Yamaha MX-70

[bekim]

Hi


Yammie amp has blown right channel, 4-5 output transistors, many small trans, zeners, small caps, and so on. The output trans. are 2SB1362 & 2SD2053 which, I cannot find them originaly, and what replacement should I put there..??
I was thinking for MJL3281/1302 replac. and considering that this amp has 10 output devices, there should be all trannies same, and perhaps I should change trannies in left channel as well in order to have symmetrical amp. Drivers are 2SA1306 & 2SC3298.
Big condensers, 4 of them, are 25000uF/50V - two of them are "puffed up" slightly, is this problem...? It looks that this amp will cost me a lot of money, because I have to replace a lot of electrolytes, semis, and so on...

Any schematics, help, part list or similar are welcome.

thanx
bekim

 

[ostie01]

HI, I think sub could be for the 2sb1362 ---˙>2sa1106
2sd2053 --- >2sc3182

Normally around $2.00 each

 

[bekim]

idle adjusting

I manage to fix right channel and is ok, but left channel seems that is having idle voltage problems, or is oscilating (!?)
On TP in good working channel is 10mv +/- 1mV, but on faulty one minimum voltage is 25mV and after 2-3 minutes is increasing around 40mV. On the scope, I can see sine wave on amp output and trannies are getting warm. I had chance to check TP from one identical amp which is working.
I just don't know what to do with it..

anyone, any idea..

 

[XEAGLEKEEPER]

Get some freeze mist and start hitting the transistors while monitoring voltage to see which one causes voltage to drop back to 25mv . Also recheck your bias voltage maybe to high this would cause transistors to heat-up faster than normal.

Big condensers, 4 of them, are 25000uF/50V - two of them are "puffed up" slightly, is this problem...?

Yes they are probably internally shorted and will eventually swell up more and start leaking fluid if they haven't already.

Dave

 

[bekim]

Hi,

yes, the problem is that, the bias voltage is high on TP. In working channel, max voltage is 30mV, but the normal is 10mV +/- 1mV.
Even though condensers need replacement, I can listen right channel without any problem, and protection acts normaly when both channels are connected or even only one channel is connected on the mainboard.
I've desoldered all old solder and I put new silver high-grade solder just in case, as well all electrolytes I changed due to age of the amp. But, still I cannot adjust the bias below 30mV after 2-3minutes when amp is on.

pic 1 volts/div set on .2mv time/div set on .1ms
pic 2 volts.div set on .2mv time/div set on .1us

 

[bekim]

pic2