Power Supply Hum

[riverrat373]

I have an Astron RS-35M power supply that supplies the correct voltage to my equipment but has a hum after it has been running for a while. Would the problem be in the transformer and how would I test it? I have not had the cover off yet.

 

[Blackswan73]

Most likely electrolytics breaking down after heat builds up

 

[prcguy]

Is the hum coming from the speaker in some radio equipment or from the power supply? If its making a hum in the speaker its probably a bad diode in the bridge rectifier and not the electrolytic caps since you would have to loose most of the capacitance to cause a hum at low current like a radio in receive mode. If there was no hum in the speaker of a transceiver during receive but hum on transmit during high current draw then it could be electrolytic caps going bad.

If the hum is coming from the power supply it could be from the transformer mounting bolts being loose or from something else being loose (case screws?) in close proximity of the transformer. Worst case would be the transformer not being properly dipped in varnish and placed in a vacuum chamber to get the varnish pulled between all the laminations during manufacture and the laminations are buzzing, That's hard to fix without taking out the transformer and running it through the right process.
prcguy

 

[zz0468]

...but has a hum after it has been running for a while....

Maybe it just doesn't know the words.




Sorry. =)

 

[Blackswan73]

Is the hum coming from the speaker in some radio equipment or from the power supply? If its making a hum in the speaker its probably a bad diode in the bridge rectifier and not the electrolytic caps since you would have to loose most of the capacitance to cause a hum at low current like a radio in receive mode. If there was no hum in the speaker of a transceiver during receive but hum on transmit during high current draw then it could be electrolytic caps going bad.

If the hum is coming from the power supply it could be from the transformer mounting bolts being loose or from something else being loose (case screws?) in close proximity of the transformer. Worst case would be the transformer not being properly dipped in varnish and placed in a vacuum chamber to get the varnish pulled between all the laminations during manufacture and the laminations are buzzing, That's hard to fix without taking out the transformer and running it through the right process.
prcguy

It only takes one leaky cap to cause a 120hz hum. The 120hz ripple is already present on the output of the rectifier

 

[riverrat373]

The hum is coming from the power supply and not the receiver.

 

[zz0468]

The hum is coming from the power supply and not the receiver.

That might be loose laminations in the transformer core.

 

[prcguy]

The Astron RS-35M has around 70,000uf of capacitance and is designed to provide up to 35A with minimal ripple. If you had hum in a receiver that's only going to be an amp or so draw from the supply and a few thousand uf would probably provide adequate filtering.

Do you think its reasonable to think a 70,000uf cap has lost that much capacitance due to being "leaky"? Anyway, sounds like the problem is mechanical and not electrical according to the OP.
prcguy

It only takes one leaky cap to cause a 120hz hum. The 120hz ripple is already present on the output of the rectifier

 

[cmdrwill]

That might be loose laminations in the transformer core.

Bad transformer, not that uncommon.

 

[Blackswan73]

Weak capacitors is a common cause of hum in these supplies
Repairing Astron 13.8V Linear Power Supplies

 

[Blackswan73]

The Astron RS-35M has around 70,000uf of capacitance and is designed to provide up to 35A with minimal ripple. If you had hum in a receiver that's only going to be an amp or so draw from the supply and a few thousand uf would probably provide adequate filtering.

Do you think its reasonable to think a 70,000uf cap has lost that much capacitance due to being "leaky"? Anyway, sounds like the problem is mechanical and not electrical according to the OP.
prcguy

Amperage draw has nothing to do with hum in an audio circuit. The hum is passed as part of the audio. For example, your body can act as an antenna and pick up the 60hz waves of the electricity around you. You can touch the tip of a guitar cord that plugs into a guitar, while it is plugged into an amp and hear the 60hz hum your body is picking up.

 

[zz0468]

Weak capacitors is a common cause of hum in these supplies
Repairing Astron 13.8V Linear Power Supplies

That will cause ripple on the output voltage, and hum in the devices powered by the supply. Physical audible hum from the power supply itself comes from the windings and laminated core of the transformer. They will physically vibrate.

A bad transformer will have the glue and varnish that binds the coils and laminations crack so that the vibration becomes annoying. It can eventually wear away the insulation and cause the transformer to actually short out and fail.

 

[Blackswan73]

Sorry, after rereading the thread, I see where I missed the OPs second post where he posted the hum was emanating from the power supply. I thought the hum was in his speakers. I stand humbly corrected.

 

[moonbounce]

That will cause ripple on the output voltage, and hum in the devices powered by the supply. Physical audible hum from the power supply itself comes from the windings and laminated core of the transformer. They will physically vibrate.

A bad transformer will have the glue and varnish that binds the coils and laminations crack so that the vibration becomes annoying. It can eventually wear away the insulation and cause the transformer to actually short out and fail.

Back when I was in the TV repair business when a transformer would hum or give off high frequency noise we used to paint them with Corona Dope, that would silence them.

 

[SCPD]

Mine did the same thing. I replaced it.