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Hello everyone.
I'm fixing Sunn 1200s and have reached point when i have problem that can not understand.
I've bought it `not working` on ebay for small price and it was in really bad condition - few resistors on board were burned to ashes, some tracks on board burned, some elements were removed by someone before i got it. Amplifier had only one mains fuse and damage was really terrifying.
So i started a reconstruction - removed everything suspitious from board and checked every element in power amplifier. I have attached image with failed components - they marked with red. I'm pretty sure that i have double checked every element and they all are good now. I have discovered also that R185 and R185 (aroung Q18) were swapped in places - i think it is factory mistake.
I have replaced 2sd1857a / 2sb1236b pair with 2sc2383 / 2sa1013 transistors, all other were easy to buy in my area.
After this replacement amp started to sound, and it is loud and sounds good, but i have another problem:
Q10, Q11, R168 are too hot, more than 80 degree C with no signal few minutes after start. Voldage drop across R168 is 70V, so it means that across R168 and R166 drop is 94v - all supply voltage! So, my question is:
What do transistors Q12 and Q13 do? Should be any dc voltage drop between emitter and collector for theese transistors? What could be reason makes Q10 and Q11 run so hot?
Thank you.
I'm fixing Sunn 1200s and have reached point when i have problem that can not understand.
I've bought it `not working` on ebay for small price and it was in really bad condition - few resistors on board were burned to ashes, some tracks on board burned, some elements were removed by someone before i got it. Amplifier had only one mains fuse and damage was really terrifying.
So i started a reconstruction - removed everything suspitious from board and checked every element in power amplifier. I have attached image with failed components - they marked with red. I'm pretty sure that i have double checked every element and they all are good now. I have discovered also that R185 and R185 (aroung Q18) were swapped in places - i think it is factory mistake.
I have replaced 2sd1857a / 2sb1236b pair with 2sc2383 / 2sa1013 transistors, all other were easy to buy in my area.
After this replacement amp started to sound, and it is loud and sounds good, but i have another problem:
Q10, Q11, R168 are too hot, more than 80 degree C with no signal few minutes after start. Voldage drop across R168 is 70V, so it means that across R168 and R166 drop is 94v - all supply voltage! So, my question is:
What do transistors Q12 and Q13 do? Should be any dc voltage drop between emitter and collector for theese transistors? What could be reason makes Q10 and Q11 run so hot?
Thank you.
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