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The internal power supply on my Yamaha keyboard was broken while moving house (PCB is nearly snapped in half) and because it's 30+ years old it's not easy finding a NOS part replacement. I've found the schematic for it (attached), but I'm confused on one part.

The output pins are labeled (and what believe they are):

  • +12 (+12V DC)
  • AG (No idea - assuming ground???)
  • -12 (-12V DC)
  • +5 (+5V DC)
  • DC (DC Ground)

Can someone please clarify the function of the AG for me? I can see it's connected via rectifier diodes to the +12V and -12V rail, but since I only have a basic understanding of electronics, I don't know what it's actually for.

I'm considering swapping in a pico ATX PSU since they're easily obtained and have the necessary +12, -12, +5 rails, but I've got no idea how (or if it's even possible) to wire in this AG. So secondly I'd like someone to clarify if this idea would/wouldn't work.

Yamaha YS200 Power Supply

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The +12V and -12V are used to power the audio sections of the keyboard.

If you measure the voltage between +12V and -12V, you will find 24V total.

AG is the audio ground.

If you measure from +12V (red lead to +) to AG, then you will find 12V.

If you measure from -12V (red lead to -) to AG, then you will find -12V.

This known as a "dual rail power supply."

I wouldn't use the Pico power supply. The ground for the +-12V and the ground for the 5V is combined. They are separate in your keyboard. Besides which, I expect the computer power supply will have some "whine" kind of noises on the voltage output. That wouldn't sound good.

The easiest (though ugliest) solution is to get two 12V DC wallwarts and one 5V DC wall wart.

Connect the 5V DC wallwart to the 5V and DC ground connections.

Connect the two 12V DC wall warts in series. That is, the + wire of one to the - wire of the other. That junction is AG - connect it to the AG connection of your keyboard. You then have a +12V and a -12V - connect those to your keyboard and it should work.

You'll probably want to get old fashioned regulated wall warts. The kind that provide a regulated voltage, but use a heavy transformer rather than the modern switching power supplies. The modern ones are lighter, but they produce more electrical noise (whine and whistle.)

Your keyboard expects a clean DC supply, and modern power supplies may be noisier than the rest of the analog circuitry can deal with.

It might also handle it just fine - but the only way to know for sure is to try it out. I tend to do what I know will work rather than try one questionable thing then try the sure thing. It's cheaper.

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  • \$\begingroup\$ Brilliant, Thank you! Yes there's a hefty transformer on the broken PSU; seems to be what caused the PCB to crack. The noise sensitivity explains the sizable caps on the board. Will do some more research, might just end up getting a reprinted PCB (on much thicker board) and rebuilding it that way since it's a simple enough design. Plus I get to learn more about electronics along the way. \$\endgroup\$ – RunPotato Mar 21 '20 at 10:30
  • \$\begingroup\$ If you want to do it, you could recreate the schematic in Kicad then draw a new layout for it. You have the (broken) original at hand to get it correct. You could then have a new PCB manufactured (that would probably cost around $50 US) and move all the parts from the old board to the new one. The new one will be fiberglass and epoxy, so it'll be stronger than the old one (probably made of phenolic resin.) \$\endgroup\$ – JRE Mar 21 '20 at 11:11

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