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Old 17th December 2010, 11:27 PM   #1
spooney is offline spooney  United States
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Default Full rail voltage on output terminals

Got an amp here that had a severely blown power supply and shorted outputs in one channel. I put in two ps fets and new gate resistors for a partial power supply and clipped the shorted output transistors out of the bad channel. The amp powers up fine and plays cleanly through one channel but the one with the shorted outputs has full rail voltage on the output terminals of the amplifier. I only removed the shorted transistors for that channel (ktb688) but left the complements in (ktd718) . I know they need to be replaced before the amp is put back into service but for the time being I left them in simply because they weren't shorted. Is is possible that even though these outputs aren't shorted they are bad enough to pass full rail voltage to the output terminals?
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Old 17th December 2010, 11:33 PM   #2
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In my experience transistors usually go short D-S or C-E.

I have had the odd one do something unexpected like a G-S short.
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Old 17th December 2010, 11:47 PM   #3
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How many b688s in the blown channel?
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Old 18th December 2010, 12:56 AM   #4
spooney is offline spooney  United States
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2 total b688's . both were shorted .
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Old 18th December 2010, 01:23 AM   #5
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It's extremely rare for two parallel outputs to fail. Did you check them after you cut them out of the circuit?
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Old 18th December 2010, 02:41 AM   #6
spooney is offline spooney  United States
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I'm pretty sure I did but I will re check them. I believe they are still laying on my bench somewhere.
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Old 18th December 2010, 03:07 AM   #7
spooney is offline spooney  United States
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As usual you are correct Perry. One of the b688's is not shorted and appears to test normal with my meter
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Old 18th December 2010, 03:17 AM   #8
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Reinstall it and see if the amp will produce audio in the blown channel. If it draws excessive current pull the two 718s from that channel and check them for leakage. Reinstall the good one. Does the amp produce clean audio?

If not, find the defective driver or resistor. If it does, you know that it only needs outputs.
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Old 18th December 2010, 05:43 AM   #9
spooney is offline spooney  United States
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Awesome. Thanks for the advice.
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Old 18th December 2010, 07:39 AM   #10
spooney is offline spooney  United States
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Well I put the b688 back in and powered the amp up. It did not draw excessive current but it still had full rail voltage on the output terminals. I pulled the d718's and checked them and they seemed ok so I put them back in. I started comparing resistance measurements from the good channel to the bad channel and I think I am close to finding the problem. One of the driver transistors is giving me some crazy resistance readings. I pulled it to check it out and between the outer two legs of the transistor I am only seeing approximately 70 ohms of resistance. The part in question is a 2SD667 .
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