My digital keyboard turns off a few seconds after turning on. If i'm quick enough I can hit the keys and hear the sound before it happens.
I have: time, multimeter and basic understanding on how to use it. Where do I begin?

 

Welcome to AAC!

The first step would be to find a Service Manual or circuit schematic for the P-95. If none is the available, the second option would be to use the P-85 Service Manual as a reference.
https://www.synthxl.com/wp-content/uploads/2020/02/Yamaha-P-85-85s-service-manual.pdf

There is already a thread on P-95 repairs.
https://forum.allaboutcircuits.com/...-95-sound-problem-speakers-headphones.140229/
If you have the time you can read through some of the success stories there. Do not add any more posts to that thread. Instead post your progress here.

What is the extent of your knowledge and experience working with and repairing electronic devices?

 

Welcome to AAC!

I have: time, multimeter and basic understanding on how to use it. Where do I begin?

If you're asking where to start, you probably don't have the skills to repair it. Having a basic understanding of how to use a DVM doesn't mean you understand how circuits work well enough to troubleshoot a problem to the component level. Don't feel bad. I knew a number of technicians who were just board swappers.

What do you mean when you say it turns off? Is it completely off, as in no power to the circuits? Or does off mean something you want to do doesn't work?

For convenience, the P85 schematics.

 

Thanks for your reply. Yes, it completely turns off. I know what capacitors and resistors are and soldered a few things back in the day, will that help? What module should I start debugging with? Anyone with a similar symptom?

 

What module should I start debugging with?

Since it's turning off, I'd start with the power supplies.

 

It depends on what you mean by turning off. I would try to distinguish between power going off vs sound going off.
You want to know if it is a power issue or just the sound going off.

Are there any power lights or indicators that stay on?
Probing the power supplies with a voltmeter is always a good starting point.

 

It depends on what you mean by turning off. I would try to distinguish between power going off vs sound going off.
You want to know if it is a power issue or just the sound going off.

Are there any power lights or indicators that stay on?
Probing the power supplies with a voltmeter is always a good starting point.

Yes, the on light goes off. Power supply shows 14V (it's a 12V 1.5 amp)

 

Check for +5VDC at the output of the voltage regulators IC401 and IC402.








This photo shows where to connect the BLACK probe of your DMM for a ground connection.
The RED arrows indicate where there ought to be +5VDC from the two voltage regulators IC401 and IC402.

 

I am able to test 5V on both for a brief second that it's on.

 

We are going to focus on the power control signals PSWON and PSWCNT into transistor TR401 that controls the 12VDC supply feeding into the two voltage regulators IC401 and IC402.





There are two test points shown in RED below. We want to know the voltage at each test point with respect to ground, immediately after turn on and then when the keyboard power shuts down.

The top red arrow is at the junction of D402 and R401 (PSWON).
The lower red arrow is connected to base of transistor TR402 from R405 (PSWCNT).

 

R401 - goes to 500mV, then to 100mV

R4506 (what the arrow points to in your image) - goes to 50mV on power on, then falls to -200mV

 

There is a red power indicator and power button on the top left corner of the keyboard.



When the power adapter is first plugged in, is the red LED continuously on?
After you press and release the power button, does the LED go off after a few seconds?
What happens if you do not release the button, i.e. keep pressing the button? Does the keyboard play?

 

>When the power adapter is first plugged in, is the red LED continuously on?

No

>After you press and release the power button, does the LED go off after a few seconds?

Yes, turns red when I press power button, then off on 2-3 service with and audible pop in speakers.

>What happens if you do not release the button, i.e. keep pressing the button? Does the keyboard play?

Stays red, but the power keeps cycling, with clicking. Can only play in brief intervals when it's on.

 

There is a part of the power-on indicator circuit that I don't understand.
ZD1, UDZS6.8B is a 6.8V 200mW zener diode.

/PSWI signal is normally pulled up to +3.3V via R519.
/PSWI will go low if TR501 is turned on. This would only happen if +5D is higher than +7.25V.
I cannot see how this will happen and hence the purpose of this circuit.

I am posting this hoping that some other member might have some insight.

 

Here is my understanding of the POWER ON sequence.

When the 12VDC adapter is plugged for the first time, TR401 is turned off and no +12V is available to the 5V regulators IC401 and IC402. The Power ON LED is off.

When the POWER button is pressed, /PSWON is grounded. This enables TR401 and +5D and +5A voltages are active. The Power ON LED is turned on. Power ought to be enabled while the POWER button is continuously held down.

The master CPU IC101 outputs high +3.3V on pin-51, PA3, PSWCNT to enable TR401 even when the Power ON button is released.

The CPU monitors the condition of +3.3V power via pin-50, PA2, /PSWI and /PSWON. If the power is already on, it turns off power if the button is pressed again.

 

Unplug the 12V adapter.
As a temporary test, place a jumper as shown in red. This will bypass TR401.
Plug in the 12V adapter. The keyboard ought to power up. You will not be able to power down the keyboard while the adapter is still plugged in.
Don't run this test for too long in case there is a voltage fault still existing somewhere.

 

After clipping the jumper: red light is always on. The system clicks and cycles. Sometimes staying on for up to 10 sec, where I can actually play the keys, the restarts again.

 

There are a couple of places to investigate:

1) A voltage regulator is shutting down, perhaps from an overload condition.
2) System reset IC105 on DM board.

I am going to place my bet on a power smoothing capacitor failing on one of the power lines.

For starters, look at C404 and C408, 100μF/16V.
With the power jumper still in place, go back and measure the two voltages identified in post #8.
We want to know the voltages when the keyboard is working and not working (as it cycles on and off).

Are you capable of reading circuit schematics and trouble shooting from there?

 

Tested with the jumper:

Ic401 fluctuates between 5v when on and 3.5 when off.

Ic402 I was not able to get piano to cycle. It's on now, even without the jumper, and everything seems to be working.

I am confused...what could be my next steps?

 

I am going to guess that there is a current overload causing one of the voltage regulators (perhaps IC401) to shut down. A possible culprit is a bad smoothing capacitor.

One possible trouble shooting technique is to use a thermal imaging camera in order to identify the hot spots in the circuit. Some smart phones are capable of doing this (maybe with an attachment). I have yet to confirm this.

On another thread a user had found that C112 had failed as a dead short across the +3.3D power rail. Anything is possible at this stage.