when do you replace capacitors?

So, the equipment is going to "explode in boiling fire"? Or is it that the cost of replacing a blown cap is less than renting a variac?
I can understand if the item is a bit of medical equipment attached to your heart pump. but usually old stuff that was stored for ten years is not the usual Ferrarri material.
I would not be afraid to use the cheap and dirty method on ANY product i owned.
Any yeah, if the product means a LOT to you, and it is really valuable, then splurge if it makes you feel better. But the item is not going to explode with the turn on method any more than with the variac.
This is like do you take your car to the dealer? or a local guy who you know.

Perhaps I just like to live dangerously?
I deliberately bought a SinglePower MPX3 because the notorious 'exploding with brimestone and boiling fire, killer/blinding headphone amp was just too good a conversation piece to pass on. Aparently some dude over at headfi got in a tizzy over his product being shoddily made and declaired all SinglePower products to be timebombs waiting to maim and kill the owners, and everyone needed to stop using them immediately, or risk immolation.
I have used my SinglePower now with it on the higher bias setting and so far have not been killed. (though some wish I was dead..sigh.)
Or is it I just am practical?
I stand up for cheap HiFi. and the turn on is fine by me: cheap.

A qualified technician can apply a variac and reform the caps over time if they have been sitting for awhile. However, the tech must first inspect the caps to see if leakage has occured. Caps can be different. There are paper caps, oil filled caps, etc. So, they will age and fail differently. This is why the best caps are stupidly expensive vs. cheaper caps. Personally, for a device that has sat unused for many years, I would apply a variac and slowly bring up the voltage over a period of days to reform the caps and see if some other component has failed. If everything is okay, I would not change the caps unless they are leaking or won't reform. Also, another area people absolutely don't know about or forget is that power transistors typically use silicon transistor heat sink grease between the transistor insulator and the heat sinks. This grease definitely hardens and fails over time and for older amps is the main reason (other than the owner switching interconnect or speaker cables without turning it off first) why amps blow. For a really nice older amp, I typically will test all the pre-driver and output driver transitors, see if they are within spec and if not replace them (or replace them on general principle with newer more linear transistors) and absolutely replace the silicon grease. Of course I am an Electrical Engineer and also work on amps/components and upgrade them all the time. However, a good technician can do this also. Again, if you manage to grab a Mark Levinson ML3, 23.5, Bedini, etc. amps absolutely check the caps and change the silicon grease. But, if the caps reform and aren't leaking, I wouln't replace them just yet.

enjoy.

Thanks Minori, I was unaware that the silicone grease can degraded over time.

The silicon grease will dry out and causes the transistor body to come into direct contact with the heat sink and shorts out the transistor and may cause a cascade effect with the other output drivers. For TO3 type metal can transistors there is a rubber or mica, or really cheap plastic insulator between the transistor and the heat sink and silicon insulating grease is applied between the device and the heat sink. the cheap insulators will crack causing a short to the heat sink also. So, I typically take all the output drivers off and replace them with more linear better transistors and replace the insulators and the silicon grease, check the capacitors for signs of leakage and replace if necessary. More detailed upgrades can turn a very good amp into a really great amp.

enjoy

MinOrl,
What transistors are best and more linear
I have a pair of DNA1 dlx amPs