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Help tracking down 60 hz hum!!!

wlzed279

wlzed279

Active Member
Hello All, I have been pulling out what is left of my hair trying to track down an annoying 60 cycle hum in my newly recapped system. I created an earlier post about this issue that I thought was solved but it is not. I have leaned on the community before and you all have helped me out considerably. I am hoping you can pull me down off the ledge on this one.Below are the stats and symptoms:

KLH Model Twenty System
Fully recapped including the power supply board and filter capacitors

Symptoms:
faint 60hz hum on all inputs in both channels
Can be heard with the volume turned all the way down
Not controlled by the volume control. If I play music, the signal will drown out the hum.

Weird symptoms:
If I wave my hand over the control board, the hum gets louder...almost like an additional hum added to the original. This will go away if I touch any part of the chassis.

If I move the chassis ground wire on either the power board or the left channel power amp board, the hum will change in amplitude????

What I have looked at already with no results:
Reversed the two prong plug into the wall socket
Reflowed and inspected the chassis ground connections on all boards
Tried shorting the rca connection on the Aux input.
Tapped around all the ground connections to locate any loose connections.

I am completely stumped on this one. With these symptoms, should I focus on the Power Supply board since it appears on both channels and is not controlled by volume. Where on the power supply should I check for AC that should not be there? Is it possible that the rectifier diodes are leaky? How would I check those? I am pretty sure the new caps are in using the correct polarity.

Since I recapped this system it sounds absolutely amazing. It is just this low level hum that is driving me crazy.

Thanks as alway for your assistance.
 
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I saw this in the schematics. If this didn't have any issue before with hum I'd be looking into the grounding to see if something got messed up maybe. I hate ground issues if this is that. Out of curiosity did you try flipping the plug around in the outlet. I had that cause an issue in an old SAE tuner.
 
hamrules said:
View attachment 1897812

I saw this in the schematics. If this didn't have any issue before with hum I'd be looking into the grounding to see if something got messed up maybe. I hate ground issues if this is that. Out of curiosity did you try flipping the plug around in the outlet. I had that cause an issue in an old SAE tuner.
Click to expand...
Hello ham rules, thanks for the reply. I did try flipping the plug around..no help. So if I read this correctly, the if and rf chassis should not be grounded to the rest of the chassis. Is that a place that I should look?
 
wlzed279 said:
Hello ham rules, thanks for the reply. I did try flipping the plug around..no help. So if I read this correctly, the if and rf chassis should not be grounded to the rest of the chassis. Is that a place that I should look?
Click to expand...

I’d be looking at the schematic at all grounds versus the actual unit first, maybe get lucky.
 
hamrules said:
I’d be looking at the schematic at all grounds versus the actual unit first, maybe get lucky.
Click to expand...
Hello hamrules. I am still a novice at this hobby so bear with me. I do have a copy of the schematic. Do you mean to reference the schematic at every point that it indicates a chassis ground and test that point for continuity to the chassis?
 
I would try to rule out an external source. Do you have flourescent or LED lighting nearby? When you said you tried different outlets, was it in a different room?
Did you ‘check your work’ for faults. Possible solder bridges or reversed caps from the recapping? I sometimes take a plastic tuning tool or a chopstick and press or tap on devices to try to duplicate troubles from broken solder joints or damage.
 
Down load an emf detector on your phone.
I'll try to make a long story short.
I had a hum in an amp. Soundcraftsmen ma5002
Left channel only mainly.
Chased it like a coyote on a rabbit.
I would take the preamp down to the floor, pull the lid, search, search ,search. Everyone said ground loops, chassis, ground ground ground, switch this try that.
A few times I was sure I found it, left and right quiet
Turn the preamp volume up and no hum.
Restack everything all happy I found it and wtf humm is back.
One morning with everything full volume no input I lifted the preamp and 3 inch up hum went away.
6 inch and totally silent.
Put 2 and 2 together the preamp transformer sits right on top of the left channel.
Emf detector on phone shows high at the transformers.
Just rearranged and I have zero hum, max gain on amp and full volume.on pre amp with no input
Quiet as a church mouse.
 
Jimbo40 said:
Down load an emf detector on your phone.
I'll try to make a long story short.
I had a hum in an amp. Soundcraftsmen ma5002
Left channel only mainly.
Chased it like a coyote on a rabbit.
I would take the preamp down to the floor, pull the lid, search, search ,search. Everyone said ground loops, chassis, ground ground ground, switch this try that.
A few times I was sure I found it, left and right quiet
Turn the preamp volume up and no hum.
Restack everything all happy I found it and wtf humm is back.
One morning with everything full volume no input I lifted the preamp and 3 inch up hum went away.
6 inch and totally silent.
Put 2 and 2 together the preamp transformer sits right on top of the left channel.
Emf detector on phone shows high at the transformers.
Just rearranged and I have zero hum, max gain on amp and full volume.on pre amp with no input
Quiet as a church mouse.
Click to expand...
I’m curious about the app. What do you use as a probe? What’s the frequency range?
 
One item I did not see addressed yet are the board attachments to the chassis. Since the unit has been recapped, meaning a lot of handling, have the screws been checked at the attachment points?

Typically these are ground connections to the chassis as well. If they are loose, a high impedance connection can result.
 
El Rubio said:
I would try to rule out an external source. Do you have flourescent or LED lighting nearby? When you said you tried different outlets, was it in a different room?
Did you ‘check your work’ for faults. Possible solder bridges or reversed caps from the recapping? I sometimes take a plastic tuning tool or a chopstick and press or tap on devices to try to duplicate troubles from broken solder joints or damage.
Click to expand...
Thanks for the reply El Rubio, I did try it in different rooms. I have not gone back to inspect every board that I touched. I am afraid that will be my next step.
 
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