Need Help for my Fender Stage 112 SE

[PCL]

I'm Peter from Manila, Philippines. I'm a new member here at the music-electronics forum. My goal is to find a solution to my defective amp which electronic technicians around my area have continously failed to solve. I joined this site and went through forums about the Fender Stage 112 SE. I've read through various threads and I'm optimistic that professional fellows here at this forum can help me solve my amp problem.
I have a Fender Stage 112 SE and I've been using this amp for more than 7 years without a problem. Last December we were having a party at home. We went on a short break and I left the amp still on though with the volume turned down zero. After a few minutes when I went back to start playing again, I found out my amp has no power at all. Turning it on and off didn't work. My aunt told me that my niece played with a cable and plug it into the amp and the amp's power went off. After asking my niece I"ve found out she played with my spare instrument cable and plug it into the POWER AMP IN jack of the amp and she heard a pop. That started the trouble.
Brought my amp to an audio technician and told me that some power transistors are shorted. The technician repaired the amp and it was working again. but after 2 weeks it stopped working. Brought it back to the technician as a backjob. Got the amp again after more than a week. It played okay and after another 3 weeks it went dead again. Called the technician and told me I have to spend for repair again. Curiosity and anger prompted me to open the chassis and tried to look inside and find out what's wrong though I have limited knowledge with electronics repair. The fuse was blown. I brought it to a friend who has some knowledge with electronics though not a professional tech. We decided to do things ourselves. We found out that 1 TIP142 was missing at the far left end of the amp. The reason why it was left missing by the previous technician was a puzzle.
We replaced all TIP142 and TIP147 with new ones including the missing TIP142 and also some aging condensers with new ones as well. Again the amp worked fine. and I'm back playing with it.
It lasted for about less than 2 months when it smoked and stopped working during a gig.
Pulled the amp out again and found out a blown fuse, shorted power transistors and burned resistor at R76 and capacitor at C49. Replaced the burned resistor and capacitor with the same values respectively and resoldered L1 (Coil), likewise replaced all the power transistors with new ones again. Before reassembling the amp back to the cabinet, we tested the amp without load. Just turning it on. We've noticed the left most and the second TIP142 is becoming hot in about 3 minutes and too hot to touch with bare finger while the TIP 147 are warm and touchable. Though my friend says something is still wrong I still put the amp back and started using. It's working well like before though holding the bottom of the amp I can feel the heat generating from where those power transistors are located. Yes indeed the left side is turning very hot while the middle up to the right end where the transformer is located is just warm like normal. Undisturb I still use the amp from time to time. In less than a week the amp stopped working only with a blown fuse this time. I'd presume some of the power transistors gave up again and I'm suspecting the TIP142s which gets hotter than those TIP147s.
I'm getting too weary about this amp and feeling hopeless if I can still have my amp fixed. I'm currently using an old Marshall valvestate 8080. Clean and powerful, though the sound of this Fender amp still haunts me a lot. I just love the Fender amp's sound but I don't know what's the next step and where to go for assistance until I found this forum. Hoping the experience and expertise of fellows of this forum can help bring my Stage 112 back to life.

[Enzo]

OK, whenever you replace a TIP in the amp always check all those 0.47 ohm 5 watt sment resistors - one per transistor. Replace any that are open or grossly off value.

Since only a couple transistors are getting hot, we need to find out why. Check the voltage at the base of each 142. Also check at the base of each 147. The 147s are running cool enough you say, but the 142s are getting hot. The base voltage on the 147s and 142s ought to be about the same, ignoring polarity.

Your bias string is between +16 and -16VDC, through R92,93 and the four bias diodes CR13-16. Make sure both 16v suplies are close to voltage value, and measure voltage drop across each diode. ANy out of tolerance? Are R92,93 OK?

Q1,Q2 are limiters, and they operate on the TIP bases through diodes CR17,CR28. During testing, unsolder one end of each diode and lift it out of its hole to remove it from the circuit. No need to completely remove them, just lift one end. This will eliminate those Q1,2 curcuits. If that makes a difference, then something is bad in their circuit. Remember to restore the connection once we are done.

Does your tech have an oscilloscope? R75,76 are part of the stability network, and of one of them burnt up, your amp was oscillating hard at frequencies above audio. That might also explain overheating transistors and repeated failures. We scope the output to look for RF oscillation.

There are some things to start on.

[PCL]

Many thanks Enzo for your detailed guidance. I'm glad we have now a direction in having my troubled amp repaired.
Darlington transistors sold here locally in our area aren't trustworthy nor credible. Hence, we've ordered from Music Parts Guru for another set of TIPs 142 and 147 (again) through a friend in California. This may take about 2 weeks for us to recieve the parts before we can start working with the repair.
Meantime, are there any other parts that we can check while waiting for the TIPs to arrive?
Yes R76 burned up, that was when I found out TIP142 at Q3 was missing. This was probably done deliberately by the first technician my amp visited after my neice's accidental plugging into the amp in jack which shorted the amp. Don't know why the tech did that but the amp works alright for awhile with one missing TIP142 at Q3, for about 5 gigs until R76 burned. Just wondering did he found something wrong with the circuitry along that area of Q1, Q2, CR17, CR28 ..? Or the technician just don't have a spare TIP142 handy to replace and left Q3 empty instead? Sorry to think this way but we do have a lot of rogue technicians around here.

[Enzo]

Or it could even be an honest mistake. The amp has six output transistors, three per side. The amp will operate with just one transistor per side. It won't have full power capacity, but it will function. I sometimes do that during tests. The guy MIGHT have removed a bad transistor and then ran it on the remaining ones during test, then forgot to replace the missing one, since the amp would sound normal. But that is just a theory, he also could be dishonest...


Make sure to check the voltage drops across those bias diodes and related circuits.

[PCL]

Dishonest indeed he was. I remembered calling him inquiring about the TIP after I've discovered Q3 missing. I told him "you've managed to made me run that amp with an uneven pair of transistors?" He replied "it is Fender's design and that Q3 is a spare slot intended for a different model using the same board".
Here in my area, it isn't easy to discuss with technicians and give them an idea that you have some little basic knowledge in electronics as this would annoy them. Not all though but most do in my area. And if you don't give them any hint about that little knowledge of yours, bet you'll be their milking cow for the day. No offense meant though but these things simply happens here.
Thanks for the education Enzo. As mentioned that my amp will still function even with just one transistor per side .. Does that mean I can detach the bad ones and leave one working TIP each side so I can proceed and continue testing while waiting for the new TIPs to arrive?

[Enzo]

Yes, but don't load the amp or ask it to produce any power. Limit yourself to static tests and measurements, and signal flow with no load on the output.

[PCL]

Thanks a lot Enzo for your prompt reply. Will do that in my spare next week. I'll keep you posted with the test results and observations.