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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Apr 2021, 1:59 pm   #1
vinrads
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Default NAD 3150 amplifier protection light on !

All the output transistors read ok with load resistors connected , energize the relay with 12v one channel ok the other a fault giving about 3.5 volts , DC on the output , don't have a cct yet . Mick.
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Old 22nd Apr 2021, 3:27 pm   #2
snowman_al
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Default Re: N.A.D amplifier protection light on !

If the protection IC is something like a TA7317P they respond to DC off sets at +-1.0 volt.
Normally a leaky cap pulling the 0 volt line out of spec is enough to trip them. There should be a pot to trim the off set after you find the fault.
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Old 22nd Apr 2021, 4:18 pm   #3
vinrads
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Default Re: N.A.D amplifier protection light on !

Having checked all the transistors ,all ok I have a discrepancy in the voltages on the drivers ,I think I need to lift some capacitors ,thanks Mick.
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Old 22nd Apr 2021, 4:20 pm   #4
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Default Re: N.A.D amplifier protection light on !

For what it's worth, there's a couple of circuit diagrams / service manuals on the hifiengine.com site - rough scans, but may be helpful.

Alan
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Old 22nd Apr 2021, 5:31 pm   #5
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Default Re: NAD 3150 amplifier protection light on !

One here:
Attached Files
File Type: pdf Nad-2150-3150-4150-7150-Service-Manual.pdf (3.85 MB, 147 views)
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Old 22nd Apr 2021, 6:17 pm   #6
vinrads
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Default Re: NAD 3150 amplifier protection light on !

I decided to check all the coupling capacitors ,just lifted one leg ,all ok on checking a few resistors with the Avo 8 , better on this sort of job where there may be a small voltage on the cct , even after discharging the smoothing capacitors , Its not definite yet but I found the mid point adj pre set reading near 500k should be 500 ohm , I have two on order , fingers crossed , A big thanks for the circuit Snowman al , Mick.
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Old 4th May 2021, 2:39 pm   #7
vinrads
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Default Re: NAD 3150 amplifier protection light on !

Having replace all four pre set , with a dummy load and the relay energized both channels set up ok , both outputs working ok , but when removing the earth from the relay , the protection light came on , I decided is needed a chip ,It came this morning , fitted it and guess what not cured , so I had read somewhere regarding decoupling capacitors , replaced about five some read ok but some read above twice the value , switched on and after a few seconds the relay click sound was heard , all ok now , Mick.
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