Popping Fuses. Op-amp replacement question...

[J M Fahey]

Hi jdparsons.
You are right, the TIP147 that shows zeros between any of its pins is shorted.
When you buy it, order new micas, some grease, plastic nipples if needed, for the 4 of them, I don't trust very much on re-used micas.
To test the resistors: use the lowest ohm scale (200 r ?) and short test pins together.
You will *not* get a zero reading, but something like 0.3 to 0.7 r.
When you measure your wire ceramic resistors, substract what you measured earlier from your actual reading.
If they show under 1 ohm, they are probably right, the error lies on the measurement rather than on the component itself (in this particular case).
Be very careful when reseating your TIPs to keep them insulated from tha chassis or heatsink.
Good hunting.

[jdparsons1]

Thanks! this forum is great. Everyone is very helpful and tolerant to amateurs. Thats what makes a great community! Before i did these tests i actually ordered four new transistors and 4 new resistors so i will probably just replace with all new but its good to know that one was actually bad. The whole order came to $11 + shipping from Mouser.com.

This guitar amp is not worth the time i have put in to it but its a fun challenge to get it working and will make a good guitar amp for my kid if it works out. I'll let everyone know if it works.

[jdparsons1]

Got my transistors and changed them out with the old ones. Got everything put back together and put a fuse in and the amp powered on. I was so geeked!!! Hooked up a guitar and played about 5 chords through it and it made a loud pop and shut off. Now its back to popping fuses again.

Man,,, what a disappointment! To get so excited only to be let down like that.. I have no idea where to go with this thing now.

[J M Fahey]

Hey JDparsons, , you're *almost* there, no kidding.
Your *main* problem was burnt output transistors, you changed them, the amp worked.
That's 99% in my book, very good.
Yet there was that 1% left.
Unfortunately you will have to buy more transistors .
Maybe only one or two died, but I'd replace all.
Here the components are the cheap part, spending another time around 10 bucks is nothing compared to $50 an hour bench time, or more.
Please build a lamp limiter, and get two bulbs, a 25W one and a 100W one.
Replace what you find burnt, use new micas and nipples (that is one possible cause), check that the transistor tabs are insulated from chassis, and turn in on, with no signal in and all controls on "0", no speaker, USING THE LAMP LIMITER WITH A 25W BULB.
I suspect either the bias is very wrong , perhaps preset AP1 open or very dirty, or the amp is oscillating, or your re-used micas failed, there is *something* that burnt it.
The lamp limiter will let you measure.
*IF* you find nothing wrong, all voltages check, hook the speaker and play a little.
If all is well, use the 100W bulb which will let you play quite loud, all the time looking at your amp (chassis on the bench) for smoke, sparks, loud clicks on the speaker, whatever.
We are going slowly, step by step here.
If it works for half an hour or more, playing quite loud, only then you can risk hooking it straight on the wall, no limiter.
Any abnormal thing you see, turn it off and post here.
Check the bias as instructed on the schematic, first with the 100W bulb and later fine-tuning it without limiter.
EDIT: in this particular amp, IC3 *is* part of the power amp. Is it socketed? Hope not, a socket here is a *bad idea*.

[jdparsons1]

Thanks. I dont know if you read above but i am kind of an amateur when it comes to electronics.. What are "micas and nippes"?

In regards to the lamp limiter. Is there any kind of online tutorial that shows what that is or how to build?

[Enzo]

He refers to the mica insulator between the power transistor and the heat sink. And by nipples, I THINK he refers to those plastic shoulder washers around the transistor mounting screw.

When you have replaced any more blown transistors, personally I'd say move beyond the bulb limiter. It may not tell you the parts are working but running hot.

I would connect my meter set for AC current in placce of or better yet, in series with the mains fuse. SO you can monitor the current the unit draws from the wall outlet. Be prepared to turn the bias control. Power on and immediately set the bias for the lowest current you can. The amp SHOULD settle down to a pretty low current. If not, there is more trouble.

Things I would check:
there is a zener diode between the 40v rails. It sets 39v across the 5532 op amp. is that what is there, or is it open, leaving higher voltage between pins 4 and 8 on the IC (or simply across that zener.) That is D10.

Each output transistor has a 0.33 ohm 5w resistor associated. Are any of those open?

Check little Q10, Q11, either shorted? And their diodes D11,D12. SHorted?

Is the 5532 new? Really should be.

[jdparsons1]

Not sure if this will help but heres a pic. I took it in decent resolution so you can zoom in if needed.

http://jparsons.homeip.net/img_4286.jpg

Is the bias adjustment the small blue square pot with the white adjust slot on the top?

[Enzo]

Probably. Does it say AP1 next to it?

[jdparsons1]

No. It says P12. I dont see anything labeled AP1. I've looked the whole board over about 10 times.

[jdparsons1]

Im getting a little closer on this. One troubleshooting step to note is with the power transistors out of the circuit the amp turns on and does not blow the fuse. So does that mean that its either something to do with the transistors dead shorting or something after the transistors on the schematic?

[olddawg]

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?

The .33 ohm resistors are the emitter resistors for your output transistors. You test them with an ohm meter on the 1 ohm scale. Usually they will be good or open. If they are heat stressed I would replace them. Also check those 200 ohm resistors. There are not a lot of components in this amp. Yous should be able to check most of everything by lifting one leg of each component. If you got the amp for nothing and do not have a lot of experience you might want to just shotgun it. Relace all of the outputs, the driver transistors, the voltage amp IC, the diodes, and all of the lower value resistors. You also will probably want to use new micas and mounting screw insulators on the output transistors. If you don't, at least check that the collectors are not shorted to the heat sinks. Use new heat sink compound. Mark the original position of the bias pot. Move it a few times and test it with a meter. Bit it back to the original position and check the bias when the amp is repaired.

[jdparsons1]

How do you check the bias?

[jdparsons1]

i got the amp working! Replaced all of the parts that all of you recommended. Transistors, Diodes, IC3, Bridge Rectifier. I sat here and played it for 10 min or so and it doesnt blow or feel like its getting hot. I also used Arctic Silver compound on the transistors and heat sink and replaced the mica and shoulder washers.

One problem left though. as im playing i will get static sometimes. Could that be a component problem or is it more likely something like a dirty/corroded 1/4 inch jack? Every time it does it i get scared its gonna pop and i shut it off.

[jdparsons1]

I think it might be time to pitch it. I just sat down to play again and as soon as i turn it on its getting a loud static. I shut it off and unplugged the guitar and turned the volume all the way down and then turned it back on and it get the same thing. Nothing but loud static and pops. But it isn't blowing the fuse.

[52 Bill]

Unhook the speaker and check the output leads for any voltage.

Did you follow the schematic instructions and set the bias level?