Problem with sound board of a digital piano. Dynatone DPS-330.

Thread Starter

franrebo

Joined Nov 13, 2021
5
Two weeks ago a friend give me for my daughter a digital piano. It is a Grandtone DPS-330. When you turn on the piano it does the check and turns on the LED lights but no sound comes out of it neither through the speakers nor through the sound outputs nor through the headphones. Additionally I tested the midi output and only the twelve keys in the center give a signal. Doing tests inserting and removing the headphones from the sound outlet for two or three seconds, sound came out of the speakers with a lot of background sound but only from the middle keys (the 12 midi out ones). After some research I finded out that is a rebranded Dynatone DPS-330. I wrote to the Korean site and they sent me the service and use manual of the piano (I send attached). When I opened the piano i saw the piano has been tried to fix and there are three capacitors replaced and there is a leakage arround Q505 and C526 look like mising. I asked my friend and he told me that they replaced the Q505 but that did not fixed teh problem so the leave it as it is and gave to me (i send pictures attached).
What do you think is there any chance to fix it, i just keep the cabinet and the pedals, i am newbie to electronics but i have iron, multimeter, time and no money for buying a new piano for my daughther. Any help would be appreciated.
Thank you very much in advance and sorry for my english (is not my mother tongue).IMG_20211220_093619_046.jpgIMG_20211220_093558_804.jpgIMG_20211220_093604_500.jpgIMG_20211220_093609_317.jpgIMG_20211220_093614_650.jpgIMG_20211220_093619_046.jpgIMG_20211220_093637_286.jpg
 

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MrChips

Joined Oct 2, 2009
26,097
I would have said that we had a good chance of fixing this until I see that the circuit schematic is not included in the Service Manual.

See if you can find the circuit diagram from somewhere.
 

Thread Starter

franrebo

Joined Nov 13, 2021
5
I would have said that we had a good chance of fixing this until I see that the circuit schematic is not included in the Service Manual.

See if you can find the circuit diagram from somewhere.
I would try but it was like a miracle to obtain the service manual (I sent an email to korea in english)
 

MrChips

Joined Oct 2, 2009
26,097
Ok, there is still some hope of fixing this. It would be nice to get a hold of an oscilloscope. Failing that, you will have to get a sound amplifier. A powered computer loudspeaker is perfect for this.
 

MrChips

Joined Oct 2, 2009
26,097
Confirmed, there is no posibility of obtaining the circuit schematic.
Es una pena!

Your first step is to prepare your computer speaker to become an audio probe. It might be dual channel. You only need one working channel.

You need to put a capacitor in series with the signal input. This will block any DC signal from reaching the speaker.

1640096543694.png


This is a diagram taken off the internet. It shows two capacitors, one on the signal input and one on the ground connection.

1) Any value from 0.1μF to 10μF is suitable. Working voltage is not critical. Don't worry about capacitor polarity.

2) If the computer speaker is battery operated you only need one capacitor. If the speaker is powered from an AC outlet with no ground connection then one capacitor will do. The second capacitor on the ground side has been added to prevent a short circuit via the ground connection if it exists.

3) The diagram shows headphones. You do not need this if you have loudspeaker output.
 

Thread Starter

franrebo

Joined Nov 13, 2021
5
I have been working a little bit with the board and I think there are two problems I installed the missing Q505 and for 2 or 3 seconds there is sound but only in the keys between 37th and 48th inclusive (the only ones that have midi sound) after that there is a pop and no more sound, that only happens the first time I installed the board. For me it is curious that only the central keys of the piano work, it could be because some voltage regulator is giving less voltage than it should?
Pd: I will buy the 2 capacitors for buiding the audio probe.
 

MrChips

Joined Oct 2, 2009
26,097
Before you do anymore testing, we need to fix up the changes people before you have made.

Why are there wires on the bottom side?
I see a ripped track on Q505.

DPS-330 bottom_1.jpg


We need some better focused photos around Q505.
Make sure that your photos are sharply focused before posting. Try different angles so that we can read the writing on the board.

DPS-330 Q505 missing.jpg


We need to know what capacitance and voltage was used in those caps that were replaced.

Before you start testing and worrying about which keys are playing, get out your DMM and start measuring all the supply voltages.

Check that all the supply voltages are present and within ±5%.

DPS-330 Supply Voltages.jpg
 
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