Calling all gurus Pioneer SX-1050 problem

Bought this Pioneer SX-1050 for the bargain price of $10! took it home and had loud crackling from the right channel. I proceeded to set the DC offset and Bias to spec, the noise stopped and the amp played nicely all day. The next day I turned it on and bang. Found output transistors fried. Ordered more and replaced them, and bang, they went up also. Proceeded to go through the amp, put a 100W incandescent bulb in series with the supply and checked every transistor in the amp board, found a couple buggered so proceeded to use all good parts to get one channel working. Checked all voltages at power circuit, these appeared ok. Powered the unit up with the bulb, it dimmed and the amp fired up, bias and offset seemed fine, test voltages on the amp board all within specs, played music. Plugged the amp into the power and this seemed to put it over the edge. DC offset shot up to 40V and power transistors fried AGAIN! Its like something is going into runaway and killing the bias or something. Maybe the protection circuit? I really am struggling with this as I only really played with tube amps, but liked the sound from this amp so much I really wanted to get it working. . Have a box of buggered output transistors now which brings a tear to my eye. I'd appreciate any suggestions and help anybody might be able to provide,Much appreciated. :D
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Couldn't help but wonder if these were fake, bought from Taiwan and they looked legit. NEC branded but who knows.... I'm sure there is something I'm missing elsewhere.
The varistors on the heatsink are still in the circuit and test fine.[IMGDEAD]https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-xpt1/v/t1.0-9/12494774_505689939616251_2273306258352229989_n.jpg?oh=160413c0e95bead7580890d86085c7da&oe=57782F93&__gda__=1464505495_b3c69eba707488c4006ea7e991cedb8f[/IMGDEAD]
 
Thought I'd go over the speaker protection circuit, found 3 very leaky capacitors. Replaced them. And all resistors and transistors and diodes checked out. I've heard people say check the protection circuit but I'm unsure how this could affect the amp bias or other amp variables....
 

Welcome

Member
2013-03-11 10:52 am
So only the side with Taiwan transistors blow? Those are definitely fake, then. Also look at the printing - is it uneven across transistors? NEC has higher quality standards than that.

Fakes are ultra-widespread, especially outside of Japan. Pretty much guaranteed to be fake, especially since they keep blowing.

Also, a respected manufacturer wouldn't supply transistors on styrofoam - that's an ESD disaster waiting to happen.
 
So only the side with Taiwan transistors blow? Those are definitely fake, then. Also look at the printing - is it uneven across transistors? NEC has higher quality standards than that.

Fakes are ultra-widespread, especially outside of Japan. Pretty much guaranteed to be fake, especially since they keep blowing.

Also, a respected manufacturer wouldn't supply transistors on styrofoam - that's an ESD disaster waiting to happen.
No sorry all the transistors blew, the original lot went first which was why I bought replacements. Having worked for NEC for a number of years I recognise the writing on the transistors being the older NEC and the newer NEC style. About late 70s and the other early 90s. So pretty sure they are legit. I'm open for suggestions how to test these at rated voltage and current to ensure they are ok.
As for the styrofoam they are high voltage transistors not eproms we're talking about. Surely that wouldn't be an issue?

It's so damn hard to check the circuit without the power transistors in because all the voltages depend on them being in circuit. Each time I thought I had corrected the fault and put a new set of transistors in I blew them. I will contact the supplier and double check the authenticity of the transistors anyway.:xeye:
 
FAKES~!!!!!!! no wonder I blew the new transistors up! originals on the right, fakes on the left. They use a similar transistor to give a reading with a DMM that seems ok, but power it up and POP! the VERY worst part of this story is I mixed and matched old and new and the ******* new transistors took the good old ones with them! so bloody angry right now. Guess I have my answer.[IMGDEAD]https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-xfa1/v/t1.0-9/12321565_505727119612533_6634623925139419870_n.jpg?oh=9e95010e5f0aacb37e4a0aa65dba225f&oe=5798FC14&__gda__=1469165858_06840ebfb4bf2cbe1dcc5b9796d908dd[/IMGDEAD]