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Old 17th May 2012, 10:14 PM   #11
mjona is offline mjona  New Zealand
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Location: Upper Hutt
Hello Mike,

OK - you didn't change these parts,however as you say it would be wise to check the integrity of the resistor to take instability out of the frame. (from this source)

It is possible to line up the transistors with the thermal washers and test for short circuits with the heatsink and everything is fine until you recheck everything after the screws are fully tightened down.

It is useful to tighten each transistor in turn and recheck when the job is complete. I find it easier to back track that way.

Sometimes a thermal washer can be punctured and a magnifier is needed to see the cause.

Are the washers in your amplifier mica or rubber?

Michael Jonassen

Last edited by mjona; 17th May 2012 at 10:35 PM.
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Old 17th May 2012, 10:45 PM   #12
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Location: Victoria,TX
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You mentioned that everything worked OKAY until you installed the heat sink. Meaning the problem it is in the way you are installing the transistor into the heat sink. What you have to do is to check the resistance to find out if you a have a shorted. If you do then remove each transistor one at a time until you find which one it is causing the short.
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Old 18th May 2012, 12:06 AM   #13
cihtog is offline cihtog  United States
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Join Date: Nov 2006
To quote an often repeated phrase - "stop blowing fuses, use a dim bulb".

Based on the information in your first post, there is something definitely wrong with your method of attaching the heat sink. Conductive paste, bad or missing insulators, missing top hats under the mounting screws. Somewhere, somehow, your heat sink is part of the circuit, and shorting the rails to ground and/or each other.

In your schematic, the collectors of your outputs are connected directly to both power rails - the mounting points, including screw holes and heat spreader, of your output transistors, are connected internally to the collector of those transistors.

Artic Silver 5 is for computer heat sinks, NOT amplifier transistors. Regular old silicon thermal grease is just fine - some people even prefer no grease at all and instead use silicon inserts.

Last edited by cihtog; 18th May 2012 at 12:19 AM.
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Old 18th May 2012, 01:29 AM   #14
mjona is offline mjona  New Zealand
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Join Date: Jul 2007
Location: Upper Hutt

A perhaps hidden matter I forgot to mention re checking for shorts is that the heatsinks are anodised - so the layer of black material insulates the heatsink aluminium from the test lead of a multimeter. I assumed you would have covered this point in the tests you made.

If not then you need to find a place on the heatsink or a connection to the chassis earth where there is some bare metal. If you remove a screw securing the heatsink to the chassis you might find bare metal inside the screw hole.

Michael Jonassen

Last edited by mjona; 18th May 2012 at 01:31 AM.
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Old 18th May 2012, 05:12 AM   #15
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Join Date: Jan 2011
Hey guys, thanks for all the help. Got it back together tonight. It was just the paste. That is why I was unable to find a short. Doesn't show up just probing with meter. I knew everything was assembled correctly and could find nothing out of place. Cleaned all the Artic Silver 5 off ( real pain in the a$$) and replaced with Artic silver Ceramique 2. Good to go. Now I need to find the time to lesten to it and see how it sounds compared to the stock one.
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