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NAD 7155 Hum
NAD 7155 Hum
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Old 7th March 2019, 07:21 PM   #1
pjblakey is offline pjblakey  United States
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Default NAD 7155 Hum

I picked up a 7155 receiver and when powering up, the relay clicks in after a few seconds and there is a distinct low level hum from both speakers, with or without inputs connected. A little louder in the right channel. I think it is 120hz.

After about 10 minutes the hum diminishes to a point where I cannot hear it when 8 feet away from the speakers. I would like to fix it though.

I have a donor 7155 minus itís volume/balance control.
I have a soldering iron and a DVM and access to the circuit diagram.

Any ideas where I should start looking, swapping etc to try and fix it?

I sounds really fine, detailed, warmish and all inputs work.

Thanks
Peter
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Old 7th March 2019, 07:36 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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120hZ hum means main smoothing or decoupling from the power supply usually.
I would check C905/906, C916 and it looks like C907, C928/929 for ageing.
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Old 7th March 2019, 08:19 PM   #3
pjblakey is offline pjblakey  United States
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Thanks.

Peter
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Old 17th March 2019, 09:32 PM   #4
pjblakey is offline pjblakey  United States
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Iíve gained access to the amplifier board and find no swelling or leaking on any of the mentioned caps, but there is some scorching of the board in the vicinity of C907(photo attached). The center of the scorched area is at Q903.
I have a donor 7155 amp and the amp board looks very clean and no sign of overheating. The mentioned caps have been removed from the donor board ready to swap into the faulty one.
Should I address any further components in the overheated area?
Many thanks
Peter
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Old 17th March 2019, 10:32 PM   #5
Old'n'Cranky is offline Old'n'Cranky  Australia
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Swelling and leaking are only two of the visual clues.
Lack of this won't tell you what condition they are really in.

Don't use second hand parts to replace second hand parts.
Especially electrolytic's that come from the same age and model of unit.
If it's worn out in one, it will likely be worn out in the other.

That picture shows the bulk of the caps as heat stressed.
Shrunken sleeving.
What happens to an eletro when you subject it to prolonged high temperatures ?

As JonSnell said, hum will (normally) be result of failing main power supply filter caps.
It's far cheaper and easier to outright replace them rather than muck around removing, testing, reinstalling.

Spend the $50 on quality new caps.

Last edited by Old'n'Cranky; 17th March 2019 at 10:37 PM.
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Old Yesterday, 11:40 PM   #6
pjblakey is offline pjblakey  United States
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Default Diode. Remove or not?

Just purchased a dozen caps from Mouser and included replacements for all the 9xx caps in the scorched area.
Apparently someone has soldered a diode across C917. Photo attached. I have no idea why, maybe it is a band aid for a problem? Is it related to a problematic cap that I am going to replace or something else?
The donor board that I have is very clean and all components intact.
Any help would be much appreciated.
Thanks
Peter
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Old Today, 12:35 AM   #7
pjblakey is offline pjblakey  United States
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Join Date: Mar 2009
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Default Diode. Remove or not?

Just purchased a dozen caps from Mouser and included replacements for all the 9xx caps in the scorched area.
Apparently someone has soldered a diode across C917. Photo attached. I have no idea why, maybe it is a band aid for a problem? Is it related to a problematic cap that I am going to replace or something else?
The donor board that I have is very clean and all components intact.
Any help would be much appreciated.
Thanks
Peter
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Old Today, 01:32 AM   #8
Old'n'Cranky is offline Old'n'Cranky  Australia
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Could be a running change from the factory.

Then there's it being a nad.
The schematics do not always match the unit on the bench.
For some models there may be 3 or more different schematics for the same model.

Personally I'd leave it as is and just replace the caps.
Don't forget to go over the whole board for bad joints.
Another thing nad (and others) are known for.

Oh, and be gentle and careful with the soldering.
Tracks lifting can be fun to repair....
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Old Today, 01:54 AM   #9
pjblakey is offline pjblakey  United States
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Thanks for your help.
The identical donor amp does not have the diode, and my knowledge of why it would be there is nonexistent. Or how it affects the circuit.
I’ve been scrutinizing for dry joints.
The hum I experience is quite minor when the amp has been on for a few minutes. I’d rather not have it, and if this doesn’t fix it, I will tolerate it. I bought it along with some Thiel speakers that I have fallen in love with, and when I first heard the system, didn’t even notice the noise. Otherwise, I like it.
Peter
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Old Today, 02:02 AM   #10
pjblakey is offline pjblakey  United States
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The only circuit diagram I have found online I only partly legible in that particular area of the board. Maybe a sign that someone else had problem there?
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