It may be a failure too, if they did not sing before. If ripple current increases or frequency changes, noise may appear. With modern equipment where high ripple current is present like graphics cards, it may change from batch to batch or manufacturer. Even though the same reference PCB was used. Just unlucky combination of parts and parameters and capacitors start singing.
Well with a linear supply, what could possibly happen that leads to this condition? All that comes to mind is something with the diodes, the transformer windings shrinking (inductance change), and perhaps down stream decoupling capacitors having lower capacitance. Its hard to imagine those things leading to excess capacitor noise (unlike a switch mode power supply). Maybe if I get another one of these singing linear devices I will try replacing the rectifier diodes first, to see if the noise is related to that. I can't help but wonder if I am just masking a problem by putting foil capacitors down to mute it, especially if the ceramic caps are fine, but are improperly loaded by some circuit condition. How do you damage a damn ceramic disk?? it seems invulnerable.
I wonder if its just insufficent filtering at the input because the power grid got nasty compared to 1960 in terms of harmonics because of all the switchers (solar, car, small smpsu, furances, hvac etc). So maybe if you brought one folward in time, it would be just as loud because it was not designed to handle such a chaotic power grid situation.
Maybe a big beefy low noise EMI filter would be a good piece of test equipment, to see if plugging it in between the PSU and the wall would help.
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Just to clarify how I got down to the bottom of this last year (I confused myself in the thread by now)
1. Replace all power transistors and put new thermal grease (one was blown), disassemble and clean all the potentiometers, repolish all screws, grease all contacts with deoxit grease after careful polishing to get rid of tarnish.
2. replace all electrolytic capacitors with good quality ones from digikey (including using axial and radial as needed)
-still hear noise, blamed transformer (somewhat quieter)
3. Desolder transformer and vacuum varnish it (after cursing a lot)
-part of the noise went away, but the high pitched tinitus one was still there
4. fix PCB trace and repair amplifier because solder joint cracked on transformer replacement'
5. Snoop around with tube to determine the ceramic caps were buzzing, check performance of amplifier and it seems fine (no excessive electrical noise, good regulation, plays audio music through a speaker fine)
5. replace ceramic caps with film and enjoy silence
So I feel like I basically replaced everything that could have been making the capacitors act badly. I measured alot of the parts when I was going through it when the trace broke, all the diodes looked OK on Vf.
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Wraper,
Could you give me a plausible combination of factors that could lead to the capacitor singing based on drift of various part in a typical linear amplifier?? (assuming the above list was checked where the power transistors, electrolytic caps, potentiometers and interconnects were serviced). Maybe you need a small series resistance between the filter caps and the rectifier or something?