Yamaha EMX 640 power amp fault

[maxoche]

Hello you all,
A little help please for a broken P.A amplifier.
here, I have a Yamaha emx 640 that has a fault after powering on in bridged mode with speakers plugged in the L-R outputs. The amplifier turned on, with audio passing though then rapidly went down, with a blowed general fuse that now keeps on blowing. I checked the amp insides and it's very clean with no burned parts or looking strange. Just the main reservoir caps cans a little domed, with no leaks.
The power transformer is ok and clearly, the small+/- 15v fuses are ok. it's the output section that is down. I am surprised that the protection circuit didn't detect the fault.
my question is on the way to approach the fault. where should I look first? test each of the darlingon output transistors separatly ? should I try to power the amp with the output transistors out of the circuit and get those back pair by pair? Maybe you guys know about the usual suspects...

thanks a lot,

Maxime

 

[Khron]

https://sound-au.com/troubleshooting.htm

 

[maxoche]

thanks Khron. That's a nice compiler for the diy/repair man. Here my concerns are more centered to my question with these output transistors.

 

[maxoche]

thanks Khron. That's a nice compiler for the diy/repair man. Here my concerns are more centered to my question with these output transistors.

 

[maxoche]

So I tested the output darlington transistors and 3 out of 12 appear to be dead. some being shorted between emitter and collector.I was also trying to test the surrounding transistors, in case they failed too. these transistors appear to be high voltage epitaxial switching type and the readings are confuse as I only get 2 diode voltage readings out of of the six different meter testlead positions, instead of 3. All these transistors have the same readings(2SC4488- 2SA1708). what do you think of that ? should I change those too ?

 

[JohnRoberts]

I recall those amps when they were new... they used a trick boosted rail PS for higher efficiency.

With power amp repair, obviously replace all known bad devices. With questionable driver transistors you must ask yourself do you feel lucky? A faulty driver transistor can take out all your new output transistors so when in doubt replace em all....

JR

 

[maxoche]

sure ! that's wise. It seems that the surroundings are ok. I ll take your advice and check each one of the drivers thoroughly.

 

[maxoche]

thanks, JR !