My longest repair took about eight years.
My CD-player (a Swedish Primare D20) started spitting out white noise randomly and intermittently, needing a power cycle to make it stop doing that.
It started around 2002-2003. At first I had the shop that sold it to me have a go, which cost me € 150 but didn't cure the problem. Then I decided to have a closer look myself, starting
this thread in 2008, only to find the real cause two years later.
This player has quite a few separate power sections, the DAC chip alone getting two separate 5 V supplies, one of which turned out to be faulty, intermittently. The cause was a bad wire connection inside one of the two pcb mounted transformers. Because of loud mechanical hum, I took them off the pcb and mounted them on rubber grommets inside the case. The white noise fault had already started at that point in time. After doing this it had cured the problem, but only temporarily.
When the fault returned again it became more persistent. As a result I was finally able to trace the problem to one of the 5 V sections to the DAC going out of regulation, which in turn was caused by a drop in the AC voltage supplied by one the transformer taps. This transformer being a resin sealed type meant repair was not going to be likely, so I added another one I found in a pile of scrapped pcbs at work.
It continues to work fine ever since, now being about 18 years old.
I can only assume that the fault lies within the transformer, which still works fine one the other taps, though.