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Old 20th November 2015, 04:47 AM   #1
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Default STK084 amp mild 100hz

Hi all,
I'm hoping some of you may be able to help me to eradicate some mains hum and also some interference noise present on an old Kawai DX900 organ with STK084 (x3) power amplifier.

This post is mostly amp related but does stray into the rest of the organ. Apologies if that's not cool. I'd be happy to separate the issues if that's better.
I have only basic knowledge of electronics but good practical skills and good knowledge of safety procedures and the dangers of working with mains appliances.

I bought the organ recently with faults...mainly a huge mains hum.
The -15vdc supply was reading -22vdc and the 5v was reading 8vdc.
It also had a bunch of missing notes caused by failed divider ICs.

I replaced a failed -15v regulator, a failed regulating diode (for 5v), recapped the whole PSU and amp, and replaced the divider chips fixing all the issues listed above.

After the divider chips were replaced (the most recent repair) the organ was perfect apart from a mild background interference noise coming from within the organ.
It wasn't terrible - I could have lived with it.
The next day, however, it had a mild 100hz hum which I now can't get rid of.

About the mains hum :
  • The organ, from factory, does not have a mains earth connection. (UK)
  • The buzz is not being induced from nearby dimmers, lighting, or other electrical equipment.
  • The buzz is not present on the headphone out; Speakers only.
  • The buzz is present in left and right speakers equally.
  • The buzz does not change with changes to master volume control or expression pedal although the interference drowns it out at high volumes.
  • It does not seem to be related to the line or mic inputs at all.

About the interference :

I hear a reasonably high pitched inconsistent whine (No...not the Mrs.).
It does rise and fall with master volume control and is present on the headphone outputs.
If I operate switches or change settings on the organ I can usually hear a click or pop and/or a change to the whine.
Even playing the keys a slight click or static noise can be heard, almost like a percussion drawbar.

One of the original symptoms I mentioned was the erratic synth which, as I say, is now pretty much fixed,
but the synth still has a few quirks such as erratic pitch only if portamento is set to minimum.
Some instrument selections don't activate their sustain/repeat settings (so if you chose piano there's no sound until you manually enable sustain)...minor things like that.
I wonder if there could be some damage to the synth circuitry that's causing deeper issues?

As the organ is for studio recording, Ideally I'd like to fix both issues.
That said, the mains hum is the main offender. I'd settle if I was able to get that sorted.

Apologies for the block of text - I've tried to cover as much as I know.

MP3 examples and pdfs are below - All are safe links direct to my server.
Buzz - organ and low volume, played for reference.
Interference - organ at high volume. Switches operated in 2nd half of clip.

Service manual (not great quality)
PSU schematic pdf (slightly better scan)

I can also link to hi-res photos of the amp/psu if that'd be useful.
Many thanks in advance for any help offered.

Edit: I disconnected the internal input headers on the main amp.
The mains hum remains but the interference goes away.
Fairly predicable but I thought I'd add it on here.

Last edited by steenamaroo; 22nd November 2015 at 12:28 PM. Reason: location, schematics and mp3s added
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Old 20th November 2015, 10:16 AM   #2
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Service manual? Schematic would help.
Support for Flying Mole Class D, Allen & Heath, Valve amps, older designs & 2CVs.
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Old 20th November 2015, 10:38 AM   #3
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Originally Posted by JonSnell Electronic View Post
Service manual? Schematic would help.
Service manual (not great quality)
PSU schematic psf (slightly better scan)

Apologies. These are as good as I was able to find.
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Old 21st November 2015, 11:16 PM   #4
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I added mp3s for reference too.
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Old 22nd November 2015, 03:02 PM   #5
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Sorry to spam post - Ordinarily I'd edit it in but I'm not seeing a post edit option.

Looking at psu.pdf

If I unplug the header for audio input and start the organ I still hear mains hum,
but if I replace it then unplug the header for organ power (after 7815/7915) the hum goes away.

So, would that suggest that the buzz is being generated in the PS-24 section?
In my previous troubleshooting I replaced Q2 and its diode, 7915 regulator, and the 7 elec caps which doesn't leave a whole lot...

Any thoughts are appreciated.
Thank you again in advance.

Last edited by steenamaroo; 22nd November 2015 at 03:19 PM.
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Old Yesterday, 03:26 AM   #6
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Hi again,
A little more info.

I previously neglected to measure DC voltage at the rectifier outputs.

They read +/-23.8 and +10v where they should be +/-21v and +8v.

This concerns me for two reasons.
1: The +/-21 V are used directly in the organ
2: I think 23V is the max rated for the 7815/7915 regulators....Keeping in mind that the 7815 had to be replaced when I got this organ. (type error in previous post)

Is this just normal for testing without a load, or am I on to something here?

Last edited by steenamaroo; Yesterday at 03:36 AM.
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