Our-Nick replied...Dave the bass wrote: ↑Sun May 01, 2022 5:58 pm I'm looking at a chum's Cambridge Audio amp, reported fault was sparks when the speaker leads touched!
I've found about 12 to 17V DC across the lhd speaker terminals, gut feeling is one channel of the LM4766T output device has popped. Has anyone here had an LMxxxx device produce this scenario? I'm working on it using a resistive dummy load as I don't fancy frying my bench test speakers.
Rails checked OK, I looked up price of genuine replacement chip (about £13 IIRC) then chum decided he didn't want the amp as it didn't have a Phono stage in it and just left it with me. It's sat for about a year on my workbench waiting for attention, I didn't want to scrap it because, erm, errrr I don't know, erm, I like repairing things OK
It finally got some attention, crikey, what a battle. I ordered the LM4766T and set about replacing it, on a multiple pin device I often cut the legs off it and remove it so it makes unsoldering the pins a lot easier. I cut a few and noticed the chip moved far more than you'd expect... I looked closer and noticed 3 or 4 legs had cracked at the 90 degree bend where the legs rest on the PCB! Odd. That could have caused the supply rail issue Nick hinted at but having got this far I continued to snip the rest of the legs then set about removing each pin through the PCB.
Erks, 7 (!) PCB tracks were broken too, kerrrikey! I wonder what caused that? The heatsink and chip are very well supported. The chip doesn't run hot at sensible room listening levels so I doubt its major thermal cycling.
New chip in and 7 tracks bridged later, it works, whahey!
Ta dah <jazz hands>
You'll just have to imagine Robin Trower blasting out the bench speakers OK.