LCR meter

One tool I don't have is a LCR meter for testing capacitors as they are most likely to degrade. 

I have a 1970s amplifier which started to get a line voltage hum. 

I finally replaced all the capacitors and the hum disappeared. 

I never tested the old capacitors but they must have degraded significantly after 50 years. 


Has anyone actually measured old capacitors and then replaced them and noticed such a big reduction in hum.


I replaced the caps on a 20 year old parasound but it made zero difference.  It didn't exhibit any hum so I probably jumped the gun on that one. 


Yes I have tested and replaced many capacitors in older equipment. The suspect ones are near sources of high heat generation. The caps replaced range from 20 to 80% low on my cheap tester. I would highly suggest you buy one, mine was less than $30. I have also tested caps that have been in service for over 40 years and many test within specification limits. Old electrolytics should be replaced, as I've found that even ones that are less than 20% low eventually fail. They have been known to take out transistors and other semiconductors. Many older products use components that simply cannot be replaced, so if you want to keep your equipment running and sounding good, it's better to service before you have a hard failure. One example are the V-Fet transistors made by Sony, but there are many unobtainable semi's. 

I'm not sure what the maximum uF is but I use DATS from Dayton/Parts Express for anything besides a resistor, especially if I need the equivalent series resistsance (ESR) or DC resistance of a coil  (DCR).

This is typical for older equipment (and even not so old equipment).  When the main power supply capacitors dry out, they will lose the ability to hold capacitance.  This is where that line voltage hum comes from.  This in itself is not damaging, but it is just very annoying and you will probably have weak sound (no bass and weak midrange, etc.). 

That being said, it is also a symptom that the separator/spacer material between the aluminum foil in the capacitor can be wearing thin.  If this wears down to nothing, you can have a hard failure because it creates a "short" between voltage and ground.  This can be a small failure of the capacitor itself or it can end up being a catastrophic failure that takes out a large portion of your equipment. 

So better get those old caps replaced!