yamaha emx640 darlington amp repair

[erikb]

Hi there,

I'm trying to fix the amp board of a yamaha emx640 but i really could use some help.

This is an amp that uses sanken 2SB1647 and 2SC2560 darlingtons for the amplification. I've found one pair that was shorted and two small 220pf silver mica caps that were blown (because of the transistor failure, i assume). Take a look at the picture to see which ones. The drivers still seem fine.

I replaced all the silver mica caps, because those were easy to find and were cheap, but the darlingtons are much harder to find, especially the Y variant with a hFE > 15000. But i don't really get the configuration of this output section so i have a few questions.

My questions are:
1. Can i replace just this pair, only matched to each other, but with different hFE's than the rest? Or should i replace all the darlingtons in this channel? Or in the whole amp, because the power consumption of the two channels will become unbalanced?
2. Are there any other parts that obviously need inspection after this failure? The power supply perhaps?

Thanks, Erik

 

[soulman25]

Did you check the all of the Darlingtons to see if they were leaky ? If they have leaky beta or hFE, I would replace them all. Just to be safe.