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KAWAI Digital Piano won't turn on

Discussion in 'Troubleshooting and Repair' started by Crocodile, Apr 27, 2015.

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  1. Crocodile

    Crocodile

    1
    0
    Apr 27, 2015
    I bought a used Kawai Digital Piano MR 120 recently. I guess they're pretty old, from '90 or '91, but I got a really good deal, and I don't want to give up on it.
    When I first got it, it powered on, but some of the keys would not sound. So, I opened it up, figuring the contacts should be cleaned. There had been a really bad bug infestation, like someone had left it outside, so I set to cleaning out the whole interior.
    I unscrewed the keyboard, flipped it over, and disconnected it from the motherboard. At some point, while examining the keys, I reached over the top of the lid and tried switching the power on. I pressed the keys and there was no sound. Then, I remembered that the keyboard was not connected to the motherboard, and quickly switched it back off. Because I had the lid up, I never saw if the light came on or not. But, I don't remember it turning on after this. I'm not really sure if this is what caused the problem, but it's the only thing I can think of worth mentioning.
    I unplugged the piano and took out all the contacts, cleaned them with water, dried them, and reassembled them. After everything was put back together, I plugged it in and pressed the power switch. NOTHING.
    I checked and all the connections were in place, and tried different outlets. Fearing there might have been a short circuit from some possible condensation, I examined the fuses, but they seem to be intact.
    I am really at a loss at this point. First it works, then it doesn't. It looks like a pretty simple layout, so I feel like it could be a simple problem. Any ideas? Thanks in advance. dytj.jpg rtsthh.jpg e5yu.jpg
     
  2. quantumtangles

    quantumtangles

    152
    3
    Dec 19, 2012
    Do not attempt this unless you are safety certified and have electrical engineering expertise. Wear protective gloves and do not use both hands near the circuit. Use a high voltage certified multimeter. In fact, do not attempt this repair unless you are safety certified.

    The safest option (not that there really is a safe option) is to plug the unit in, turn it on, and check whether power of any sort is actually arriving at the secondary board (the darker brown circuit board in the first photo) from the power supply board (the lighter coloured board beside the transformer with green tape on it in the second photo).

    Avoiding the power supply board completely, check whether voltage is arriving at the secondary (darker) circuit board. Make a note of the voltage arriving there. It will be a direct current (DC) voltage which I would guess would be in the region of 35v but it may be lower or higher depending on the turn ratio of your transformer etc. Set your multimeter to the highest DC setting to test this voltage and reduce the setting for a more accurate reading as appropriate.

    What is going on here is that 250v AC is entering the primary coil of the green transformer. The secondary coil of the transformer is outputting a lower AC voltage, likely about 35v though I am not sure as this depends on the ratio between the number of coils of wire vis the primary and secondary coils in the transformer (a ratio of 10 to 1 windings on the coils would mean for example that 250v is entering the primary coil of the transformer and 25v AC is outputting the secondary coil of the transformer).

    This lower AC voltage is then entering the bridge rectifier on the lighter coloured power supply board, that is, the four black and white diodes one on top of the other in the second photo which comprise the Bridge Rectifier.

    The circa 35v AC entering the four diode bridge rectifier is converted (by the bridge rectifier) into, for example, 35v direct current (DC voltage), a DC voltage which is then supplied (possibly without being reduced in voltage) along the connecting wires to the secondary (darker brown) circuit board containing the integrated circuits.

    By checking whether DC voltage is being received by the secondary darker circuit board, you can avoid going anywhere near the main power supply board (the lighter coloured board beside the transformer, the transformer with green tape on it), which is highly dangerous.

    If the wires arriving at the secondary darker board do not register any voltage being received from the power supply board, your problem is at the power supply end.

    Be careful to avoid going near the power supply board as this is mains voltage and if you must, you should wear protective gloves and take every precaution (I would not do this if you are not safety certified and lack a suitably rated multimeter).

    If the secondary board is not receiving power, I would check the voltage output of the bridge rectifier (again taking every precaution by wearing insulating gloves and never using both hands near the circuit). Work backwards, avoiding the hot end of the circuit (the power supply board) as much as possible.

    If direct current voltage is indeed reaching the secondary (darker brown) circuit board containing the integrated circuits, the problem arises at a later point and we can take it from there.

    This may yet prove to be a fuse issue (either in the wall plug itself or on the power supply board). Do not attempt this repair unless you are safety certified. Even experts get nervous and are extremely careful when dealing with hot power supplies (mains operated power supplies).

    Let us know if DC power is arriving at the second (dark brown) circuit board and what the voltage is. If not, is any voltage exiting the Bridge rectifier and if so what DC voltage is exiting it?

    If no AC voltage is arriving at the bridge rectifier (which performs the role of converting AC to DC), is power exiting the secondary coil of the transformer or indeed, is power even reaching the primary coil of the transformer? Do be careful as this is a potentially very dangerous repair.
     
    Last edited: May 7, 2015
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