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Pioneer SX-950 Repair

Jcamp

Jcamp

New Member
Hello,


I need some help with a SX-950. I received it with the following components blown


Four power transistor - Replaced all with "original" 2SB557/2SD427 from eBay
Both 2SB536
One C1628
Both Bias pots blown
150 ohm R40,R39


I have replaced these transistors
2SB536 with KSA940
2SD381 with KSC2073
2SA818 with KSA1142
2SC1628 with KSC3503

Small 2SA726S all test good

Diodes and STV4H all test good


On the power supply board AWR101 I replaced these blown components

2SA720 with KSA1013
2SD313 with KSC2334Y
2SC1318 with KSC2690
Three 150 ohm resistors


Tested all voltages from the power supply board and matched service manual. I then powered up with no power transistors and relay clicked in and no smoke. I installed the power transistors triple checking placement and pinouts, also tested for shorts and tested the transistors before installing and everything looked good so I powered up and the 150 ohm resistors started smoking. I removed the power transistors to test and they all went short and open!. The transistors I ordered looked very suspicious, they were much lighter and thinner than the originals and the silk screening rubbed off of them very easy. I ordered eight new MJ21194G and MJ21193G to replace them. I don't want to install them until I am 100% sure the amp board is working correctly.

With no power transistors installed I am now getting 50v between collector and emitter and also collector and base! same thing on the negative side -50v between collector and emitter/collector and base (I have pulled all the transistors I replaced on the board to test and they all survived. I have spent so many hours trying to figure this out and I desperately need help.

I really appreciate any help!


Thanks :)
 
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The rail voltage reading (±50V) from C-E is normal since the emitters are effectively pulled to a point of balance (halfway between the +50V and -50V supply rails) by their resistors.

For the price of just two of the replacement power transistors, you might want to consider building a Dim Bulb Tester: http://audiokarma.org/forums/index....attempt-to-fix-my-sansui.512012/#post-6656998
This sill allow you to install the output transistors which tested "good" and power the unit on more safely for further diagnosis.
 
Watthour said:
The rail voltage reading (±50V) from C-E is normal since the emitters are effectively pulled to a point of balance (halfway between the +50V and -50V supply rails) by their resistors.

For the price of just two of the replacement power transistors, you might want to consider building a Dim Bulb Tester: http://audiokarma.org/forums/index....attempt-to-fix-my-sansui.512012/#post-6656998
This sill allow you to install the output transistors which tested "good" and power the unit on more safely for further diagnosis.
Click to expand...


Thank you for confirming that!. I will power up today with the DBT and new transistors in!
 
Watthour said:
The rail voltage reading (±50V) from C-E is normal since the emitters are effectively pulled to a point of balance (halfway between the +50V and -50V supply rails) by their resistors.

For the price of just two of the replacement power transistors, you might want to consider building a Dim Bulb Tester: http://audiokarma.org/forums/index....attempt-to-fix-my-sansui.512012/#post-6656998
This sill allow you to install the output transistors which tested "good" and power the unit on more safely for further diagnosis.
Click to expand...

What about 50v/-50v between C and B? is that normal?
 
Watthour said:
The rail voltage reading (±50V) from C-E is normal since the emitters are effectively pulled to a point of balance (halfway between the +50V and -50V supply rails) by their resistors.

For the price of just two of the replacement power transistors, you might want to consider building a Dim Bulb Tester: http://audiokarma.org/forums/index....attempt-to-fix-my-sansui.512012/#post-6656998
This sill allow you to install the output transistors which tested "good" and power the unit on more safely for further diagnosis.
Click to expand...

I forgot to ask. Can you confirm if 0.5W pots for the bias is enough. Also on first power up should I have them at 0 ohms (all the way counter clockwise) like the service manual says?.

thanks!
 
Jcamp said:
What about 50v/-50v between C and B? is that normal?
Click to expand...

That isn't normal, but it's also probably skewed due to the missing outputs' junctions pulling down that voltage. It could also indicate driver problems.
 
Oh, and ½ watt trimmers should be sufficient. The fixed resistors in that circuit (like R35) are only rated at ¼ watt anyway.
 
Watthour said:
That isn't normal, but it's also probably skewed due to the missing outputs' junctions pulling down that voltage. It could also indicate driver problems.
Click to expand...

Driver transistors are good and correct pinouts also checked all resistors and diodes on the amp board again. Power transistors are back in. Do you think this 100w bulb will save these expensive power transistors if I try it?
 
No details? I would suspect that installing REAL output transistors and properly adjusting everything may have made a difference, but that's merely a guess.
 
Watthour said:
No details? I would suspect that installing REAL output transistors and properly adjusting everything may have made a difference, but that's merely a guess.
Click to expand...

I pulled the amp board again and double checked everything and it all looked perfect so installed the new 0.5 watt trimmers and set to 0 ohms, plugged the receiver into the DBT, measured for DC at output with nothing, plugged in speakers and everything works. I measured the bias and offset and didn't have to change a thing. With the transistors in I am getting 41/-41V Between C and E, 41/-41V between C and B, 0V between E and B.
 
Hello, because is very hard find the original transistor I change the power amplifier module for other called "zener_cuasi_200w_st" by contruyetuvideorokola and sound very nice!
 
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