SS help

[JC@]

Hi, I have a problem with a Laney Concept CR-150 PA Head and I haven't found the solution yet.
The power amp makes a loud sound like a semi-authomatic gun: pam pam pam pam. Really loud.

It is not in the preamp because I have it disconnected.
It is not the power transistors.
The rectifier and filter caps I believe they are ok.
All solder points have been resoldered.

Does anybody have some experience with this?

[Enzo]

It is motorboating - power oscillating at that low frequency. Hard to say why. I would be looking for something open, like a connection somewhere, rather than a bad part.

How did you disconnect the preamp? Sometimes the input of a power amp receives its ground from the preamp input.

[JC@]

Enzo, the power amp board has a ground wire bolted to the chassis.
The circuit looks simple though it has a few smd components, including some transistors, under the board.

[Enzo]

I am not worried about a large wire grounding the circuit to earth. I am more concerned about a possible open path to circuit ground for a porttion of a circuit. In analogy consider what would happen if the cathode resistor on a Fender preamp stage didn't make it to ground because one of the little jumper wires from the eyelet board was broken. I would be looking at the board closely. A ground trace might be burnt open at a narrow spot, almost invisibly. Of course an open resistor could do the same as an open trace.

This will just have to be a lot of troubleshooting and isolating. MAke sure all decoupling happens

[JC@]

still here

About this Laney Concept CR-150 PA Head
It stopped making that loud pam-pam noise but I have loud hum and 37volts dc on the output.
Both power transistors and tip31 & 32 test ok
One problem is that I don't have rework equippment for SMD components, and there are quite a few transistors and a fet under the board. Anyway I believe there are no shorts in them but I have a doubt about TS5 and TS6. What are these, limiters?

Any help or should I just quit/let it pass?

[Enzo]

No shame in bailing out if it is beyond you.

The output is being turned on somewhere. COUld be an open part further back. In my experience the sm stuff is less likely to be what blows up in a power amp. I hate the stuff, myself.

I don't have it, but you need to get the proper schematic for better troubleshooting.