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Old 16th February 2010, 03:28 PM   #1
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Default NAD 2100 - Shorted Transistors

A "broken" NAD 2100 amplifier landed in my lap. Now, I'd like to get it working.

The protection light is on full time. I popped the lid and noticed all power supply fuses are blown (F503/504/505/506 for those who have a schematic).

On further examination, I found 3 shorted transistors (Q434, 440, 442). All 3 transistors are from the Left Channel.

I'm not experienced in amplifier repair. Is the Transistor failure the cause of my problems, or are they the effect of some other problem? What other diagnostics should I do before replacing transistors/fuses?

I'm going to try and upload the schematic to this post.

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Old 16th February 2010, 08:22 PM   #2
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Location: Carlisle, England
If the amp doesnt have short circuit protection then a short on the output could have popped the transistors.
Possibly overheating might have popped them.

Simply replacing them might work but sometimes there is something else blown that either caused the problem or was caused by the problem.
Murton-Pike Systems PCBCAD51 pcb design software.
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Old 16th February 2010, 09:57 PM   #3
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The amp has a "protection circuit", but I have no idea if that protects the unit or simply alerts the user to a short circuit.

My concerns are exactly as you described. I don't want to go through the effort of ordering and installing new transistors, only to damage them again.

I have a good multimeter and I know how to check caps, resistors, and diodes. Without any specific suggestions, I'll just start checking everything...but that's tedious and easy to miss things.
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Old 16th February 2010, 11:04 PM   #4
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This should link to the schematic if anyone is interested. I circled the failed components in red.

Last edited by seabass23; 16th February 2010 at 11:06 PM.
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Old 4th May 2010, 01:10 PM   #5
MI-9448 is offline MI-9448  United States
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Got one of these too, Same prob.Changed some shorted outputs and what seems to be the Driver on + supply side which i believe is referred to as the source side and all is well so far.They were 2SD1238,2SA1386,2SC2344 AND 2SC3519.Drive and output seems similar to the old Dyna 400 but this has a much more elaborate and nicely balanced input and pre-drive stages.
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