Fender Stage 112 SE

[glebert]

Where did you get the power transistors from? Anything on ebay is suspect now, I've seen fakes from US sellers in addition to the ones from China.

[nwguy]

No.... they all came from Mouser

[DrGonz78]

We’re they the same 4 transistor that blew before? Same 4 locations on the board. Did you check the driver transistors Q1 & Q2? Also check CR13-15 to make sure the bias string is intact. With those flying rail amps I noticed a lot of diodes in the power amp section failed when many of power transistors failed. In this case I would think the diodes were fine if you got signal out the amp. However, parts can be stressed by failures leading to trouble later, such as with driver transistors.

[nwguy]

I haven't started checking things yet but here is what I found from checking before the fuse blew again. CR-13 to 16 tested good. I was told that Q1 and 2 were not driver transistors but some type sensor network so if an excess of voltage appears at the emitters of the output transistors, the circuit gets shut down. Also was told that U7 is what actually drives the output transistors. Was I told wrong?

[DrGonz78]

I haven't started checking things yet but here is what I found from checking before the fuse blew again. CR-13 to 16 tested good. I was told that Q1 and 2 were not driver transistors but some type sensor network so if an excess of voltage appears at the emitters of the output transistors, the circuit gets shut down. Also was told that U7 is what actually drives the output transistors. Was I told wrong?

It's strange looking at the schematics of a flying rail design. Yes I see how the 4560 opamp is driving the output stage of the amp after all. So you were told right.

[Enzo]

Remember, those output TIP142/147 are darlingtons - dual transistors, so the driver transistors are right inside the output transistors.

Q1,2 are common output current limiters seen on many many SS power amps.

[g1]

I don't know if the 112SE has the 'bar' style heatsink that mounts to the chassis. If so, it's possible to mount them incorrectly in which case the output transistors will burn up pretty quick.
Since you picked it up in defective condition, you need to verify things were put together right by whoever was in it last.

[Enzo]

Yes. I always called them "wedge chassis" in my shop, perhaps no one else does. On front it has the narrow front panel, and in the rear is the tall rear panel, so the thing is sorta wedge shaped.

The chassis is part of the heat sink. The board has a flat heat sink for the transistors. Then a square shaped bar runs the length, maybe half inch square or so? ANyway, that bar must fit flat against the chassis, And it must fit flat against the board heat sink. There are clearance holes for the chassis screws, so if the screws sticking out are in the way of getting it all flat, you have the bar upside down.

And never fire it up without the heatsink screwed down.

And in all the models with that style chassis - all are solid state - there are the two main filter caps sticking up from the board. They are in the middle, right next to the heat sink. They are large-ish and sticking up like that vibrate and shake. A very common fault is that those caps crack their solder. REsult is either hum or intermittent hum.

[DrGonz78]

Yup I always have marked those heat sink bars with a sharpie marker to remember the correct orientation. Also I always put a good helping of thermal paste between both sides of that bar for good heat transfer.

[nwguy]

I would agree that it would make sense to check the correct transistors in the correct spot, which I did not do. However, the amp worked perfect 3 separate times and then it didn't, and blew the fuse. Scratching my head.

[Enzo]

Just occurred to me I have the file open, the part is officially "Heat sink bar" p/n 039203

That is just for anyone keeping score at home.

Now I am back to watching the Saints-Raiders game from Las Vegas. Currently 24-17 Raiders with the fourth quarter still to play.