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Old 9th February 2014, 08:44 AM   #1
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Default NAD 2100 - single channel Hum Problem

Folks

I need some help and direction: I've refurbished (recapped, MF resistors, new speaker bindings, new relays) a NAD 2100 power amp. It sounds terrific and I've been playing music through it for ~2-months with no issue.

Recently, a background hum has started at initial switch-on, in the left channel only. The hum fades to nothing over ~30 minutes and it seems to go more quickly when the amp is playing music (~15-minutes). The hum sounds slightly different to a typical earth hum - slightly more edgy and aggressive! It is independent of any volume control input - it hums even when there is no pre-amp connected.

I have carried-out some fault finding, including desoldering and testing the output transistors, but I really don't know what to look at next:
Hum seems independent of any music, i.e. it is not distorting the left channel. It is present when there is no pre-amp connected. Switching to Bridged mode or Soft Clipping makes no difference. Switching to the Normal Input path (signal path via an opamp and volume control) reduces the level of hum slightly, but it behaves in the same way.

I do have a simple handheld scope, so can conduct basic analysis with that.
I've attached a schematic which another forum member kindly posted previously (the red marked transistors were for the historic post, not this), where the Left channel is the top one in the schematic. I really hope someone has experience of this type of problem - the forum didn't throw up many helpful results - I'm pretty sure that this is not power supply related, because the scope shows no ripple and the hum is on the L channel only rather than both...

Many thanks in advance for any advice you can give

Will

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Old 9th February 2014, 11:50 AM   #2
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Just stating the obvious; a 30 min period before the hum disappears suggests a thermal issue. IME this is an electrolytic that is ageing but
there are other possibilities too. Did you replace only the main electrolytics or what did you mean by mF - millifarads? (just the main 10 mF power electros) or microfarads? (as in all electrolytics) There are lots of them!

Generally, a raw sounding hum is rectified AC - sawtooth waveform ripple on DC. That's what the power suppy puts out. Capacitors provide a low impedance path to shunt the AC component to ground, lowering the hum voltage on the power rails, wherever they disperse their current. If the caps are ailing and slow to reach their capacitance rating, the hum is evident. As they heat up (and it likely gets worse over time) the cap becomes more functional and passes AC again. Eventually though, it will fail completely.

So what caps have been replaced? I would like to suggest comparing voltages as you have a good channel to compare but 30 mins is not long enough to check much for someone not experienced in servicing. You may well need a 'scope to trace this kind of fault too. Often though, substituting a good new cap over the old can find the culprit - if you have a stock of parts near to the required values.
Some may find this manual easier to read: http://www.hifiengine.com/library/nad/2100.shtml
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Last edited by Ian Finch; 9th February 2014 at 12:19 PM. Reason: add link
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Old 9th February 2014, 12:01 PM   #3
Mooly is offline Mooly  United Kingdom
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Its worth comparing output stage bias between the channels and also confirming that the Class G rail switching is working correctly. In other words, that the output stage is being fed from the "lower" voltage rails on both channels. That's four voltage to measure for that one.
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Old 9th February 2014, 12:49 PM   #4
tvrgeek is offline tvrgeek  United States
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After caps, could be a cold solder joint. When I had issues in a NAD receiver, the rail switching effected both channels. It was thermal.
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Old 9th February 2014, 08:53 PM   #5
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thanks both - i'll take a look more closely again - be great if it was as simple as a cold-soldered joint! I'm away all week, but will have a go at end of the week and post my findings.

Will
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Old 4th September 2014, 06:29 PM   #6
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Have you corrected this problem? i have exact problem right down to the lab/normal switch. i have changed all elkectrol;ytic caps expect 4 ac ps caps.
with a scope connected to the left output you can see a very small sin wave . if you blow thru a straw on the area of the Q401,Q403 the hum gest louder and the sin goes from 2mv to .5 volt. have reflowed the the entire area(several times). swaped out some diodes , caps and resistors, no luck. also swapped q401,403 from left to right channel still hum in left.
pc boards don.t seem to have any crack. press on the board at various spots with no change.
hum is louder on high humidity days, hence blowing with straw might be moisture not heet/cold thermal problem.

any ideas?

tnx mike
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