I own a Naim Nac62 and Nap140 which I bought in the mid 80s. I used them continuously up until about 10 years ago, then put them in storage until 6 months ago when I set them back up again. They’ve worked fine up until a few weeks ago when the fuse in the power amp blew on switching it on.
Given that the amps are way over 30 years old, I decided to have a look inside, I quickly discovered that one diode in one of the rectifier bridges was short circuited. I’ve bought the exact same part, but I’m wondering what caused it to fail? Old age, or are the capacitors in the power supply circuitry on their way out?
One thing that puzzles me is that since I set up the amps again 6 months ago, when I switched the amps on, there was a thud in the speakers, but I can’t remember if the amps always made a thud on switching on? Is it normal for the amps to make a dull thud in the speakers when switching on? And if not, is this a sign of failing capacitors?
An unserviced amp of that age is well overdue a recap. DIY is outside the scope of this forum, but you should deal with this before you use the amp if you don’t want to risk more expensive damage to your speaker drivers.
I expect the diode failed through over curernt. I.e something downstream ( most likely the cap) has done short and allowed the excess current draw. Shame the fuse didnt provide protection. Anyway, time for some Class A action. And then another 30 years of fun.
The thud getting louder is a sign that the amp needs attention, but the worst case scenario is that the output dumps DC current into the speaker and cooks the drivers, which can be much more expensive to replace than the caps in a power amp.
As others have pointed out, electronic DIY discussion falls outside of forum AUP. If the NAP140 has never been serviced then it’s way past due. Note that a proper Naim approved service is more than just a “re-cap” and will have the amp back to performing as new again.