Fender Stage 112 SE

[nwguy]

I just picked up a Fender Stage 112 SE. When I power it up, I get a few seconds of hum and then the fuse blows. Here is what I have found: When I disconnect the +/- 45 volt transformer taps, the fuse holds and in fact the pre-amp section seems to work. I have tested all 6 power transistors Base to Collector and Base to Emitter and they are all good. The +/- 45 volt section of the power supply section seems to be good. I currently have components R113 and CR31 and CR32 under suspecion at I seen to get a short from the + 45 to - 45 rail. What is really throwing me off is trying to figure out how this power amp operates. It seems as though the signal comes out of U7A, flows through CR13 /14 and CR15/16 and into the bases of Q3/4/5 and Q6/7/8. How it then flows to the speaker is a mystery. Any thoughts here would be helpful. Schematic attached

[Enzo]

Sounds like you forgot the main test of the transistors, C to E. If C is shorted to E, the two other tests can still seem OK. But an EC short will pop fuses in an instant.

Disconnect any speaker or load until the amp is stable and not producing DC on the speaker wires.

This is a flying rails amplifier - some call it grounded emitter. The speaker load is between the center tap and filter cap common in the power supply, and ground. The five 0.22 ohm resistors in the negative speaker lead are in its ground return as a current sense.

Any part can fail, but the two diodes almost never fail. Please check the EC status of your outputs, shorted there would explain the 45v rail to rail short.

Took me a while to find R113, but it isn't the problem, resistors don't short.

[nwguy]

Enzo.... thanks so much for the help here. I went back and re-tested the power transistors and found 3 bad (2 TIP 147 and 1 TIP 142) testing C to E. Funny I thought B to C and B to E would have done it. Won't be making that mistake again. Guessing that a shorted out speaker is what caused 3 output transistors to fry. Anyway, appreciate the explanation on the amp also. Seems a bit convoluted to me but I guess that's why I don't design amps for a living.

[Enzo]

Actually we can make the amp arrangement more clear, but seemed not appropriate at the moment,. Actually, transistor failing cause a lot more speaker deaths than the other way around.

It is comforting to say "AHA! THIS thing cause that failure", but the truth is often as not we never know.

That loud hum you heard was DC on the speaker. In fact if you do that long enough, it will melt your speaker voice coil.

[nwguy]

As I go through this, and while waiting for the the replacement power transistors, what would happen if I connected just two good power transistors instead of all 6? Would it harm the amp at all? Not expecting it to sound great but would confirm that everything else is working. Obviously would have to be one on the plus side and one on the negative side.

[J M Fahey]

Maybe, one NPN and one PNP, but use a lamp bulb limiter, 40 to 70W bulb, not larger.
A real filament bulb, not LED, CDL, etc.
We need a little piece of tungsten wire getting red hot.

[vintagekiki]

About TIP142/TIP147 Darlington Transistors

https://www.onsemi.com/pub/Collateral/TIP140-D.PDF

Replacement and equivalent transistor

https://www.el-component.com/bipolar-transistors/tip142

https://www.el-component.com/bipolar-transistors/tip147

[nwguy]

Well, it just got weirder. I connected 1 good PNP and 1 good NPN and powered it up. Got the dreaded hum so I immediately shut it down. Took those two transistors and disconnected them. Figure I would double check voltages on my +/- 46 volt rails. First connected the light bulb limiter and disconnected the speaker. Powered up the amp and across R113, read about 67 volts DC. Figured that I wanted to then see what the voltage would read if I bypassed the LBL. Read about 90 volts and after a few seconds, the fuse blew. So, Unless I am mis-reading something, the only possibilities are any of the following shorting.... C52, C53, R113, CR31, CR32. Or, the amp is haunted and should be a door stop.

[Enzo]

First off DO NOT CONNECT A SPEAKER until the amp is stable and not making DC on the speaker leads. That is a good way to destroy the speaker. Also it makes fuses more likely to blow.

If your amp puts 45v on the speaker wires and no speaker is there, it often as not will not blow a fuse. WHen you connect a speaker, now of a sudden that 45v has to flow the current the 8 ohm speaker demands at that voltage. SO the fuses blow.

Forget R113. It is wired from +45 to -45, so it will have 90v across it normally. Resistors don't short, and if it were shorted anyway, then there would not be 90v across it, there would be zero.

The whole point of the bulb is to protect the amp. it prevents fuses blowing and the excess current of failures in the amp. Until the amp os stable, nothing good will come of bypassing it, any more than bypassing your seat belt will make car travel safer..

With power off, you can measure across CR31,32. Are they shorted? I bet not, but if they are you can remove them. Same with R113 and the two big caps, do they read shorted? LIkely not, remember you got 90v for a moment.

Please determine if the fuses don't blow -or the bulb doesn't glow bright - with speaker disconnected. WHen the bulb glows brightly, that means a fuse would have blown otherwise. If the light flashes for a moment at power up but settles down to dim, then fuses would not have blown. We need to know this first step.

[nwguy]

OK.... I did some further testing per Enzo's instructions. I am using a light bulb limiter with a 60 watt incandescent bulb. All Power transistors have had their collectors disconnected from the circuit. The speaker has been disconnected. When turning the amp on (through the LBL), the lamp glows very brightly. There is no DC voltage on the speaker terminals. When I disconnect both transformer secondary red leads, the light bulb goes out. Reconnecting one or the other red lead and the light bulb again glows brightly. With power off, I tested both diodes with the diode testor on my digital multi meter. Diodes remained in the circuit. They both read almost exacly the same. 1.9 in one direction and reversing leads read .54. For grins, I read R113. it started out at 0 and rose up to about 85K ohms

[nwguy]

Sorry for the error on the above, I disconnected the power transistors emitters.

[nwguy]

Update:
On a whim, I disconnected the collectors too. Now the only part of the power transistors in the circuit are the bases. Powered it up and the LBL didn't light at all and the fuse held. Checked the speaker output and found 2.3 volts DC. The voltage here crept up slowly so guessing a cap was charging. I decided the bypass the LBL and the fuse held and no smoke. Still however, I am getting that DC voltage on the speaker terminals. So..... is this DC voltage normal? Is there anything else left to do / check or do I just replace all 6 power transistors? Thanks for everyone's help.

[Enzo]

Resist the urge to bypass the bulb until the amp is working as a complete thing. With all output transistors removed, the output kinda floats, so if it wanders to a couple volts DC offset, it doesn't tell me much.

But since it holds with no outputs, that ought to put an end to R113 and the main caps as suspects, or the two big diodes.

If I had four of my six outputs shorted, I would certainly replace them all anyway.

Make sure to retain the little mica insulators under each one, and use fresh heat grease on new parts.

[nwguy]

Just wanted to post an update. I replaced all 6 power transistors and it is now working. My very big thanks to all who helped me out here with their advice.

[nwguy]

Think I spoke too soon. Replace all the power transistors and the fuse held. No DC on speaker leads. Took it off of the LBL, still worked fine. Connected a speaker and plugged in a mike.... perfect everything worked including reverb. Today I decided to plug in a guitar to see how it sounded. Got a hum and fuse blew. Unplugged the speaker and plugged in the LBL..... bright light. I'm ready to give up and toss this thing.