Author Topic: electronic piano power board  (Read 740 times)

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Offline neel

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electronic piano power board
« on: January 05, 2019, 09:24:52 am »

I'm trying to repair an electronic piano. When I power it I just see the power led lit and no other led is working and it make no sound.

My first test was to measure the power supply output voltage, it is a 10v 1A and I have measured an open output voltage of 15V (I find this a bit high considering the piano is rated for 9V DC).

My second test was to power it through the battery input (the piano can use 6x1.5V batteries as power) using another power supply with selectable voltage (I have set it to have 8.7V open voltage) but the result is exactly the same, I only get the power led lit.

Then I have opened it to have a look at the power board. Visually all the caps look fine but this piano is at least 20 years old I think, so I have decided to measure all the caps from the board.

They are all at nominal capacity excepting a some that are a few percents below nominal but I think nothing out of tolerance range. However I am wondering about a few measures for which I had the measured capacity decreasing slightly on each successive measure (without discharging the cap from the previous measure). Like a 47uF was first measured at 46.xx, then 45.xx, then 44.xx. I don't know if this is symptomatic of a bad health for an electrolytic cap ?

Finally I have noticed something weird: all the electrolytic caps have a marking for the negative pole on the PCB, they were all soldered according to the polarity indicated on the PCB execpting one that was soldered reversed.

I have also measured that one an the capacoty is nominal and the cap has no visual damage (and the piano has been working for years like that).

Below are the pictures of this power board. You can see at the top the caps that was soldered reversed. It is near a small pcb soldered perpendiculary to this main power board and on the track side picture you can see there is a wire connecting the positive pin of this cap to a pin at the extremity of a  Mitsubishi component labelled M5222L 4Z04.

So I'm wondering if this cap was soldered reverse on purpose ? Maybe the design of this board has been modified so that it makes sense this cap is soldered reversed compared to pcb marking (all the caps on this board have the negative pole toward the same direction excepting this reversed cap).

Also my reasoning is this cap would have died if it was soldered reversed, although I have read an aluminum electrolytic cap can be powered up to 1v in reversed polarity (but I don't know if there is any usage for doing that).

I now plan to solder everything back (maybe replacing the caps that have the measured capacity decreasing over successive measures ?) except this reversed cap, then power the pcb and measure the voltage at the location this reversed cap (good idea ?).

Maybe some people with more electonic knowledge here can understand the logic of this board using the pictures below and tell me if it is normal this cap is soldered reverse compared to all other caps (and compared to the pcb marking). The reversed cap is the 16v 47uF you can see the location in the middle of the picture of the daugther board.

Thank you !

Main power board:

Daughter board:

« Last Edit: January 05, 2019, 09:27:14 am by neel »

Offline fzabkar

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Re: electronic piano power board
« Reply #1 on: January 05, 2019, 09:58:14 am »
If the positive terminal of the capacitor connects to pin #8 of the M5222L IC, then that's OK. Pin #8 is the (+) Supply.

Dual VCA for low voltage electronic volume control:

I notice that the PCB is marked "PSR-60" and "PSR-70".

FWIW there are service manuals for those Yamaha pianos here:
« Last Edit: January 05, 2019, 10:01:19 am by fzabkar »

Offline Armadillo

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Re: electronic piano power board
« Reply #2 on: January 05, 2019, 10:12:39 am »
The legend on the board is Wrong.
The reverse installed cap is correct.
The front diode clearly reviewed that it is correct.
The IC4069 is connected as sink further confirmed it.

Offline neel

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Re: electronic piano power board
« Reply #3 on: January 05, 2019, 11:22:06 am »
Thank you, I will solder it back that way then.

Also after a 2nd round of measuring all the caps I realized the decreasing successive measures were just due to decreasing temp as I was measuring right after unsoldering (on the 2nd measuring round I had all successive measures increasing as the caps were colder and their temp increasing from my finger holding them in place).

I'm also going to have a look at the service manuals you kindly shared the link of (mine is psr-70).

Any test idea is welcome as I'm not very skilled in electronic and just do basic repairs.
« Last Edit: January 05, 2019, 11:42:55 am by neel »

Offline Armadillo

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Re: electronic piano power board
« Reply #4 on: January 05, 2019, 11:32:58 am »
1st Lesson: Learn to read the schematic or study the circuit.
2nd Lesson: Do not unsolder anything until 1st Lesson.
3rd Lesson: Identify the faults and symptoms. Learn to ask with symptoms and faults clearly described. Do not be a "CAP' only man, those are for the uneducated.
4th Lesson: Divide circuit into functional part and Learn in-situ SAFE measurement without taking anything OUT. If you making this for living then time is essential. See attached.


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