Digital Piano Low Volume

[zifey]

Hey guys,

I have a Korg SP 250 that I picked up very cheaply at a pawn shop. One speaker was crackling when I bought it, so I just unplugged and continued to enjoy my cheap keyboard.

Recently, the volume would become very quiet while playing, no matter how high the volume slider is set. After a reset, the volume would return to normal for a few seconds before rapidly becoming quiet again. The only way to get acceptable volume is to hook it up to an external amplifier.

What do you guys think the problem may be? I feel like it would be obvious if I had more experience. How might I go about fixing this? I have opened it up and checked all the capacitors, but they are all visually fine. It seems like a problem with the amplifier board, but I can't find anybody that is still selling those for this model.

Thanks in advance

 

[duke37]

A crackling output would make me suspect a capacitor. Varying volume could also be due to a capacitor in the power supply. I suggest that you do not use it until the fault is found. You do not want to take out the mains transformer.

A photograph of the relevent area and a schematic would be a great help.

 

[zifey]

duke, thanks for your response. I initially suspected capacitors as well and was kind of hoping to affirm my suspicions before wasting too much time on it. The keyboard uses three boards, a main, an amp, and a control board. I've attached the schematic, and will add pictures when I can the keyboard open again.

 

[73's de Edd]

Sir zifey . . . . .

Your problem will very likely be the potted POWER Audo Amplifier Module that I have marked up in the top right of the schematic with points of initial interest in [YELLOW].
Get that 250 board acessible for AC metering and initially put a continuous tone / note thru the unit and monitor the two YELLOW X Circles to see if you have a constant tone making it through the unit at each of those points.
With a suspected level of 100's of millivolta AC on up to a volt.
If so, you should have sound coming out of the speakers if both sections of that LA4708 IC are good.

IF not, initially inspect the individual 18 pins of that IC/Module to see if it possibly has developed some floating pins within their solder blobs, due to a poor wave soldering of the board at manufacture. It takes a vewy-vewy bwight light to examine, and a strong magnifying glass would certainly help.

The other markups are relevent to other signal paths, if they seem to be involved later on.
I certainly think that IC2 or its support circuitry and components is your most likely problem.

MARKED UP REFERENCING:

http://i.imgur.com/FFdZWC7.png

73's de Edd

 

[duke37]

Check that there is 12V on pin 18 of the LA4708.
Have a good look at C38 and the voltage on it.

 

[zifey]

duke and 73's de Edd, thanks so much for your advice. I've had a pretty good look at LA4708 and all of the voltages are within the expected ranges. C38 is visually fine and has ~ 6V DC going to it. I do not have an oscilloscope but am tempted to purchase one in the spirit of learning.

I did notice that the output pins have ~6V DC on them, I'm not sure if that was expected.

I think it's worth noting that the headphone out is also very quiet. The only way I am able to get acceptable volume is by going through the headphone out to an external amplifier. This leads me to believe that the issue is prior to the LA4708 chip. What do you guys think?

I plan to look for the expected values of CN10-6 and CN10-3 wires (circled in green in Edd's marked up diagram) and check them out.

 

[duke37]

The 6V on the LA4708 outputs is reasonable since there are two amplifiers working in anti-phase.

You could use a little amplifier to trace the signal, before and after the LA4708. Note that the LA4708 has a mute connection, check again that this is above 2.5V

 

[Johannes v I]

Did you solved the problem? I have a problem that the Piano is switching on badly. when you push the ON/OFF button after some seconds the lights come up and the piano play badly. Maybe you had some solution? I made note of the comments above and will try this weekend. someone any other suggestions, on the fault I described, please let me know. Thank you

 

[StevenVerlinden]

It is mentioned above but do check your supply voltage. I had exact the same problem as Zifey describes: piano working but at a very low volume with slider all the way up. It turned out that my adapter was faulty and produced only about 8V, although the power indicator led was on. The digital parts is fully functional because the 8V input is enough for the power module to generate the internal 5V supply (IC4). For the amplifiers the 8V resulted in my case in the very low volume. At even lower values, several parts of the circuit will gradually fail, which might result in any strange result. If you checked your adapter and it is fine, I recommend to take the following steps;

1. Check the supply voltage on the board. You can access (board top side) them without removing the board (you'de have to open the keyboard obviously).
- Ground: e.g. JMP36 (bare metal wire, somewhere in the middle), your 0V reference.
- 12V : The cathode end of D14 (big component, close to the power connection)
- 5V: JMP23
- 9V: JMP16
- V/2 (6V, the analog ground): R2 terminal facing the middle of the board

If 12V is not there, check the anode end of D14 (the voltage here should be about 0.7V higher), although this component is unlikely to fail. If there is no 12V then you most likely have a problem in the on/off switch or in the power input plug.
If 12V is ok, then continue;

If 5V is not there, the problem is most likely in IC4 (7805).
If 9V is not there, the problem is most likely in IC3 (7809).
V/2 is derived from 12V by a resistive voltage divider: it is very unlikely to fail but if you have the 12V and not the 6V for V/2, then you could try replacing C1 or even C1,R1 and R2

2. If all supply voltage are ok and still no good sound
- check MUTE: you can measure this signal on the R62 terminal closest to the edge of the board. It should be close to 0V (certainly less then 0.5V). If the voltage is higher then about 0.6V (most likely it will be above 5V in that case), the signal from the synthesizer board is blocked.
- check AMP_MUTE: you can measure this signal n the R10 terminal closest to the middle of the board. It should be above 2.5V.

