MOSFET problem

[Grumpy2]

Sorry to be a leech but I’ve got a problem and don’t know where to turn to for help. Here goes…

I have a Yamaha piano that doesn’t work, I’m ok with taking stuff apart and basic diagnostics so that’s exactly what I’ve done.

I’ve tracked the problem down to a mosfet which seems to have failed but I can’t source a direct replacement and this is where my ability runs out. The mosfet is marked J20A10M, neither Farnell or RS seem to have any.

So, is anyone kind enough to help me source the part or a suitable alternative from a UK supplier?

Many thanks in advance.

 

[bertus]

Hello,

The mosfet is likely a TJ20A10M.

Bertus

 

[Grumpy2]

Many thanks Bertus, I found that supplier but none are available for at least a month (probably longer I suspect)

cheers

 

[AnalogKid]

Do you have any information (or feel) about what the part is doing in your device? For example, is it part of the power supply? If so, then it could be a part of a switching circuit, or a linear regulator. If it is acting as a DC switch or linear regulator, then there are many workable substitutes that probably will work without knowing the exact circuit details. If it is part of a switching regulator, then more of that specific device's parameters become relevant, and picking a substitute is more complex.

Piano model number / production year? There are several sources of user and service manuals on the innergoogle that might produce a schematic.

ak

 

[bertus]

Hello,

As @AnalogKid said, try to retrieve the manual.
Perhaps the model is given on one of the pages in this thread:
Looking for SERVICE MANUAL FOR SYNTHESIZER & ELECTRONIC MUSICAL INSTRUMENT

Bertus

 

[Grumpy2]

Thanks again for the replies

yes I have downloaded the manual, the piano is a Yamaha P45B and the MOSFET is on the power board. I've isolated the problem using the criteria in the manual and then removed if from the circuit and tested it on the bench and it looks like its failed. ie there is a resistance of approx 400 ohms between source and drain with or without the gate energised.

 

[AnalogKid]

Connect the gate to the source and remeasure in both directions.

ak

 

[Grumpy2]

will do, in about 30 mins.

thanks

 

[bertus]

Hello,

It looks like the mosfet is used as power switch:

You could test if the rest of the piano is working by shorting the pins D and S of the fet. (without the fet).
The power switch is disabled and always on.

Bertus

 

[Grumpy2]

Yes I believe it’s acting as a switch - ‘hot wiring’ d to s may make it stay on all the time? If so then I’m ok with that as I can switch it on and off at the mains.
Gate to source is infinite in both directions.
Thanks

 

[WHONOES]

You need to check source to drain with the gate connected to the source. Leaving the gate unconnected whilst testing will give erroneous results.

 

[Grumpy2]

Update, hot wired and all the test points in the service manual check out correctly, they didn’t before so it’s fair to assume the MOSFET was faulty. However the piano still doesn’t work so I’m really no further forward.
Many thanks all for your help

 

[bertus]

Hello,

When you say that power is available, but the rest is not functional, there must be a signal missing.
Looking at the powerswitch schematic, there are 3 signals.
One to turn TR101 on and two other signals.
Place the mosfet back and remove TR101 and bridge the C-E points and see what happens.
If the voltages are back, the mosfet is OK and there is a problem with TR101.

Bertus

 

[Grumpy2]

MOSFET replaced and tr101 removed. All voltages comply with those in service manual but still no sound. Not through speakers or headphones.
Power light not on either but I’d expect that I think? Voltage definitely present.

 

[Tha fios agaibh]

remove TR101 and bridge the C-E points and see what happens.

Did you also jumper C (pin2) and E (pin1) like Bertus asked?

I would test your 12v supply under a load to verify something isn't inhibiting current flow.
Sometimes a volt meter will show a voltage but will collapse under a load because of lack of potential.

Try using a small wattage automotive light and test along various points with respect to ground.

 

[Grumpy2]

Sorry, yes C-E bridged and all voltages are correct. Supply previously checked with car bulb and negligible voltage drop. Tr101 and MOSFET reinstalled and test voltages below acceptable levels.
To summarise

with Tr101 and Mosfet installed voltages outside range, no sound from piano
With just tr101 removed and bridged voltages within range but still no sound
with just mosfet removed and bridged voltages within range and no sound

all tried with two power supplies, power supplies tested with 12v bulb

 

[Grumpy2]

Ps, of the link works this is the service manual I’m using and the test criteria is on page 61

https://elektrotanya.com/cgi-bin/do...d=330293&file=yamaha_p-115b_p-115wh_p-45b.pdf

 

[bertus]

Hello,

I have made an overview of all parts that are involved by the power switch:

The hardware switch pulls down the mosfet by a diode.

Bertus

 

[Grumpy2]

Thanks. I understand the diagrams and can follow the test procedures but after that my skill runs out.
Thanks for your help and patience but I think I’ve reached the limit of my abilities
all the best for 2022