Yamaha P-120 Keyboard

[lowell]

No power here. I opened 'er up and the AC jack for the wall wart had a broken trace. I resolder, and now still no power. The wall wart puts out the specified 16v so it's good. When I plug it in, there is no voltage when I test that point on the board. There is a component that is in series w/ the 16v, it's the first component in the circuit. It looks like a metal film cap but it says 30 U250 on it. I found something online that says it's an auto-resetting polymer fuse?? If that's the case then the 0dc resistance reading I'm getting is ok.

[Enzo]

Try putting a load on the wall wart. This wall wart puts out AC, doesnt it? It can be fooling your meter. The winding could be open, and your meter is picking up the same as it would if you grab the probe with your fingers. Whatever current capability it is rated for, find a resistor that would draw maybe 3/4 of the rating and connect across the wart. if it can power the resistor, it is OK, but if it collapses to nothing, it is bad.

In any case, assuming a good wart, (now there is a sentence I would never have predicted I would write.) go right to the solder terminals of the wart connector on the instrument. Is voltage present there? If not, then either the wart is still bad, or the connector is broken. And if voltage is present across the connector, then follow it down the traces. I cracked trace can be hard to see.

[lowell]

hmm. that's interesting to know. I put my monstrous dummy load 8ohm resistor across it. The rating for the WART is 2.4amps. 16v/1.8amps=~8ohms. There is NO voltage. Good call on the meter being fooled by no load.

[Enzo]

Unplug from the wall. Now measure resistance/continuity through the wart secondary. Open, I'll assume.

Most likely the wart just failed on its own, most likely failure point would be a broken conductor inside the wire where it leaves the wart body. But it is possible something serious is wrong inside the keyboard that killed it.

Some yamaha warts have screws in them so you can open them up. If so, you might get lucky.

Do you have some shop AC you could connect to the keyboard? Doesn't have to be 16v, even a 12v supply ought to light up the P120. It might be hummy, but close enough to determine if the P120 has some fault or not.


I remember a long time ago, I was working on pinball machines. Pinballs are chock full of #47 or #44 lamps, the same bulb as behind the power light on most Fenders. A typical machine might have a couple of 20A 6.3VAC circuits just for the bulbs. I could not get the lights to come on. There was a steady 6.3VAC coming from the transformer, the wiring had continuity. Everything semed OK.

After a while I found out the 6v winding was open, but it just worked out that the stray voltage my meter was reading came out to exactly the voltage I wanted to see. I had my 6.3v alright, but with the open winding the available current was zero.

I took a while figuring that out, and I never forgot that lesson.

[lowell]

not sure I get this. How does the meter get a voltage reading if the winding is open? Also I'm getting a 2kohm reading on the secondary. It's a DC supply by the way. I tried my 12vDC wall wart and it worked.

[Enzo]

Oh, I thought we were using an AC wall wart.