amplifier gone weird

[goodsguys]

Hi and thanks for reading this post.

My amplifier has been playing up on me and i noticed a swollen capacitor which i replaced with a new one of the same voltage and capacitance.

When reassembled the amp would not work at all, after a few hours of experimentation i noticed a row of transistors which are clipped onto the metal chassis of the amp, obviously this was done by the manufacturer to keep the transistors cool and i had to unclip these to get the board out.

When the transistors are clipped onto the chassis of the amp so in effect they are all sharing one large heatsink the amp does not work, i gave each transistor its own heatsink and now the amp works fine.

Can anyone tell me what is going on. Is there some kind of interference between the transistors, i was thinking perhaps the manufacturer used a thermal paste which effectively insulated the transistors from each other.

Many thanks in advance. John.

[Firebottle]

Is there a little more to this tale John?
Have you been blowing any fuses when all the transistors were on the common heat sink?

There are usually NPN and PNP transistors used for the output stage (not all amplifiers) and depending on whether the amp is single or dual supply rail, some or all transistors are normally isolated from the common heatsink.
This can be achieved with thin mica or silicon pads under the transistors.

Are you missing these?

A picture may be helpful,
Alan

[Ali Tait]

Wot Alan sez. Looks like you forgot to replace the pads that isolate the transistors from the heat sink.

[UV101]

bit like these

[greenhomeelectronics]

Just to add a bit to what has already been said - most amps have a protection circuit so hopefully no damage will have been done. The circuit (if fitted) should detect a problem and stop any damage occuring. If you look at the back of the transistors they will have a metal tab, if these tabs are being shorted together by being in contact with the heatsink, that would explain your problem. If all fuses are intact you have most likely got away with it thanks to the protection circuit.

[Dingdong]

As has been mentioned you've probably shorted the transistors out. Thankfully it does not seem to have caused any damage.

The transistors should have little 'pads' that provide electrical insulation but heat conductivity.

[goodsguys]

Hi. Thanks for the replies.

Unfortunately there were no pads on the back of the transistors. The only thing between the transistors and the chassis was a layer of thermal paste. I checked the chassis and it has nothing connected to it, no ground wires, no mains earth wire, no phono ground.

When i reassembled the amp i did not put on a new layer of paste, just thought i would power it up for a few seconds to see if the recap had fixed the problem, the power supply made a high pitched whining noise.

So i applied new paste and the whining noise went away but now there is no sound, perhaps the old paste was having an insulating effect and separating the transistors from each other and the new paste i am using does not do that.

Can you tell me anything about the pads you mentioned, is this something i should buy what should be my next move here.

Many thanks. John

[Dingdong]

You will need to measure the continuity between the transistors and the heatsink. If any are shorted you will need to fix that.

Which amp is it?