Fender stage 100 DSP Head need help

[Jam]

I have a Fender stage 100 DSP Head and when I hook it up it will play fine and then once in while 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 effect come in and out. I dont know where to start

[booj]

bad connection somewhere! I've found that putting the amp on its side makes removal of the chassis easier. If your hip to bench work, that's when the fun begins. If not just keep bangin' on it.

[Jam]

Fender stage 100 dsp now blows fuses

I hooked it up and it worked but then it stop working and blew the fuse and so I took it a part to check it out everything looks ok then I put it together and it still blows fuse??

[olddawg]

I hooked it up and it worked but then it stop working and blew the fuse and so I took it a part to check it out everything looks ok then I put it together and it still blows fuse??

See if you shorted out the outputs.

[Jam]

Stage100

What do you mean by that ?

[olddawg]

What do you mean by that ?

I don't have the schematic in front of me but I am assuming this is an SS amp. It has either discrete bipolar transistors for the output stage or a monoblock IC. Either way, if you are blowing fuses you are drawing excessive current. That symptom in SS amps usually means shorted output devices. Check them with an ohm meter. It could be a rectifier or smething else shorted but that is where I would start. If none of this make sense you should not be working on the amp. It's dangerous.

[Jam]

stage 100 fuse blown

It is a SS amp when you say out put transistors are those the three that have the neg rail of -45v and three +45 V what is the best way to test them with no power?

[olddawg]

It is a SS amp when you say out put transistors are those the three that have the neg rail of -45v and three +45 V what is the best way to test them with no power?

Generally there would be 2 or 4 output transistors in push/pull class AB configuration. Each set would have a PNP and An NPN transistor. I don't know why you wolud have 3 unless one is a driver or a regulator transistor. I doubt that this amp uses Darlingtons in the output stage. Yes, check the transistors with the power off, preferably out of circuit. Check for shorts and foreward bias the junctions with an ohm meter on the diode scale if it has it. Your meter may even measure hfe. Most new ones do. You are looking most likely for a direct short though if you are immediately blowing fuses. A variac would be helpful. Don't keep blowing fuses. Each time you do you risk damaging more components. Was there any smoke or a burnt smell? If you shorted one or more output transistors you could have also damaged driver transistors as well as current limiting resistors, diodes, etc. Bottom line, if you are blowing fuses you have a short somewhere. It is an SS amp. You can run it without a load, but if you shorted the output stage you could have serious problems. It is simply going to require real troubleshooting. If you do not understand how the amp works your task may be difficult.

[Enzo]

It is a push pull. All it needs is the same number of xstrs on each polarity. COUld be one pair or two pair or the three pair he has. Step up to higher power amps like PA power amps adn you might have five or six pairs or more. They are most likely TIP142 and TIP147, three of each. Total of six.

Your original problem was very low output, right? And only NOW is it blowing fuses, right? In other words i0t wasn't blowing fuses until you started trying to fix teh low output, is that right?

If so, then you have either managed to leave out some insulator, or gotten some bit of hardware like a screw or nut under a circuit board, or some cable is in the wrong position in a conector, or you have managed to short out a power transistor.

Each power transistor has three legs. Turn the power off and unplug the amp and let it sit for ten minutes or more. Now use your ohm meter and check for a short between any two legs on each power transistor.

[Jam]

Fender stage 100 head blown fuses

It does keep blowing fuses. Ok I think I have six power transistors three are are -volts and three are + volts. I tested all of them but when I test Q18 and Q15 I would get a reading on the left two legs but not the right two does it matter what two legs you check and you should have a reading on both sides? and you use diode on the meter ? So does that mean the two transistors are bad TIP 142 and Tip 147.

[Enzo]

Uh... No reading? Do you mean zero ohms?

Yes, Q14-19 are your output transistors. You are not taking readings to ground, you are taking them from leg to leg on these things. There are three legs each, so there are three combinations of legs. At each part, test all three combinations. Call the legs 1,2,and 3. SO check resistance from 1-2, from 2-3, and from 1-3. If any of those reads like zero ohms or 1 ohm, then it is shorted.

Here is a quick check, look for diodes D25, D26. They are just in front of Q18 and Q15. Does either test shorted? If not, then the power transistors are likely not either.

But you didn't answer my question. Is the fuse blowing something new? Was it not blowing fuses when it had the weak output? And has it ONLY started blowing fuses since we opened it up? The point is we look for different things when it is just a failure of the circuit than when we have a new problem caused by our work.

Did you at any time take the screws out of those transistors? DId you at any time demount the main circuit board? Look under teh main board from the end of the chassis, is anything under there that could touch the circuitry?

[Jam]

stage 100 head dsp

Yes I had zero ohms on the 2=3 leg of Q18 and Q15 . D25 and D26 Seem to get a reading on it both ways but I did not pull it out to test it. If the transistor is bad does it mean the diodes D25, D26 are bad too ? The blown fuse is new it did not do it when it had a weak output. I did open it up to look at it and I did demount it to look for any cracks in the solder joints. when I was done doing that I pluged it in to test it and it played fine and did not act up but little bit later it had blown the fuse. the amp did seem pretty warm to me too

[Enzo]

I only mentioned teh diodes since they are in parallel with the power transistors. If the power transistors short, then teh reading across those diodes should read short too. It is pretty unlikely the diodes are damaged.

Now then, each power transistor has a 5 watt rectangular power resistor next to it. In fact there is one beside each of those diodes. They should each measure as very low resistance. Make sure they all are OK. if they measure like they are almost shorted they are OK. They are supposed to be half an ohm. When power transistors fail, those resistors sometimes burn out.

If you remove those two shorted transistors, the rest of the amp may still be OK. It can run without them at reduced power - for testing in other words.

[52 Bill]

Jam:
When you plugged it in to test it after checking for bad solder joints, did you remount the board and the heatsink block to the chassis before testing?

Enzo:
Is this one of the amps that will go into thermal runaway and overheat unless the outputs are bolted down to the heatsink?

[Enzo]

No, it is not THAT unstable, I run them off the chassis all the time. Throw a towell or newspaper across the chassis and flip the board over onto it to expose the underside for example.

I am concerned some insykator was lost or something mechanical. Of course it could just be coincidence the output blew now.