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Old 13th August 2007, 01:54 PM   #1
Lad is offline Lad  United Kingdom
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Default Help with a NAP 150

A recent and rather ‘over enthusiastic’ party has left me without music!

Somebody (who is now Naimless i.e. me) cranked up the volume on my Naim CD5/112/150/FC2 into Linn Index Plus speakers and we had fun for a short time. End result, amp and one speaker drive unit out of action. So, let the repairs begin.

The output stage of one of the channels on the NAP 150 has a few dead components, a couple of scorched resistors, a vented electrolytic cap and I’m sure the output stage has seen better days. Anyway, I’m handy with the soldering iron, so, swapping the components is no issue and I can use the good channel of the amplifier as a reference for components values etc.

Couple of questions:

1) A schematic for the NAP 150 would really help – I know the general circuit topology is similar to diagram on Neil McBrides site, although the devil is in the detail and Neil’s site doesn’t detail the protection cct that is on the NAP 150– Can anybody share any additional information on this cct?

2) Having changed out the damaged components for new ones, how do I set up the amplifier and to what values?

I assume, it’s just output bias voltage – Do I measure this with the speakers attached or not?

3) Will changing components on just one channel affect the balance of the amplifier - That is, are the main components matched across both channels?



Many thanks in anticipation.
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Old 13th August 2007, 02:03 PM   #2
Did it Himself
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I have a schematic somewhere, not sure it is exactly following the component IDs or has the exact same values of components on though.

You main problem possibly is that the output transistors are Naim specials. So you might need to replace the ones on the good channel as well.

Out of interest, how did it blow? No sound from the broken speaker then the speaker started smoking? i.e. shorted speaker took out the amp. Or did the amp seem to go first and fry the speaker?
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Old 13th August 2007, 02:45 PM   #3
Lad is offline Lad  United Kingdom
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The output stage uses Sanken 2SC2922’s rather than Naim specials. I have owned this amplifier from new, so these are the genuine components not replacements.

With respect to how did it blow ~ I assume speaker first; the coil is visible burnt out. The DC resistance across the damaged speaker is approx 1 ohm ~ I assume this then damaged the amp.

A schematic, even if not the most update would be useful.
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Old 13th August 2007, 03:09 PM   #4
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Either case I suggested would result in the same end happening - burnt out coil.

I'll see if I can find the schematic at home. I will be really busy tonight though and I'm away for the rest of the week.
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Old 13th August 2007, 09:44 PM   #5
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Only just remembered.

Click the image to open in full size.

Note I don't know which model this belongs to, but Naim seem to use the same basic circuit for all their amps.
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Old 17th August 2007, 08:26 AM   #6
Lad is offline Lad  United Kingdom
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Many thanks for the cct details - I have been working away for most of this week, so no cahnce to getting working on the fix

I'll let you know how things go....!

Once many thanks for the details.
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