NAD 7120 Transformer Hum/Buzz

[Hi Fi Fan]

Hello,

I have an NAD 7120 receiver with a noticeable physical hum emanating from the power transformer. A low level hum/buzz is also audible through the speakers with the volume off, it increases slightly at full volume with no inputs attached.

I have already tried isolating the transformer from the case with rubber grommets. Any other suggestions? New filter caps? Some sort of AC filter? Varnish soaking the transformer?

Thanks in advance!

 

[Reliving.]

I’ve just change my 40 year old Smoothing Capacitors and was advised to slightly up the capacitance and the Voltage. This has made a big difference.
They smooth the DC current ripples These peaks are the Hum and the good quality capacitors smooth the top of the ripples....
As a beginner this is how I read it .

So I believe them to be quite important?

The first link in the chain so to speak .

When from 4700uf to 6800uf This jump I have learnt to make the line above the DC ripple straighter meaning less hum....

Have I got this right Genteman ....!

 

[dimanag]

Hi,
I am not familiar with this model but you can lift the secondary(s) of the transformer and check for hum! If not, change the main power capacitors and the bridge rectifier (or diodes) and check again!

 

[Hi Fi Fan]

I’ve just change my 40 year old Smoothing Capacitors and was advised to slightly up the capacitance and the Voltage. This has made a big difference.
They smooth the DC current ripples These peaks are the Hum and the good quality capacitors smooth the top of the ripples....
As a beginner this is how I read it .

So I believe them to be quite important?

The first link in the chain so to speak .

When from 4700uf to 6800uf This jump I have learnt to make the line above the DC ripple straighter meaning less hum....

Have I got this right Genteman ....!

Thank you.

 

[Hi Fi Fan]

Hi,
I am not familiar with this model but you can lift the secondary(s) of the transformer and check for hum! If not, change the main power capacitors and the bridge rectifier (or diodes) and check again!

Thanks a lot. I think I can identify the primary and secondary based on the helpful link: https://leetsacademy.blogspot.com/2017/04/how-to-identify-primary-and-secondary.html?m=1

Just to be clear, by "lift" you mean to physically disconnect the secondary leads from the main board, presumably to take the board filter capacitors etc. out of the buzzing troubleshooting process?

Thank you.

 

[dimanag]

Yes , you have to remove the secondary windings from the pcb! You don't need to remove both cables of the secondary just one of them!

 

[Hi Fi Fan]

Yes , you have to remove the secondary windings from the pcb! You don't need to remove both cables of the secondary just one of them!

Thank you!

 

[dimanag]

Let be more specific!
Download the service manual from hifiengine or electrotanya. Both are free , hfe need a subscribe.
From the service manual you can remove fuse f1,f2,f3,f4
Also you can unsolder on end of fr1 and fr2.
This way your transformer is free without any load.

 

[Hi Fi Fan]

Let be more specific!
Download the service manual from hifiengine or electrotanya. Both are free , hfe need a subscribe.
From the service manual you can remove fuse f1,f2,f3,f4
Also you can unsolder on end of fr1 and fr2.
This way your transformer is free without any load.

Thank you!!!

 

[Hi Fi Fan]

Let be more specific!
Download the service manual from hifiengine or electrotanya. Both are free , hfe need a subscribe.
From the service manual you can remove fuse f1,f2,f3,f4
Also you can unsolder on end of fr1 and fr2.
This way your transformer is free without any load.

I removed fuses 1-4 as suggested. Unit did not hum under power. Nor did it hum with fuses 1 and 2 installed only. When fuses 3 and/or 4 were installed (along with 1 and 2) the unit started humming. Filter caps?

Thank you.

 

[Watthour]

Probably not filter caps causing this. More likely the winding is loose on the bobbin (core) and/or the laminations of the core are loose.

If the laminations are loose, they may have gaps or be moving, and this can be made apparent by polishing or rust between the mag iron plates. If you observe this, try cleaning any rust from between the loose plates, coat with insulating enamel (Glyptal) and tighten the hardware or clamp the laminations together to set. Bare plates allow random eddy currents, causing unnecessary heating and efficiency loss, not to mention the noise.

If the laminations don't appear to have gaps or are not moving, the winding may be moving a bit. The winding can work loose, and it may be possible to shim it back into a secure mounting using solid plastic shims wedged into the space between the winding and laminated core plates. Place the shims at the center of the core if at all possible and let the outer portion of the winding float a little if you must. If shimming quiets the unit, apply some insulating enamel or varnish over the shims and winding area once they are securely in place.

 

[Hi Fi Fan]

Probably not filter caps causing this. More likely the winding is loose on the bobbin (core) and/or the laminations of the core are loose.

If the laminations are loose, they may have gaps or be moving, and this can be made apparent by polishing or rust between the mag iron plates. If you observe this, try cleaning any rust from between the loose plates, coat with insulating enamel (Glyptal) and tighten the hardware or clamp the laminations together to set. Bare plates allow random eddy currents, causing unnecessary heating and efficiency loss, not to mention the noise.

If the laminations don't appear to have gaps or are not moving, the winding may be moving a bit. The winding can work loose, and it may be possible to shim it back into a secure mounting using solid plastic shims wedged into the space between the winding and laminated core plates. Place the shims at the center of the core if at all possible and let the outer portion of the winding float a little if you must. If shimming quiets the unit, apply some insulating enamel or varnish over the shims and winding area once they are securely in place.

Thanks a lot, I will investigate this.

 

[dimanag]

Hi!
Very good advice from watthour and i agree with him, but not always is it posible to do so because of the way transformer is build
So if your investigation got you nowhere and based on your first post regarding hum on the speakers, i will sugest to change some capacitors with new fresh general porpose ones. If you solve the proplem then you can change for whatever boutique capacitor you want!
So if i was you i would change :
C307/308 4700/50 to new 4700/6800 uf /50-63 v
D301 bridge rectifier and associate c319-c322 0.022/250v capacitors.
The above are for power amp supply (F1,F2)

For the pre and mute circuit supply (F3,F4)
C309,310 1000/50v
C312,313 220/35v
C311 100/35
Check R311,312 10ohm 1w if in spec and check if burning marks.
Check zener diode D304 (36v).

All these parts are really cheap.

 

[Hi Fi Fan]

Hi!
Very good advice from watthour and i agree with him, but not always is it posible to do so because of the way transformer is build
So if your investigation got you nowhere and based on your first post regarding hum on the speakers, i will sugest to change some capacitors with new fresh general porpose ones. If you solve the proplem then you can change for whatever boutique capacitor you want!
So if i was you i would change :
C307/308 4700/50 to new 4700/6800 uf /50-63 v
D301 bridge rectifier and associate c319-c322 0.022/250v capacitors.
The above are for power amp supply (F1,F2)

For the pre and mute circuit supply (F3,F4)
C309,310 1000/50v
C312,313 220/35v
C311 100/35
Check R311,312 10ohm 1w if in spec and check if burning marks.
Check zener diode D304 (36v).

All these parts are really cheap.

Thanks a lot!

I will investigate the transformer today, if no progress is made I will order these parts as suggested.

I really appreciate the guidance, while I am reasonably good with a soldering iron, I do not know enough currently about circuit design, etc. to be able to identify the possible culprits.

Thank you!

 

[Supernaut80]

Thanks a lot!

I will investigate the transformer today, if no progress is made I will order these parts as suggested.

I really appreciate the guidance, while I am reasonably good with a soldering iron, I do not know enough currently about circuit design, etc. to be able to identify the possible culprits.

Thank you!

Hi

Did you manage to sort the hum issue out?