Checking Kawai K3 Power supplies..

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Checking Kawai K3 Power supplies..

Postby Synthetech » Fri Apr 25, 2014 5:30 pm

I was hoping some of you tech guru's could take a peek at some scope pics of a Kawai K3 PS.

Here's a link to the thread.
(CAUTION. HUGE PICS attached to the thread. Not slow bandwidth friendly.
i'll try to use smaller dpi next time I post pics here.)

viewtopic.php?f=24&t=75211&p=723656#p723656


That thread went dead on me and I noticed that we have two areas with repair threads.. so I'm not sure which is the correct one to ask for repair tech help. Even a mod moved one of my repair threads from that forum to this one recently..

Anyway, here's the schematic for the PS

Image


I am not sure what to think of the readings.
The schematic says I should have 9v AC, but I have 5v AC from the transformer instead.
The 0v AC is also 5v AC.

The "18v AC" taps are a bit odd too. One is clipped, the other more like a sine wave.

I have raw rectifier outputs shown.
I did not bother to put final outputs from the scope, there was no AC to be seen in the DC outs. Only got a reaction from the scope when I put it in DC mode.

Final output V's are stated in the thread as well.



If any of you have opinions if this PS is still working right, I'd love to hear your thoughts.
Thanks!
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Re: Checking Kawai K3 Power supplies..

Postby synthroom » Fri Apr 25, 2014 6:53 pm

I looked at the scope plots in the other thread - it's really weird that you have any voltage in the second photo which I take to be the 0V tap on the transformer. Where exactly is your ground connection for the scope?

Have you tried taking the fuses out of the path from the transformer to the diode bridge on the +18/-18V side so that you are just checking the transformer by itself?

What's the AC voltage look like going into the transformer?

Kirk
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Re: Checking Kawai K3 Power supplies..

Postby Synthetech » Fri Apr 25, 2014 7:16 pm

Hey, thanks for the reply..


I think I was clamped onto chassis for G.

Seems I need to take some more scope pics for unfused outs and the incoming 110V. I'll get them posted this evening.

According to the schematics, there are two posts from the transformer that are 0v's
One below the ~9v and the other between the two ~18v's

Any other areas to check?
Where is this ripple I am suppose to check for?
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Re: Checking Kawai K3 Power supplies..

Postby synthroom » Fri Apr 25, 2014 10:06 pm

The chassis should usually work for ground... but I think with this PSU I'd hook the ground lead to the analog ground for testing the +15/-15V circuit, and the digital ground for testing the +5V circuit. For testing the incoming AC, the chassis should be good for that.

I'm no electronics expert, but here's how my understanding is:
The two sets of taps off the transformer are going to be separate from each other. The +18/-18 VAC taps have a center tap, and that center tap should be "zero" relative to the +18 and the -18 taps. The +9/-9 VAC taps don't have a center tap, so they are kind of floating/isolated relative to the +18/-18 set. There is no "ground" for them.

So you need to check each set of voltages coming out of the transformer relative to their respective grounds - analog supply vs. analog ground and digital supply vs. digital ground. Take your voltmeter and measure volts DC across the digital ground to the analog ground and I bet you find about a 4-5 volt AC difference. Also measure from the digital ground and the analog ground vs. the chassis just for fun.

I don't think that "zero" on the schematic off the +9/-9 VAC tap is really a "ground". It's just the negative AC coming off the other side of the transformer tap from the "9V" side. Ground for those taps would be a voltage halfway between the +9 /-9, and not the -9 or "zero" side...

Ripple should be an AC voltage on top of the DC lines, so it "ripples" on top of the +15, -15, and +5 lines. To see it, measure VAC on each of those lines relative to their respective ground. The voltmeter in AC mode will not see the DC voltage, and only show you the AC ripple.

Keep in mind, I'm far from being an electronics expert...
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