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1st December 2020, 04:28 AM
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#12 |
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diyAudio Member
Join Date: Sep 2013
Location: Phoenix, Arizona
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Hello,
I carefully checked emitter resistors, and replaced all 10 outputs in the channel in which the bad outputs were found. I reassembled the amp for a bench test. The same symptom is occurring - when remoting the amp on, the power led just pulses (and my lamp dims like current is being drawn.)
__________________
DMM (Fluke 115), good power supply (100 A, 12 to 16.5 V), soldering irons and skills. |
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1st December 2020, 04:49 AM
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#13 |
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diyAudio Member
Join Date: Sep 2013
Location: Phoenix, Arizona
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Now I've found at least one bad output in a group on the other side of the amp.
I don't remember seeing these last time, I guess I just missed them. Is it possible to blow outputs without even connecting speakers? I suppose I should just lift those out a see if it powers up.
__________________
DMM (Fluke 115), good power supply (100 A, 12 to 16.5 V), soldering irons and skills. |
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1st December 2020, 05:05 AM
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#14 |
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diyAudio Member
Join Date: Sep 2013
Location: Phoenix, Arizona
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OK I lifted the lowest base-emitter resistance device, rest of group read OK.
Did power-up test and it appears to power up normally now.
__________________
DMM (Fluke 115), good power supply (100 A, 12 to 16.5 V), soldering irons and skills. |
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1st December 2020, 05:09 AM
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#15 |
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diyAudio Member
Join Date: Nov 2003
Location: Louisiana
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Was it another 6487/6490?
It's possible to have virtually any component fail without a load. If it's another output transistor, they could be failing due to age.
__________________
Anyone who purchased the repair tutorial, email me (babin_perry@yahoo.com) if you want to download the latest version. |
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1st December 2020, 05:11 AM
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#16 |
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diyAudio Member
Join Date: Sep 2013
Location: Phoenix, Arizona
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Yes, and just one in the corresponding channel on the other side of the amp.
This one is a 90, before it was and 87. The channel I replace still reads OK after letting the amp stay on idle for a few minutes.
__________________
DMM (Fluke 115), good power supply (100 A, 12 to 16.5 V), soldering irons and skills. |
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1st December 2020, 05:23 AM
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#17 |
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diyAudio Member
Join Date: Nov 2003
Location: Louisiana
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The outputs can overheat at idle if not clamped down. There should be no bias current (and no heating) but the biasing can drift.
__________________
Anyone who purchased the repair tutorial, email me (babin_perry@yahoo.com) if you want to download the latest version. |
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1st December 2020, 05:47 AM
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#19 |
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diyAudio Member
Join Date: Nov 2003
Location: Louisiana
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You should make absolutely sure that the transistors lay perfectly flat on the heatsink. If not perfectly flat, the cover may not push them flat.The edge of the transistor can dig into the insulator and leave the transistor off of the sink (attached).
__________________
Anyone who purchased the repair tutorial, email me (babin_perry@yahoo.com) if you want to download the latest version. |
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