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Old 9th June 2014, 06:34 PM   #1
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Default NAIM NAP 110 amp, high DC offset and mute channel

Sorry, posted this by error while checking for similar threads and I can't cancel it.
Anyway I've just bought a 2nd hand NAIM NAP 110 amp, it has high DC offset on the left channel and no sound from that side, should I check the output transistor?

Last edited by Calamaro; 9th June 2014 at 06:40 PM.
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Old 9th June 2014, 07:36 PM   #2
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Output transistors would be the first to check, but there may be more bad parts (particularly drivers and the feedback-side transistor in the input pair, but dead Rs, Ds and Cs are not unusual either). The mean thing about DC-coupled power amps of a certain complexity is that they tend to blow up again in no time if you overlook the tiny little thing that cause the fault in the first place. Bulb tester highly recommended.

Reportedly the basic circuitry for the NAP 110, 140 and 250 is essentially the same, so a schematic for any of these should do. Pretty standard circuit AFAICS, except for a quasicomp output stage.

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Old 9th June 2014, 07:47 PM   #3
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Forgot to mention: tested with light bulb tester, strangely enough everything seems fine.

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Old 9th June 2014, 08:30 PM   #4
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That would indicate that the output transistors are not dead short at least. One of the previous stages may have a problem, or maybe a feedback resistor has gone open circuit.

What is the magnitude and polarity of the DC offset?
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Old 9th June 2014, 08:31 PM   #5
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It was about +200 mV but I've checked again and it's about 50mV, sounds comes now from the channel but at very low volume and very distorted. Don't know if this makes sense for you.

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Old 10th June 2014, 10:53 PM   #6
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Three dead resistors (two burnt, one shorted).
Transistors seem ok. Would you someone suggest me a proper replacement for the power resistors please? I think I should replace all of them and in both boards.
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Last edited by Calamaro; 10th June 2014 at 11:22 PM.
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Old 11th June 2014, 04:10 AM   #7
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These are probably close to the original type, which are 3W in all the old Naim amplifiers. http://www.welwyn-tt.com/pdf/datasheet/WA80.PDF

It doesn't really matter what brand WW as long as it fits the PCB but its a good idea to mount any power resistor a few mm clear of the PCB to avoid fire when accidents like that one occur. You could use 2W metal film even but Metal oxide or flameproof types would be safer. I'm puzzled about the shorted resistor, though. Only a few turns could short and reduce its value by a few percent maybe. Are you certain it is the resistor itself that is shorted?
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Last edited by Ian Finch; 11th June 2014 at 04:28 AM.
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Old 11th June 2014, 09:04 AM   #8
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In the basic standard NAP schematic they are 0R22 but I've measured them with a DMM and I've read 0R5 (?)
I wonder if someone has a NAP 110 schematic...
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Old 16th June 2014, 12:26 PM   #9
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2 possibilities:
Is your meter accurate at low resistance values, have good batteries and does it read zero R when probes are shorted?

Do all the 8 WW resistors read the same value? There are actually 4 WW resistors in each channel when the standard
circuit shows just 3 resistors. That's one resistor from each output transistor (emitter on the positive rail side,
collector on the negative rail side) to the output common rail and 2 in parallel from that point to the speaker terminal.

However, NAP 110 is an early model (1979-87)and may have some minor differences. I would not expect the output
resistor to be larger than 0R22 though. Look at the PCB tracks beneath or use some backlight to see what the actual
circuit is.

BTW, There is only one schematic for all early NAP models so you won't find one specifically drawn up for any model,
apart from wiring diagrams for the bridged models - unless another DIY or service technician drew them up.
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Last edited by Ian Finch; 16th June 2014 at 12:56 PM.
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Old 17th June 2014, 09:56 PM   #10
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Ian, you're right: my DMM isn't that accurate. It reads R3 when probes are shorted: the resistors are marked R22 and so they are. Yes there are 3 of these R22 resistors, the fourth one is 8R2 that go to ground in series with a 220nf capacitor as in schematic. And checked again, no shorted resistors they are just burnt (do I need a new meter?). Transistors, diodes, etc under the probes behave like the ones in the other channel, so first I'm going to replace just the faulty resistors.

Last edited by Calamaro; 17th June 2014 at 10:21 PM.
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