Power Supply help - synth - Kawai DX900

Steenamaroo

Steenamaroo

...
Hey all,
Calling out to any members who might know about old home-organs.

I have a DX900 here which is in great shape but it's going to need some work.

It has a permanent mains hum on the output.
It also has problems in the synthesiser section. The sound is garbled and not predictable.
Repeat tapping a key on the solo keyboard sounds a bit like sample and hold on a dirty synth patch.
Maybe I can do some recordings to give you a better idea.

Anyway, where to start? Recap the PSU and take it from there?

Many thanks in advance.
 
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Well I'm going to replace the smoothing caps in the PSU and take it from there.
I'm assuming that will sort out the hum but I'm not particularly optimistic about it having any affect on the synth section.

Either way, I'll post back when I have it done.
I also have access to a service manual now.
Will share with anyone who thinks they may be able to offer help.

Thanks again.
 
Hi Arcaxis.
I was lucky enough to get the full service manual from a guy on another forum. Thanks for taking the time to look!!


I've uploaded the PSU section here and can make the rest available too.

I'm thinking I start off replacing the 5 large smoothing caps at DBA 100,60 and 60.

I'm hoping they're the cause of the hum but not sure if that could have a knock on effect to the voltages for the synth too?

There are a few additional minor faults with this unit but maybe it's best to start at the start?
If you want a full breakdown I'll do that, incase they're related. :)

I don't have any test equipment other than a basic multimeter.
 
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Well, full PSU recap didn't help but I'm sure it'll do no harm.

I bench tested output voltages and the +15V rail is running as high as 23/24.
I'm replacing +15v regulator and taking it from there.

The +5v rail is also running high at 8V, but it's all off the back of the same regulator, as far as I can tell.
 
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15v regulator replaced and 15v supply restored. :)
Hum/buzz is reduced somewhat (definitely thinner sounding) and the synth is behaving much better overall.
Still the occasional glitch though - I'll see if I can establish a pattern.

Also, I read 6.7 across the 5.6v zener diode. I guess it's cooked so I'll replace it (and q1/2) asap.

View attachment psu.pdf
 
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