voltage of a few hundred volts, it may have great leakage making it unusable. If you have an oscilloscope, you can probe the PSU capacitor connections and see if 50Hz ripple is coming through, indicating the the capacitor is not doing its job properly. I recently had motorboating on my 4A, and was able to see the failure on my oscilloscope at the junction of the choke and C24. Replacing this dual can (from Cricklewood) cured the fault. If you have 'The Manual of the Ferrograph' you can use it to diagnose the problems by reference to the DC voltages referenced in the text, then go from there. The danger with taking things out and testing them individually is that you can introduce a raft of new faults with replacement components without knowing if the originals were part of the existing fault.