Fender stage 100 DSP Head need help

[Jam]

stage 100 dsp

No I did not mount the board and the heatsink block to the chassis before testing? When you say heatsink block do you mean that thing that mount under the board that has grease stuff. DId I something wrong ?

[Enzo]

What we are concerned with is did the foil side of the circuit board or any component leads on teh top side touch the chassis while powered? I am not concerned it went without a heat sink for a couple minutes.

The heat sink is the metal strip the power transistoprs are screwed to, then there is a long square metal rod under that. The screws come through the chassis through teh rod and into the heat sink to hold it to the chassis. SO thermally the rod and chassis are part of the heat sink, but when you take it apart, they separate.

Did you insulate the board from the chassis when powered while out?

[Jam]

stage 100 dsp

What I did was turn it off and unmounted it and looked at the bottom then when I was done I set the board back it place and but I did not put the screws back in on the heatsink. What do you mean insulate the board from the chassis when powered while out? Do you think that the bottom of the board was touching the chassis I dont think it was.

[Enzo]

I am trying to think of ways that we could have damaged the circuit while working with the board. If you didn't touch it to ground, fine, it is a possibility I had to explore. As long as teh bpard was sitting up off the chassis on its big bar, and the bar was lined up with the screw holes, then that scehario is OK.

[Jam]

stage 100 dsp

But when I tested the first time I was playing it and then I had no sound and then the fuse blew and I could feel the amp getting and the transistors were really hot I dont know do they get that hot ? could the transistors cause my firt issue? and what step should I take now?

[52 Bill]

But when I tested the first time I was playing it and then I had no sound and then the fuse blew and I could feel the amp getting and the transistors were really hot I dont know do they get that hot ? could the transistors cause my firt issue? and what step should I take now?

You will need to remove and replace the shorted transistors. You should check all of the output transistors, emitter resistors, etc. before you proceed to powering it up again.

[Jam]

stage 100dsp

What is the right way to replace the out transistors? emitter resistors are those the big white ones what setting do you tse them with ? do you have to pull them out to a good reading . just check those two ?

[Enzo]

You unsolder the old transistor, clear solder from the holes, form the leads (bend them) on the new ones and drop them into the holes. Screw them to the heat sink, making sure to include the mica insulator and insulated washer on the screw. Smear some heatsink grease on both sides of the mica. (If they used the gray rubbery things instead of mica, then you don't need grease.) Now flip it over and solder the leads where they come through the board.

Those resistors are only hal;f an ohm. They will measure more or less like a short, like a piece of wire. if they measure open or more than an ohm, something is wrong. Yes you can test them while in circuit. If you are not sure about one, pull it and test it loose.

[Jam]

stage 100

I replaced the two output transistors and now the amp turns on and does not blow the fuse but do you think this was the issue I had in the start ?

[Enzo]

I have no idea, how does the amp work now?

[Jam]

stage 100

Sorry should of gave you more detail. The amp turns on and it sounds normal but I didnt know if that would fix my first promblem where it gets like weak sound then if I tap the head it will go back to normal. and some time it will be fine and then it gets weak and you can hear effects come in and out?

[Enzo]

Well, either that original problem was related or it was not. So all we can do is now re-explore the amp and find out if the original problem still exists. It COULD have been a power transistor in the act of failing, or it couls imply be a coincidence. Only using the amp will tell you.

[badraven]

Fender Stage 100 DSP intermitent problem

I know this subject is quite old but new info can help people having a similar problem to the one mentioned here. My sons Stage 100 DSP wouldn't play. I removed the chassis and began testing for power etc. Long story short, I replaced the coupling capacitor between the preamp section and the output section. The amp began working perfectly. Being all happy with the results I put the amp back together and played it - everything was fine. I took the amp back to my son the next morning, hooked it up and nothing. The amp was on but no sound. Well actually it played, but it was so low it was like it wasn't playing. I made the assumption that the output transistors were takin' a poop because that's what my FM212R did a couple of years ago. - time pasted, procrastination and all - I took the amp apart tonight, set the chassis on the bench and plugged it in and interestingly enough, it played perfectly. I was like WTF? I sat there playing it, grabbed my Multimeter and began checking test points against the schematic. At one point I bumped the chassis and the amp quit working. It was on but no sound again. I bumped it again and back to playing it went. So something was loose. I went to tapping things like the big caps, the large 5w resistors etc. and then I touched the heat sink for U11 and it teetered. Its supposed to me soldered in place to the board. I looked closed and the transistor mounted to it was moving with the heat sink (like I would expect). I removed the motherboard and flipped it over. I made a video and I will upload it here if this server will allow it. If not I will add a link to my youtube page. Anyway I wriggle it from below and I could see the terminals tipping back and forth. So I heated up the soldering gun, added some liquid flux and re-soldered the pins to the chip and the heat sink to the board, adding fresh solder to each point.
Now the amp works perfectly. Just thought I would share my findings. And yes, there are six output transistors, three for the - voltage and three for the + voltage.

https://www.youtube.com/watch?v=Bq4Jfj64mBk

[Jazz P Bass]

Sometimes you get lucky & it is an obvious fault.

U11 is the +5 voltage regulator by the way.