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tranny problem, looking for advice on a power amp
john11
post Mar 23 2013, 08:47
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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 back 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.

Many thanks in advance. John.
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Glenn Gundlach
post Mar 23 2013, 10:39
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You do NOT want individual heatsinks for the transistors. Parallel bipolar transistors are not exactly matched and one of them will run higher current than the other(s). The higher current will increase its temperature which increases its sensitivity and cause it to take MORE of the load current which will make it get hotter and hotter and hotter until it fails from over temperature. BTW even if you had perfectly matched transistors, the warmest one will eventually take the load and heat up more. The only solution is to thermally link them and add degeneration resistors in the emitter circuits to distribute the load currents. I'm not sure what happens in FET power transistors but if the manufacturer had it working on a common heatsink it should continue to work that way.

Did you replace the heat sink grease (if used) and get the devices securely anchored?

As for capacitors, there is more than just uFd and Voltage. You want low ESR (Equivalent Series Resistance / highest ripple current) to minimize internal heating as heat is what kills electrolytic caps. This is mildly a problem at 60 Hz but critically important in switching (high frequency) power supplies. The DigiKey site lets you easily sort for things like ripple current / ESR.

G
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DonP
post Mar 23 2013, 14:56
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Uneven temperature would not be an instant effect, so if with the single chassis heatsink the amp is just dead, then there must be an electrical difference. Were some of the transistors originally isolated from ground?(sometimes done with a ceramic spacer)


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DVDdoug
post Mar 24 2013, 05:44
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QUOTE (DonP @ Mar 23 2013, 06:56) *
Were some of the transistors originally isolated from ground?(sometimes done with a ceramic spacer)
Right... Or, sometimes it's a mica insulator. If all of the transistors are on the same heatsink, they will have to be isolated. The "cases" of the transistors don't all share a common connection.
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john11
post Mar 24 2013, 08:53
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Hi. Thanks for the replies.

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 transistor cases having to be isolated which you mentioned what should be my next move here.

Many thanks. John.
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Glenn Gundlach
post Mar 24 2013, 10:03
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That depends on the case of the transistor. Many (most) power transistors have a metal back side that is connected to the collector terminal meaning there is power or signal on it. This needs to be insulated from the heatsink though there are rare instances where the entire heatsink is 'floating' and the transistors have no insulators to the heatsink. Also, there are some transistor cases that are insulated and require no extra insulators. The only way to be sure is to check service information and transistor data sheets and pay close attention which you apparently did. Is there any chance you put in an electrolytic backwards (reverse polarity) ?

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john11
post Mar 25 2013, 08:36
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Hi. Thanks for the reply, much appreciated.

Pretty sure the cap is ok as the board had a shaded half which indicates the negative terminal. The amp works when the transistors are separated off from each other and have their own heatsink, so i think the manufacturer may have picked a thermal paste which has an insulating effect.

The paste i am using is silver paste which is more expensive and apparently conducts heat away better, but perhaps conducts electricity better.

Is there anything i can buy which will insulate the transistors from each other. I popped open another amplifier and noticed the transistors have what looks like a piece of paper behind them, but the paper feels soft and silky to the touch so am not sure what it is.

Many thanks. John.
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