Lleman amp broken, help needed!

[Boscoe]

Right I got THIS amp and soldered it and it was all working good so i was listening to music on it for a while to really test it then all of a sudden the music cut out then I just got wot sounded like ineterference amplified a lot. The power resistors started to get very hot and started smoking a little. Do you think if I just replaced them it would fix it? Or would there be a cause to why this is hapening to the resistors?

Thanks

[jackand08]

If the resistors got hot, it sounds like you may have a blown output transistor or it could be something in the driver stages that is providing much too high a voltage or current to the output transistors, causing them to allow a lot of current to flow through the resistors and heat them up.

For a start, I'd check any other components for signs of burning and replace them if necessary. Also, make sure that the transistor BC547 that is meant to be in contact with the heatsink is making good contact. This transistor, if I'm right, acts as thermal compensation for the bias current. This may explain why after you had listened to music for a while (and the transistors had warmed), the bias may have become unstable and ran out of control and drawn too much current through the transistors, explaining the heating of the resistors.

As usual, you should check that all of the supply voltages are in tolerance and none of the other parts are blown as thee may have just been the cause of the problem. Also check try checking the PCB for any solder bridges or burnt traces and if you want, test the output transistors with a checker but make sure they're removed from the circuit first, otherwise you might get false readings.

Just wondering, but what heatsink did you use with this amp (i.e. the velleman heatsink or your own)?

Also, for next time, this thread should really be in the solid state forum instead as it would definitely get more attention there... Unfortunately, now that it has been posted, you can't change this

[Boscoe]

Thanks for the reply.

So it could be the transitors aswell? I set the bias to what it said in the manual. I have already ordered new resistors so i'll change them and see what happens.

The heatsinks I'm using are from CPU coolers designed for CPUs with a 95W TDP so I thought they would be adaquete as I have one on each transitor.

EDIT: Ahhh I see that trabnsitor, I thought you meant the larger ones. and no it is not in contact with any heatsink as I don't really know how I could do that but I'll obvisously have to find out a way.

[AndrewT]

Originally Posted by Boscoe

.....The heatsinks I'm using are from CPU coolers designed for CPUs with a 95W TDP so I thought they would be adaquete as I have one on each transistor.....

a blown heatsink with close spaced fins performs very differently when unblown.
Expect the dissipation capacity to drop, maybe to as low as 10% of rated capacity.

Your finger will tell you a lot about how hot the transistor is running.

[Boscoe]

Okay fair enough, do you know the tempreture these transistors are supoposed to be running at? Maximum?

[AndrewT]

I'll guess and suggest, cool enough to hold your finger on the transistor indefinately. Maybe much cooler than that.

[Boscoe]

Can I replace a BC547B with a BC548B?

[AndrewT]

the 547 has a greater Vce0.
Do you need that extra voltage capability?

Do you have the BC5xx datasheets?

[Boscoe]

Yeh link HERE.

[jaycee]

Have just seen this thread in addition to the other one in solid state

CPU heatsinks are not really suitable unless you stick 3/4 of them to a metal plate. Even then they are not very good unless you use the fans, especially if they have very closely spaced fins.

The transistor T6 *MUST* be in contact with the main heatsink. If it isn't the output transistors will go into thermal runaway, and will fry. If the two large power resistors have fried, then you can almost guarantee the output transistors have also fried.