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Pioneer SX-1280 Blowing Output Transistors

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Bobs34969

New Member
Hello again everyone!
First off, I'd just like to thank everyone for their input and suggestions on my other forum post! Your knowledge and expertise have helped me resurrect my 1280 from the dead. I've gone through about every board on this thing. Power supply board, protection board, pre-amps, output boards and have replaced most if not all of the components on each of them. I have also replaced the un-obtanium output transistors with some NJW3281G and NJW1302G. Are these the right ones?? I found those suggestions on another thread. This is the issue I'm having. This morning I finished up rebuilding the last pre-amp board. Got the idle current and DC offset dialed in and In spec. I had it playing some music for the first time in a decade! Exciting! This thing is a monster! Had it playing some Led Zeppelin at a moderate volume level for about 5 hours. I had two meters hooked up on both pre amp boards so I could monitor the Idle current. All was well. A neighbor came over and we cranked it to a little below 1/2 volume and it was jamming! Shaking just about everything in the garage.Then about 5 minutes later, complete shutdown. Traced to a shorted output on the right channel. Replaced the transistor, and the 12A fuse and re-checked everything on the output and pre amp boards to make sure nothing else was damaged and reinstalled. Had it playing for another couple of hours then cranked it up again, and complete shutdown again. It plays for about 5-7 minutes at a volume in between 1/4 and 1/2 then shutsdown. What causes the outputs to just meltdown like that? and its only on one channel too. left channel hangs in there just fine. The highest I saw the idle current go was maybe 600mv? and that was when it was cranked. At moderate listening levels its hovers between 50-90 mv and plays there all day long. I'm at a loss here as I want to crank it up but can't! You'd think it would trigger protection mode before blowing the outputs ha! If anyone who has had this issue or knows what could be causing this issue could chime in that would be great. I've checked just about everything I know how to check so some ideas would be great!! Thanks for reading, and thanks for your time!
 
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Bobs34969 said:
The highest I saw the idle current go was maybe 600mv? and that was when it was cranked. At moderate listening levels its hovers between 50-90 mv and plays there all day long.
Click to expand...
Idle current is measured with no input, not while playing.
Where did you purchase the output transistors?
 
The symptom you describe is VERY typical of COUNTERFEIT output transistors (been there, done that). Typically they come no where near the Vceo rating of the genuine devices. What this causes is a situation where the unit will come up OK, and bias will setup OK, and run at moderate volume OK, but when the unit is required to produce a large amount of output, the breakdown voltage across one of the output transistors is momentarily exceeded, and the device shorts. This will cause immediate failure of the complementary device, and then the unit blows the fuse. The other scenario is where the Vceo is great enough, but the die in the transistor is very small, and does not have enough area to dissipate the heat produced when required to produce large power. The transistors sold at low prices on Ebay, are virtually ALWAYS COUNTERFEIT. I would be sure you are getting genuine devices. I would only buy those devices from a large distributer that distributes ON (Motorola) semiconductors, like Mouser, or Newark Etc. They supply parts to major industry (big bucks), and have too much skin in the game to sell any counterfeit devices.
 
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Thanks for all the responses everyone! I had an issue with one pre-amp board idle current running away when it was under a load that's why I kept the meter hooked up just so I could monitor it and make sure no funny business was happening. I got the output transistors from Mouser! I just checked the data-sheet and they are ON brand transistors. The rated VCEO on the data sheet shows 250V. I am thinking maybe they are just getting too hot and melting down. When I get home from work I'll attach a a picture of how I mounted the transistors. I fashioned a little hold down brace out of some alumunum that screws into the existing holes. I am also using the original insulator that goes between the heat-sink and transistor body as I couldn't see any damage on them. Could it be something on the pre-amp board or any other board that overloads the transistors at a high volume? Low-moderate volumes is not a problem and can stay there all day long. I appreciate all the help!
 
For an amp of that wattage we use mjl3281/1302 parts in the larger to-264 package as it has better thermal performance. Not sure how well it fits the heatsink involved.
I doubt that it is the pre-amp but can be eliminated by using an external pre as a test by plugin into the power amp in jacks in the back
 
When I have made the output transistor mounting modification to the SX-xx80 units, I have drilled and tapped the heatsink to accept the single screw mounting of the replacement transistors.
 
Thanks for the replies everyone! For now, I think the issue was related to heat. I have since replaced the blown transistors, and installed the same ones with some thermal paste and ensured they were fastened securely to the heat sink. They seem to be holding in there for the time being! The 1280 is a very powerful unit! rcs16 thanks for the part numbers. If these end up blowing sometime in the future, I will order those from mouser and install them. I underestimated how hot those things get at a high volume level!
 
Yes, thermal paste is an absolute must unless you're using the silicone thermal pads. If you were running the outputs dry it would certainly explain their premature demise. Nice result!
 
Power transistors must be tightly mounted AND use some thermal transfer medium that covers the entire heat transfer area.

I had a 35" or so Sony CRT TV years ago that had popped it's Horizontal Output Transistor. When I removed it, only about 1/3 of it's heat area had heat transfer compound on it (to fill microscopic gaps) and the transistor itself was visibly heat discolored.

Even more years ago I had a hi current DC power supply that used 4 output transistors in parallel. One had blown and since they were in parallel it took them all down.
Looking at the one that failed, again it was severely heat discolored - but had adequate quantities of heat transfer compound showing around all the edges.
Then I unscrewed it, and it was severely under torqued - darn near only finger tight. That did it....
That was an excellent example of thermal runaway.
 
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Thanks for the responses everyone! When I get home from vacation I will tear apart both channels again and double check the thermal conductivity. I will definitely increase the amount of thermal paste I apply as I don't think I did very much when I installed a few weeks ago. Is there a thermal paste you guys would recommend? At home I have some generic non-conductive thermal paste I can apply if that works. I seriously underestimated how much of a beast the 1280 is. I originally had a 1080 I restored and have been content with it for years, but I came across this 1280 and figured why not? I am glad I did! Its a monster! The low end it provides is astounding!
 
I use Wakefield 120 thermal compound. You do not want to use too much, just enough to cover the whole device surface and on both sides of the mica insulator. Too much compound is almost as bad as too little. As mtf said just enough to fill in any voids
 
Got home from vacation and got to work on the 1280. The thermal paste seemed to have done the trick! Rookie mistake on my part! Had it playing all day today at a pretty decent volume, and cranked it up multiple times without any transistors melting down.. ha! I'm happy to hear this bad boy sing again after it had been sitting dormant for 10+ years in a box in someones garage. I couldn't have done it without your guy's knowledge and expertise! Heres the finished product. Re finished the wood, polished up the faceplate and it looks pretty good for being 40+ years old. And it does sound as good as it looks! Thanks again.
 

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