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TrevorG3VLF · 14th Feb 2019, 1:02 pm

Hi there all you young whipper snappers, you will have to pedal quite fast to catch up with me. !!

One reason for excessive hum is if the rectification is half wave rather than full wave. This could happen if a diode or one section of the transformer is open circuit.

Without power, measure the resistance (forward voltage) of the diodes, they should show 0.6 or so in the forward direction and open circuit in the reverse direction. Disconnect one end then :-

Measure the resistance of both 160V windings. If one is open circuit, then you can either get the transformer rewound (expensive) or bodge it by adding a couple of extra diodes to make a bridge rectifier and use the good winding.
A scope would soon determine if you were getting 50 or 100Hz ripple but presumably you do not have such. I have one which I have not used for many months so not really necessary.

Go to your local radio club or repair cafe and get some personal help. It is all very logical and with one good set to compare, should be possible.

14th Feb 2019, 1:02 pm	#66
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Capacitors for Trio 9R 59D Hi there all you young whipper snappers, you will have to pedal quite fast to catch up with me. !! One reason for excessive hum is if the rectification is half wave rather than full wave. This could happen if a diode or one section of the transformer is open circuit. Without power, measure the resistance (forward voltage) of the diodes, they should show 0.6 or so in the forward direction and open circuit in the reverse direction. Disconnect one end then :- Measure the resistance of both 160V windings. If one is open circuit, then you can either get the transformer rewound (expensive) or bodge it by adding a couple of extra diodes to make a bridge rectifier and use the good winding. A scope would soon determine if you were getting 50 or 100Hz ripple but presumably you do not have such. I have one which I have not used for many months so not really necessary. Go to your local radio club or repair cafe and get some personal help. It is all very logical and with one good set to compare, should be possible.