Hi all,
I have a NAD 3130 with hum on the right channel of the power amp stage, the left channel is fine. It's bad enough that you can hear it from a few feet away. I have the preamp/power amp jumper links removed.
Here's the waveform it's putting out:
I've tried the following to attempt to correct it with no success:
Replaced all electrolytics
Reflowed all solder joints
Compared voltages between left and right
Checked resistor values
Swapped all transistors between left and right
Moved the transformer away from the circuit
Freeze spray
Heat
Tapping/pressing around the board
Checked the ground connections
Shorting the power amp input
Set bias/dc offset
The service manual is available here:
https://archive.org/details/manual_3130_NAD/page/n5/mode/2up
I've traced through the circuit and the hum starts to appear from Q406 onwards, no hum before that.
Many thanks in advance!
I have a NAD 3130 with hum on the right channel of the power amp stage, the left channel is fine. It's bad enough that you can hear it from a few feet away. I have the preamp/power amp jumper links removed.
Here's the waveform it's putting out:
I've tried the following to attempt to correct it with no success:
Replaced all electrolytics
Reflowed all solder joints
Compared voltages between left and right
Checked resistor values
Swapped all transistors between left and right
Moved the transformer away from the circuit
Freeze spray
Heat
Tapping/pressing around the board
Checked the ground connections
Shorting the power amp input
Set bias/dc offset
The service manual is available here:
https://archive.org/details/manual_3130_NAD/page/n5/mode/2up
I've traced through the circuit and the hum starts to appear from Q406 onwards, no hum before that.
Many thanks in advance!
Hi mrplough,
A couple of thoughts. Does it make any difference if you install a shorting plug vs. leaving the input open?
You mention the hum appears after Q406. I was going suggest comparing with the better channel for reference. But I should also point out that most of the open loop gain in the PA occurs before the output stages, so that hum you observe in later stages may in fact have its origin in the input stage. Be suspicious of grounding paths.
I would have suggested scrutiny of the "center voltage" wiper as it would be a possible ingress path. But the good channel would have comparable susceptibility, so the wiper node is an unlikely culprit.
Keep us posted. Good luck!
A couple of thoughts. Does it make any difference if you install a shorting plug vs. leaving the input open?
You mention the hum appears after Q406. I was going suggest comparing with the better channel for reference. But I should also point out that most of the open loop gain in the PA occurs before the output stages, so that hum you observe in later stages may in fact have its origin in the input stage. Be suspicious of grounding paths.
I would have suggested scrutiny of the "center voltage" wiper as it would be a possible ingress path. But the good channel would have comparable susceptibility, so the wiper node is an unlikely culprit.
Keep us posted. Good luck!
Many thanks for the help!
When I short the input it does reduce it slightly:
I've been concentrating around Q406, since before there it looked clean. I'll check all of the grounding points earlier in the circuit. Just to confirm, it's a case of looking for high resistance (over 1 ohm) between the ground point and the chassis?
Interesting you mention the center voltage pot - the originals were broken so I replaced with multi turn trimmers. I'll try swapping them to see if it makes any difference.
When I short the input it does reduce it slightly:
I've been concentrating around Q406, since before there it looked clean. I'll check all of the grounding points earlier in the circuit. Just to confirm, it's a case of looking for high resistance (over 1 ohm) between the ground point and the chassis?
Interesting you mention the center voltage pot - the originals were broken so I replaced with multi turn trimmers. I'll try swapping them to see if it makes any difference.
I've managed to make some progress:
Swapped the trimmers over, no difference.
Double checked all of the ground points - all around 0.14 ohms between them and chassis
Checked components connected to the ground points - all measure good
I noticed that the transformer isn't the original - looks different to pictures of it and doesn't match the bolt pattern. It does have the correct colour wires and voltages, so maybe from a different but similar amp? Also noticed that the original has more shielding around it.
I took a piece of grounded (and insulated) sheet metal and moved it around the transformer. Was able to get the hum down from:
to
with the piece of metal in the right place and angle. There is also some vibration from the transformer and when the piece of metal is close it seems to vibrate magnetically.
I'm guessing I need to either keep an eye out for an original transformer, or add some permanent shielding to the current transformer?
Swapped the trimmers over, no difference.
Double checked all of the ground points - all around 0.14 ohms between them and chassis
Checked components connected to the ground points - all measure good
I noticed that the transformer isn't the original - looks different to pictures of it and doesn't match the bolt pattern. It does have the correct colour wires and voltages, so maybe from a different but similar amp? Also noticed that the original has more shielding around it.
I took a piece of grounded (and insulated) sheet metal and moved it around the transformer. Was able to get the hum down from:
to
with the piece of metal in the right place and angle. There is also some vibration from the transformer and when the piece of metal is close it seems to vibrate magnetically.
I'm guessing I need to either keep an eye out for an original transformer, or add some permanent shielding to the current transformer?
I think you're on the right path.
Was the sheet metal steel (magnetic shielding) or aluminum (electric field)? Were the above photos with shorted or open input? There may be different results. My intuition tells me that grounded steel may be better, perhaps both attenuating magnetic flux and shielding voltage fields.
Was the sheet metal steel (magnetic shielding) or aluminum (electric field)? Were the above photos with shorted or open input? There may be different results. My intuition tells me that grounded steel may be better, perhaps both attenuating magnetic flux and shielding voltage fields.
Here's the comparison again with the input shorted:
I'm using some 1mm thick mild steel sheet - seems to make most difference when it's right next to the transformer. I tried with some 5mm thick aluminium but it didn't make a noticeable difference.
Also, noticed that the steel sheet reduced the hum just as much without being connected to ground.
I'm using some 1mm thick mild steel sheet - seems to make most difference when it's right next to the transformer. I tried with some 5mm thick aluminium but it didn't make a noticeable difference.
Also, noticed that the steel sheet reduced the hum just as much without being connected to ground.
Apparently, the hum is predominantly magnetic.
You might post some close-up photos of the signal input wiring to the problematic channel. Members might have suggestions about shielding/routing to minimize susceptibility to the radiated hum fields.
You mentioned that one transformer is a replacement. As you noted, finding a NAD original might help. Probably not worth the effort/risk, but you could swap transformers to confirm the problem follows the transformer, and preview the potential improvement.
Good luck.
You might post some close-up photos of the signal input wiring to the problematic channel. Members might have suggestions about shielding/routing to minimize susceptibility to the radiated hum fields.
You mentioned that one transformer is a replacement. As you noted, finding a NAD original might help. Probably not worth the effort/risk, but you could swap transformers to confirm the problem follows the transformer, and preview the potential improvement.
Good luck.
I did some more testing, when I move the transformer away from the channel it does reduce the hum, but only if the input isn't shorted:
Transformer mounted in usual place
Transformer moved away - input not shorted
Transformer moved away - input shorted
which seems strange - I'd expect the hum to go down further with the input shorted?
Here's a picture from above:
And from below:
For comparison, here's a picture of an original transformer:
Transformer mounted in usual place
Transformer moved away - input not shorted
Transformer moved away - input shorted
which seems strange - I'd expect the hum to go down further with the input shorted?
Here's a picture from above:
And from below:
For comparison, here's a picture of an original transformer: