Popping Fuses. Op-amp replacement question...

[jdparsons1]

Ok... I got an amp from someone who said they were using an external cab on it and the amp blew. So im guessing impedance was wrong. I talked to a local repair shop and they said the first thing that usually goes in a solid state amp when impedance is wrong are the outputs. Looking at the schematic i THINK this board uses an IC for output and it looks to be IC3 but i am an amateur at reading schematics. I have good soldering and troubleshooting skills though. ;-)

Can anyone confirm for me that IC3 is in fact the output? Also, if it is, can a NE5532P opamp be replaced with something else? I have a box of opamps and found one that has a similar pinout, its a Brown Burr OPA2604AP.

I've attached the schematic for my amp.

[52 Bill]

Can anyone confirm for me that IC3 is in fact the output?

No, your amp has discrete output transistors for the final stage.

IC3 is part of the driver circuit. I doubt that it has anything to do with your fuse blowing problem.

Much more likely, you have a shorted output transistor. Check Q12, Q13, Q15 or Q16 for shorts.

[jdparsons1]

Thanks! I told you im an amateur when it comes to understanding this stuff! :-) Ok. so next question. Is there a way to test with a multimeter without desoldering from the board?

[52 Bill]

Is there a way to test with a multimeter without desoldering from the board?

Yes, but you will have to desolder to find the real problem.

The outputs are paralleled with each other, so if one is shorted they both will show a short.

Start by reading across all three pins of each output device. You might find some low readings, but there should be no direct shorts between any of the three legs.

Of course be sure to unplug the amp and discharge the filters before doing this test.

[jdparsons1]

Oh ya. no worries, i got the pcb out and on my bench. Would you mind helping me out with how to test on the multimeter? I can desolder the power transistors no problem for testing. Also, do you think if i replace those i should replace the power resistors that are in the same chain? I appreciate your help!

[52 Bill]

Would you mind helping me out with how to test on the multimeter? I can desolder the power transistors no problem for testing.

Does your meter have a diode test position?

Also, do you think if i replace those i should replace the power resistors that are in the same chain?

Check them with your meter. Are they open or really off value?

[Amp Kat]

You should pull one leg of the emitter resistors before checking the transistors to keep from getting erouneous readings. If the resistors are open they are bad and as Bill said they should be close to a couple ohms max or they may have gotten really heated. You should set the meter to diode test and get a diode drop corresponding to the correct polarity of each side of the transistor oh I'd say somewhere around .580 volts soulds about right.

[jdparsons1]

ug. i probably sound like an idiot but i dont even know if my multimeter has a diode setting. Its got a 3 way switch with V / Ohms/ Amps. I may just buy all 4 power transistors and 4 new power reistors and replace them all. Looked it up and they are total $12 online. I trust my soldering skills but not my use of a meter. If that doesnt work i will just scrap it... got the amp for free anyway. You guys have been alot of help i just dont want to be a pain when i have no idea how to even use a meter. Thanks!

[52 Bill]

Then apparently not. That's ok just use the ohm setting.
If it has a range setting, set it to the low ohms range.

Touch one lead (let's say the red one) on the left leg of the transistor and then touch the other lead to the center and then the right leg. You should either get a low reading or a high reading. If you get a high readings, reverse the red and black leads, and you should get low readings. If you got low readings, then reverse the leads and you should get high readings. Also test between the center and the right legs.

In any case, no combination of leads should read zero or really low ohms. If you find one that does read zero between two legs, then unsolder one of the two legs and test again. In most cases the short will be between the center leg and the right leg.

When you unsolder the one leg, it will remove it from the circuit so if it still reads zero, then it is bad. Be sure to test all of the outputs as often there will be more than one bad one.

If none of the outputs test as shorted, then you will have to check the power supply diodes.

[jdparsons1]

ok. on all 4 transistors, with my multimeter on the ohms setting and range set to lowest available i get the same reading no matter what combination of legs / leads i use. Readout is .004. So looking at the top of the pcb the red is on the far left leg and the black lead is on the center (.004). Then leaving the red on the left i move the black over to the right leg and get the same.

When you said "and then touch the other lead to the center and then the right leg" do you do the center and then take the lead off and then do the right. or did you mean touch the center with the lead and then make the lead touch both the center and right at the same time?

[52 Bill]

First thing, no read the center leg and then read the right leg.

What reading do you get when you short the two leads together?

If you reverse the two leads on the transistor is the reading the same?

[jdparsons1]

when i touch the two leads together i get 0's

if i reverse the two leads, yes the reading is the same.

[52 Bill]

It may be that your meter will not work for testing these transistors.

You could try to unsolder one of the output transistors and test it out of circuit to see what readings you get.

[jdparsons1]

Ok. Heres an update. I completely desoldered one of the power transistors from the pcb. It produces an entirely different set of results when i test it with the meter now. With the red lead on the left leg and the black on the far right leg i get a reading of 3.6 and the same if i put the black lead on the center leg. if i reverse and put the black lead on the left leg and then the red on the right or center i get 0. This transistor is a TIP142. Not sure if that matters.

[jdparsons1]

I'm bored today so i went ahead and desoldered the other 3 transistors. Two of them are TIP142 and have a polarity of NPN. Those two test exactly the same and im guessing they are good. The other two are TIP147 and have polarity of PNP and my guess is that those should be tested reverse of the other two. One of the two tests as i would expect it to. the other one gets zeros when black lead is on left leg and red lead is on right. the two TIP147 chips are not consistent with their reading so my guess is that one of those is the culprit. Sound right?

Next step is I want to test the four power resistors, two of them look kinda brown in the middle so i removed all 4 of them from the board as well. They are Dale RS-5 .33 ohm 1% resistors. What is the proper way to test those?