Hello,

Hoping you all would be willing to help me.

I can repair most things and I can make my own circuits. Always been intimidated by component level repairs if I didn't design the board, usually just replace the whole board instead of troubleshooting. Decided to force myself to learn so, every time I need something, I buy a "parts or not working" item off ebay that hasn't been tested. Surprisingly almost everything has worked, or just needed a fuse replaced, until now.

I bought a Yamaha P-45 keyboard and it won't turn on, nor will its power light come on. Tried several known good power supplies as a first step. Still didn't work. Then looked at the diagram. Which you can see here: https://drive.google.com/file/d/1cGRCJA-2A7nTUL22xRmJ1QUIyIVA5nOQ/view?usp=drive_link on page 109 (last page) and testing showed that I didn't have 12v out of the JK board. Fuse tested good. 12V stopped at the FET (TJ20A10M3) on the JK board. Internet search revealed that sometimes the momentary power switch fails on these units which might then not activate that circuit, tested that and it was fine. Hooked up to the 12v out from the JK board with my multimeter and tried hitting the power button over and over again. Power spikes to ~12v for a fraction of a second and then falls to the ~.6v range. Holding the switch down makes no difference.

So I figured the FET or transistor next to it might be bad since that's where the power stops. I haven't ever had a reason to work with FETs before. Couldn't find this one (TJ20A10M3) in the states (Mouser/Digikey) and didn't understand how to find an equivalent one available domestically without ordering a large volume at a high price. So ordered 10 of each from China. Parts got in today and replaced both the FET and transistor on the JK board. No change in behavior. So either I got bad parts or something else is wrong.

What would your next steps be?

 

So ordered 10 of each from China. Parts got in today and replaced both the FET and transistor on the JK board. No change in behavior. So either I got bad parts or something else is wrong.

Both scenarios are possible.

So I figured the FET or transistor next to it might be bad since that's where the power stops. I haven't ever had a reason to work with FETs before.

Blindly replacing components isn't called troubleshooting. It's called guessing.

There are a lot of different types of FETs, so it's best if you use their proper names. The FET you're referring to is a MOSFET; P channel enhancement mode to be specific.

The device in question isn't very critical. It needs to handle a drain current of at least 2.5A. They must have been concerned about the threshold voltage because they put a fairly useless voltage divider on the gate that limits Vgs to 11.8V.

What would your next steps be?

Measure the voltage on the source and gate of FT101 with the connector on CN103 removed. To get 12V on the connector CN103, the MOSFET needs to be turned on. That means there should be a voltage on PSWO that's high enough to turn TR101 on.

That part of the circuitry made me do a double take because they drew it backwards. Flow in a schematic should be primarily from left to right and top to bottom.

Thanks for posting a link to the manual.

The MOSFET is way over specified. Id=20A, Vgs=20V.

 

After writing all of the below my hand slipped while verifying all readings. Shorted FT101 and TR101 and got a spark, which surprised me at 12v but was probably from the cap. Now the only output from the main IC is 3v, which is also its power input so that's fried. Darn. Given that PSWO wasn't doing anything, and PSWO comes out of pin 150 on the main IC, I suspect the main IC wasn't working before, but it certainly isn't working now. Replacement board is over $100. And I don't think I can realistically just replace the IC since I'm pretty sure it's programmed with their custom code. At least I learned a bit. Thank you for helping me.

Although fixing it would have been ideal I think I can still make a midi controller out of it with my own circuitry so I'll try to do that. And continue buying broken stuff until I learn how to do component level troubleshooting.

Here's what the original readings were before I fried it:

I think Chinese TR101 has now died in the on position. I now have 11.75V at CN103. Piano still doesn't work even with 12V out of the power board through CN103 so either TR101 has to also be turned on for the IC board to work or something else isn't working further down the line. LED also doesn't illuminate on pressing the power button but I suppose I can say that it's not the stopping it from working since the transistor in the power switch circuit appears to be internally shorted on.

Since the transistor seems to have failed on I didn't think there was much point in testing the MOSFET, which is forced on due to the failure of TR101.

Power on PSWO is 38mV tested at the R105 connection point. I don't see a minimum turn on voltage in the datasheet for TR101 so I'm not positive if that should be enough voltage to turn the transistor on or not. https://semiconductors.es/datasheet/2SC5395.html It does say that .3v is the maximum. I'm not confident in reading this graph but looking at the "Common Emitter Transfer" graph it looks like current doesn't start to flow until the base sees ~620mV, which is ~16 times more than it is seeing at the moment. As noted, it appears to have failed on and, in turn, activated the MOSFET but the LED isn't illuminating and nothing works. Pushing or holding the momentary power switch down drops the voltage at PSWO to 34mV but doesn't appear to do anything else.

I'm ordering normal P channel enhancement mode mosfets to replace the chinese units I got due to not understanding what I needed.

Thank you very much for the information and reply.

Blindly replacing components isn't called troubleshooting. It's called guessing.

That's fair.

 

bought

I bought a Yamaha P-45 keyboard and it won't turn on, nor will its power light come on. Tried several known good power supplies as a first step.

What the capacity of power supply you tried?, it need at least 500va capacity to power on this device, means 2.17 ampere at least , what ampere of adapter you tried?

 

I tried several 2.5 and 3 amp supplies @ 12V. I think the original supply this thing used was 2.5A so that was my starting point.

 

I tried several 2.5 and 3 amp supplies @ 12V. I think the original supply this thing used was 2.5A so that was my starting point.

Okay, and lately you found shorted mosfet and transistor ?

 

Okay, and lately you found shorted mosfet and transistor ?

I messed up and shorted them myself when trying to take readings. Got a spark. The CPU board in this thing is now dead and only puts out the same 3V on every pin that it takes as power input. That isn't what it did before I shorted it out and it isn't what it should be doing. So I'm confident that it's fried. Hoping I can still use the keyboard itself as a midi controller just have to figure out how to interface the existing pickup circuitry with some kind of USB controller.

 

I messed up and shorted them myself when trying to take readings. Got a spark. The CPU board in this thing is now dead and only puts out the same 3V on every pin that it takes as power input. That isn't what it did before I shorted it out and it isn't what it should be doing. So I'm confident that it's fried. Hoping I can still use the keyboard itself as a midi controller just have to figure out how to interface the existing pickup circuitry with some kind of USB controller.

Okay, good luck with that

 

Okay, good luck with that

Thanks.