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Weird problem with my KH O110 studio monitor.

Huang Jerry

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Joined
Aug 26, 2022
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Location
Sing-a-Pore; Hangzhou.
Brief overview: Klein + Hummel O110, around 15 years old, making noises as seen in the video below.


Quite temperamental tweeter electricity noise, started around 3 months ago, 60% of the time it works perfectly fine, it is unusable when this happens.

The noise still happens when XLR cable is unplugged, on multiple (no-load) breaker, using multiple different power cable, and my other (perfectly working) speaker does not have this noise whatsoever. Thus not anything to do with signal path, software etc.
Trust me. I did my due diligence.

It's either the power amp and its PCB, transformer, bass/mids adjustment pots or the input/opamp board.

Some diagrams (very basic - schematic is non-existent)
There is no "smoking gun" anywhere on PCB.

Backplate.jpg
Inside backplate noted.jpg


Local repair shop quotes me around 500USD for repair, which is ridiculous.
I'm clearly desperate as I need this to do mixing and pay for my school fee, and I'm even more desperate as I do not have the necessary skill nor equipment to do complicated PCB repairs (I can solder a few through-hole components though).

Anyone experienced this / ANY diagnosis and help very much welcome.
Tell me if you need more info.

TLDR:
C'mon just help your bro out for once, read the thing, it's not even that long.
 
Let’s do some elimination stuff first.

Try swapping speakers left to right and right to left. Does this noise sound move with the speaker or does it remain in the speaker in the same position?

Double check the speaker wiring. You might have a frayed wire insulation that is allowing the internal wire to make contact with something metallic. Check closely where the speaker wires make connection to the speaker and to the amp. Ensure that there are no loose wire strands or loose connections.

Do you have another amp to just test with? If you do just for elimination purposes. Temporarily connect speaker to different amp and determine if it still makes this noise? If yes it’s the speaker or wiring or possibly the power receptacle is a problem. Can you use a different power outlet? Try to find an outlet that is not on the same circuit in the breaker panel.

Report back and good luck.

What is that shiney golden thingy? Is that a bare strand of wire sticking out? (Upon closer examination it could just be a bit of reflected light)

IMG_0358.jpeg
 
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Let’s do some elimination stuff first.

Try swapping speakers left to right and right to left. Does this noise sound move with the speaker or does it remain in the speaker in the same position?

Double check the speaker wiring. You might have a frayed wire insulation that is allowing the internal wire to make contact with something metallic. Check closely where the speaker wires make connection to the speaker and to the amp. Ensure that there are no loose wire strands or loose connections.

Do you have another amp to just test with? If you do just for elimination purposes. Temporarily connect speaker to different amp and determine if it still makes this noise? If yes it’s the speaker or wiring or possibly the power receptacle is a problem. Can you use a different power outlet? Try to find an outlet that is not on the same circuit in the breaker panel.

Report back and good luck.

What is that shiney golden thingy? Is that a bare strand of wire sticking out? (Upon closer examination it could just be a bit of reflected light)

View attachment 278603
Thanks for your help, The noise moves with the faulty speaker regardless of position.

Connecting the faulty speaker with the plate amp from the one that works completely solved the issue.

As I stated, I tried putting it on a complete different circuit to no avail.

That shiny thing is reflection of the heat shrink cover upon closer inspection.
 
Just so we are on the same page:
The wierd sound comes out witht the speaker just powered on and nothing else connected to it. Right?
It sounds like unhappy silicon.

When in "noise mode", do any of the controls have any effect ?

Before removing boards etc try blowing some hot air (hair-dryer will do) first on the preamp wait a few minutes and then on the power amp,
OR
swap the input-preamp boards (it looks like two screws and one ribbon cable.)?

Take a pic of the board when removed, let's see if the active parts are through-hole or SMTs.
 
It sounds like unhappy silicon.
That would be my fear, as other than a few through hole capacitors amplifier chips and some weird components I cannot recognise, everything else is surface mounted.

When in "noise mode", do any of the controls have any effect ?
Everything works perfectly.

Before removing boards etc try blowing some hot air (hair-dryer will do) first on the preamp wait a few minutes and then on the power amp,
Tried for a bit for both power amp and preamp, noise persists.

swap the input-preamp boards (it looks like two screws and one ribbon cable.)?

Take a pic of the board when removed, let's see if the active parts are through-hole or SMTs.
That would be my next course of action. Will definitely do that (and update) once I have the time.

Thanks for your help!
 
Check for cold solder joints, maybe give some components a light tap with an insulated tool.
Also adjust the potentiometers back and forth a few times and return to the original position to clear out any detritus that may have accumulated.
 
I had some time to give it a checkover, though I did not test amplifier boards seperately.
I did discover something intriguing: The voltage regulator chip in one side is cool when switched on for 20 min, while the other is scolding to touch.

I do note that one of the speakers had some of its components replaced at some point, including the voltage regulator chip, although I am unaware of which exact one it is.

My main question would be: How warm are these chips suppose to be? and
Could this be the cause of my nightmarish problem?
Broken.jpg
Unbroken.jpg

Any suggestions / diagnosis very much appreciated :)
 
Make some measurements using your DMM to check input/output voltages.The LM377T is the TI version and the LM377SP is the ST Microelectronics equivalent.
You have a best case scenario with a working version to compare.

Here's the pinout:
LM337T.PNG
 
To avoid the cardinal sin of not reporting the solution of problem encountered, here is a overview of what is done to fix the speaker.
The LM337SP is indeed not working optimally, but the problem is traced to two diodes in the control circuit shatting themselves (likely due to switching on after prolonged storage, buring the two resistors right adjacent to them). This caused the voltage output of the negative voltage regulator to output -21V, instead of the specified -15V, the difference in output potential of the (working) positive voltage regulator and the -ve voltage regulator is likely what caused the hissing.
faultered resister.jpg

I suspect this is a one-off problem as I've seen no other people report anything similar.
Or it could be that there are so few of these O110s around now that nobody give a toss anymore, which I totally understand, KH120II is great.
So I'm afraid I wasted my time writing these posts and your time reading them, thank you for sticking with me though.

Also the op-amp board design as I'm told is "very strange", I think it contains the circuitry for the O110D with digital input, and hence having a lot of things (like an AES input circuit) just sitting there doing nothing. What an interesting cost-cutting measure..
 
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