Post by Owen Y on Jul 18, 2019 19:50:26 GMT 12
My 1967-built Leak Stereo 20 is part of our Kitchen-Dining Rm family music system, assembled around 3 years ago from 'vintage' parts - Celef Monitor spkrs, Technics ST-3150 Tuner (both 40+yrs old), Marantz CD-63SE (given to me), a refurbished Quad 33 preamp (50+yrs old). 
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Post by Owen Y on Jul 18, 2019 20:25:03 GMT 12
This Stereo 20 was refurbished back in the 80s & is one of the only components from back then that I've kept - because Harold Leak's push-pull 'ultralinear', EL84 design sounds great. One of Leak's best sounding amps IMO. However, it is beginning to hummm... through the loudspeakers. The PSU caps (the round cans on the LHS top) are factory-original & have been carefully 're-formed' every now & then after any long periods on non-use. The hum is most likely a sign that the caps are finally (after 52 years) beginning to lose their 'smoothing' capacity & letting AC 'ripple' through onto the signal lines. (The liquid electrolyte inside these cans dries out over time & you will typically hear a 100Hz hum tone leaking (  ) through, twice the 50Hz mains frequency, due to the usual 'full wave' AC-to-DC PSU rectification circuit.  
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Post by colinf on Jul 18, 2019 23:11:22 GMT 12
Nice! Have you got access to an oscilloscope to look at the HT waveform to confirm it? It could even be the Quad preamp unless you’ve already eliminated that. Otherwise I’d just go ahead and replace the power supply caps. Before they leak (🙄). While you’re inside measure the resistors to make sure they are still what they should be. Replace as necessary. Also it might be time to replace the caps on the EL84 cathodes as they can leak (🙄) electrically.
AMR-iFi R&D
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Post by Owen Y on Jul 19, 2019 10:42:09 GMT 12
Good advice thanks colinf. Of course, I should properly identify the hum source. There's 300-350vdc on the HT, but I will use my x10 probe on that. And yes, the 50uF cathode Cs - I can't recall what I used at the time, Philips possibly.
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) through, twice the 50Hz mains frequency, due to the usual 'full wave' AC-to-DC PSU rectification circuit.
