Who wants to talk about mains hum?

Steenamaroo

Steenamaroo

...
Hi all,
You may remember I recently asked about a Kawai DX900 organ with a bunch of missing notes?

Well, I sourced and replaced a few of the divider chips and have restored this thing to full functionality. :)


The only real problem is there's a bit too much noise for recording, so I was hoping some of you might be able to help?


There's two different noise issues.

1:
Some background interference. Reasonably high pitched erratic whine - sort of like you'd hear with cheap PC speakers and onboard audio.
You can hear changes in the whine when changing certain settings, flicking switches etc.
The organ has a built in rhythm pattern section with variable tempo and I can hear a faint pulse/click which coincides with the tempo L.E.D.

This interference does get louder/quieter with master volume/expression changes, and is present on the headphone output.

2:
A mild mains buzz/hum.
This organ originally had a huge mains hum caused by a -15v rail which was running at about -22v. I believe this was also what blew the divider chips.
Upon recapping and repairing the PSU, the hum was fixed.
This mild hum appeared one day after fitting the new divider chips. At that stage the organ was already reassembled and hadn't been moved since, so I'm at a bit of a loss.

I have re-checked PSU voltages for safety and everything checks out.

This buzz does not get louder/quieter with master volume/expression changes, and is not present on the headphone output.





So, I guess I'm dealing with a weak/damaged ground connection somewhere...
Where to start?

The full service manual is pretty low quality, but is available here......
http://www.steenaudio.com/shared/dx900.pdf


Audio examples of both interference and hum are available here.....
(Organ minimum volume)
(Organ maximum volume - static clicks towards the endare me selecting different voices)

There's nothing nearby to interfere...tube lighting, transformers, amplifiers etc..


Many thanks in advance for any suggestions or info.
 
Last edited:
Taking a look at the mains hum today.
The schematic for the mix card (pdf link) shows where headphone output is tapped, right before the main amp card.
Since there's no mains hum on the headphone output, I guess the issue has to be after that point.
It affects L+R speakers equally.

Perhaps I should be looking at something in the q1/q2/q3 area? That would affect all power amp channels equally, but not the headphone output, right?

Main PSU, tremolo PSU and main amp are all fitted on one removable unit and I've already recapped the whole thing, with the exception of a few caps in the tremolo supply section.
Although I removed the tremolo supply fuse today to take it out of the equation and the buzz is still there.
 
Update.
If I disconnect the ribbon from the mix card to the amp the buzz remains so I guess the buzz is happening within the main amp.
I'm going to take a look at replacing the six transistors and two zeners in the main amp and see what that does.
 
I've done a bit more investigating and taken some advice on this.

I'm reading +/- 32.8vDC at the amp psu rectifier where I should read 35.
Some have suggested that this may be too low for the 32v zeners and may be responsible for the ripple I'm hearing.

I can't figure out why that voltage might be low, though.
Transformer secondaries supplying that rectifier read +/- 24.4VAC which, I think is fine...right?

q4,5,6,7,8,9 and the two zener diodes have been replaced, in addition to the full lyric recap.
 
Possibly a bad filter cap. I'd try bridging the output of the PS with a known good cap.
 
Track Rat said:
Loading? Maybe some sort high resistant short on that rail somewhere?
Click to expand...
Hmm yeah. With the lytics (and 31v diodes) being replaced I guess the ceramic caps would be the most likely place for that kind of failure?

LDS said:
Possibly a bad filter cap. I'd try bridging the output of the PS with a known good cap.
Click to expand...

All the lytics were replaced and earlier tonight I temporarily removed q4,5,6,7,8,9 and their two resistors.
I used an audio probe on the circuit in this state and there was no buzz (at rectifier and 31v diode), but it return as soon as I refitted the resistors and transistors.
At least that ruled out the rectifier and filter caps.

I also did a similar procedure to eliminate the opamps and +/-13v supply from the circuit, but the buzz remained.

If there was a problem with one of the STK084s, could that cause audible buzz on all three channels, do you think?


Thanks for the replies, chaps.
 
Ok, I've done some more troubleshooting and learned a bit more.


On the bench with speakers disconnected and inputs disconnected (or grounded) I get the buzz on the right and rotary outputs.
Now, If I remove the STK chip for the Left channel, the buzz disappears from the other two!?
The left channel chip and circuitry work fine if the other two chips are removed.

All chips have been proven to work on their own so the fault isn't with the actual STKs.

I've ruled out main filter caps, rectifier etc because a: they've been replaced and b: they would cause a global issue.

The buzz is most audible on pins 7, then 5, then 3. (R+Rot chips, loudest to quietest)
It's also traceable along the regulated negative lines for the right and rotary channels, right back to emitters of q8+9, although it's pretty quiet back there.
Interesting that it's on the negative line, though....Not the positive.

I know the transistors to be good.

Again, the schematic is here.
 
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