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NAD 2100 Amp has High Center Voltage Stuck In Protection

TubeJunkie

TubeJunkie

Master of None
Hello. I have a NAD 2100 amp that is in Protection mode. The center voltage is at -1.2 VDC for both channels. Shouldn't be no higher than 100 mvDC.... I can adjust the bias in each channel to the specified 15 mvDC. So, seems like the problem is in the protection circuit, but I've yet to find a faulty component there. Can someone here please give me some advice and guidance on this issue? Thanks in advance!

Service Manual link https://www.hifiengine.com/manual_library/nad/2100.shtml
 
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I've been all over this amp again today. Still can't find a faulty component. Does anybody have any advise? I have Good adjustable bias set to 15 mvDC, but has high center voltage at 1.2 VDC.
 
first make sure its not set for bridged then see if centre voltages change with soft clipping on or off and same with lab normal switch .
 
bias and offset aren't the same thing, 1.2 volts I'd guess is a problem somewhere around the differential pair. Its not a shorted transistor, that would push it a lot higher. I can't open HFE stuff from here, it freaks out about the VPN so I can't be any more specific but I'd probably be comparing voltages around the diff pair vs the schematic.

also would be worth checking your power supply rail voltages at the point they connect to the amplifier to see if they are equal and opposite polarity. If there is a funky ground or something thats causing the voltage to not be equal it will show up as offset at the speaker terminal
 
Bridge mode is off. No change when switching Soft Clipping or LAB input. I started pulling and testing transistors out of circuit, haven't found any bad yet. I also pulled some electrolytic caps and replaced the ones that were out of spec. Now I have -0.082 mvDC center voltage on left channel, and -1.7 VDC on right channel, while being powered with a DBT 100w bulb. Where do I check the rail voltages now? Are the differential pairs Q421-Q424 ?

Yes, I'm aware that idle bias and DC offset are different things.
 
UPDATE: It's fixed! I found a failed capacitor. C418. 220uf - 10v. Luxon brand. Never seen this brand. I researched Luxon brand capacitors, and seems they have a high failure rate. Hit or miss. Anyway, The center voltage is around 50 mvDC per channel. And the bias is set to 15 mvDC per channel after 30 minutes warm up time. It's singing nicely now, as I type this. Thanks to all of you that chimed in and gave me your time, and valuable advise.
 
TubeJunkie said:
UPDATE: It's fixed! I found a failed capacitor. C418. 220uf - 10v. Luxon brand. Never seen this brand. I researched Luxon brand capacitors, and seems they have a high failure rate. Hit or miss. Anyway, The center voltage is around 50 mvDC per channel. And the bias is set to 15 mvDC per channel after 30 minutes warm up time. It's singing nicely now, as I type this. Thanks to all of you that chimed in and gave me your time, and valuable advise.
Click to expand...
well done:thumbsup:
 
Well done, but C418 should have only affected the right channel, and you said that both channels had a -1.2 volt offset. Did you also check C417 on the left channel?
 
Before I found the bad capacitor C418, I had already replaced C417 and a few other out of spec caps in that channel. Which brought down the high DC center voltage in that channel. So then, I started puling caps in the opposite channel, and I found C417 was open, No meter reading. Thanks for chiming in
 
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