replaced the 2sc2525 with NEC C2766 007 which KGE electronic store said was the equivalent. The NEC failed and the adjacent green colored transistor 2SA1075 ended up with a hole burnt through it.
NEC hasn't made the C2766 for years. Anything being sold with that number is either going to be used or fake.
Maybe I should have changed both transistors at the same time.
Not maybe, but absolutely.
If you have a blown channel, both outputs should always be replaced.
Yes, even if the other checks OK on a meter. Also would be a good idea to replace the drivers as well. You didn't say which channel (L or R), but it will be (Q13 and 15) or (14 and 16).
They might test ok on a meter, but can take hit and might fail, after you get the output stage squared away. Good replacements would be 2SA1930 for the original A985,
and 2SC5171 for the original C2275. Just to be safe, I would check three of the 6 transistors (Q7-12) which are present in the bad channel.
It would also be a good idea to go through and check the components in the protection circuit as well.
If you don't have a service manual, get one. They are available at places for download like hifiengine.com, or the AK archive.
Below is a schematic of the power amp section. Make sure to check all circuit components in both the output and driver stages (resistors, capacitors, diodes, etc).
The last thing you need is an open resistor or shorted diode, after installing new transistors.
The original or what looks like the original are available from ALIEXPRESS.COM.
There are no new originals. If they are selling new devices with that number, they are fakes and should be avoided. Best to stick with the Sanken
devices which are currently in production. Get them from one of the trusted sources which get talked about here.
