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They are not matched. ANy chance you mounted a new part there and left out the mica insulating wafer? IE is the metal back of the transistor touching the heat sink?
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Education is what you're left with after you have forgotten what you have learned. -
No I took great care in cleaning off all the old thermal paste residue from the heatsink, transistors and wafers. I'm guessing the PC grade thermal compound I used isnt electrically insulating as well. Or there's some solder debris on the board, but I did wash with alcohol after soldering.Comment
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The center leg of those transistors is collector, so is the metal back. measure resistance from center leg to heat sink. That will tell you if they are shorted or not.Education is what you're left with after you have forgotten what you have learned.Comment
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Thanks for the help so far guys. Much appreciated.Comment
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Question - Is it possible to overtighten the transistor to the heatsink? It appears to have shorted around the screw hole.Comment
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The transistors are a plastic package are they not?
I guess they could crack.
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Or the mica failed.Education is what you're left with after you have forgotten what you have learned.Comment
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right guys -- been a busy week. It looks like the Mica failed. The burnt up transistor has been swapped and I've ditched the mica on all the transistors and replaced with silicon pads. The amp is now operational and I'm testing the circuit. The voltage is now 0.3v and seems to be stable. Relay operates as it should.Comment
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Do you mean the voltage on the output? It seems to high. There is a trimmer on the PC board to set it to 0V (SVR301). Have you tried turning it?Originally posted by andywragg View Post
MarkComment
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Trimmer is all the way down. Just left it to sit unpowered for a bit and fired it back up. The DC is back up to 0.47V but slowly falling. I've pulled the board off the heat sink and now looking to see what to replace next. Trimmer measures 500ohm which isnt too far off for a 470ohm pot -- isn't it?Comment
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Result - I've swapped out the 2 transistors Q301 and Q067 and not only is the DC voltage down to 0.009 at R341 there a noticeable reduction in background hiss. The relay actuates quickly after powered up as well.
Just want to bench test it for an hour but its getting late here so a job for tomorrow.
Thanks for your help guys!!Comment
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