If any of these 2 signals is not ok I would strongly suspect your headphones plug. It has an internal switch that may be faulty. Try to plug in and unplug the jack (several times). you should measure 12V at JMP17 when the headphones are unplugged. If that is not the case (even after a few trials), you should replace the headphone jacks.

If both signals are ok, then go ahead and check the signal paths decribed above by 73's de Edd.

 

[pianorep]

@73's de Edd, @StevenVerlinden
Your explanations + diagram are a great help.
I have a Korg SP-250 which has a very low sound when volume is at max and no sound in earphones in any case.
I tested the voltages as described in StevenVerlinden post and 1/2(6V) is at 0V (R1 has 12V, R1-R2 junction has 0V).
But this is the headphone part of the circuitry.
Is it possible that this error disturbs the speaker output or is another error to be expected?
Thanks,
EDIT: I mean, I don't use phones, is it worth it to change R1/R2/C1?

 

[73's de Edd]

Sir pianorep . . . . .


OH tay ! . . .then . . . . consider this relevant analysis.
You have a full 12 VDC supply voltage presence at the top lead of R1 and it then passes down to R2 which has its other lead end going to ground.
Therefore the 12 VDC should divide in half due to there being two equal value resistors being used.
The resultant developed 1/2 voltage of 6 VDC then gets some filtering to ground by the use of the C1 . .100 ufd cap.
Then it goes on to supply critical biasing of the + input nodes of the just above IC8 A and B thru two isolative 10 K resistors .
Any short to ground on the other side of them is going to be as disconcerting and inconsequential as a pimple on a gnats ass.
But of course, as you said, this is for the headphone amp . . . . now unused by yourself . . . . as you say.
HOWEVER . . . . look at a circuit which DOES concern you, that is being the IC7 A and B anps that are the very first aspects of
incoming audio from the master volume control and audio then passing thru the IC1 and finally making some serious noise coming out of the final IC2 power audio stage .
SO the additional feed into the + nodes of the same sourced V/2 voltage thru two additional 10K resistors DO affect speaker audio in the same way.

Considering that V/2 power source function and its component build up, the most likely suspect would be a shorted or very leaky C1 E-cap.
And of course the 0.000000000001 % probability . . . an open R1.

Meanwhile . . . . back in the jungle . . . .AND TR12 and TR13 . . . . . the presence of any turn on voltage at their bases would progressively cause sound level decline / muting as their C-E's short out incoming audio to ground..

Thaaaaaaaaaaaaaassit . . . . .

73's de Edd . . . . .


The chemical formula for water is HIJLKMNO . . . . . . . . . Well now ? . . . isn't that being H2O


.

 

[pianorep]

Yeea, lots of thanks, it's clear.
I don't have the parts.
As they are relatively cheap, would you recommend to order cap, resistors and replacements for TR12 and TR13 at the same time to avoid multiple transports?
And/or something else?

Or should I first try to remove C1, then resistors and finally TRs until it works?

 

[pianorep]

Well, I made a bet, I replaced the C1 16V 100uf by a 10V 100uf I had lying around. It works!! Thanks again guys, it's great!
So now, as the piano is open, shoult I order a new 16V one or you think I could keep the 10V one?

 

[73's de Edd]

It's only being subjected to 6 V DC.

 

[pianorep]

Ok, thanks, was not sure some tech detail wouldn't hide somewhere.

 

[ErMaro89]

Hi All, I have a similar isdue with my piano:
Left speaker is very low, right doesn't make sound (but receives power).

I cheked the values from steven and the V at pins 18 and 5 of the power amp and they all look fine.
However jmp 17 reads 8V, what could be the reason?

 

[73's de Edd]

Sir ErMaro89 . . . . .

Your referencing to j u mp er wire 17 does not show on any of the supplied references in this post . . . so it must be present only on a silk screen stamping on the PCB proper.
So o o o o o o can you take this 94yo by the hand and his white cane and show him where it is being connected to / related with the schematic.
The SIMPLEST first test, is to refer to post 4's schematic and find the YELLOW X's and then get a source of sound coming thru the speaker(s) that are working.
Use a jumper wire to short those two X points together . . .to thereby . . .then make this a monaural dual channel system.
You should then have a like level of sound coming from both speakers if the LA4708 power amp I.C. is good and doing its thang.
If not, its then on to see if the fault then might be a lazy sub power supply or in earlier audio pre amp stages . . . I'm sort of expecting the latter.
(As you can see . . . the earlier audio stages RED circle points would be used for the next jumper test session.)

Thaaaasssssssssiitt !

73's de Edd . . . . .

Just put down and finished . . . . a good book . . . .
A COMPLEAT TRAVELERS GUIDE TO THE PACIFIC
By Amelia Earhart




.

 

[ErMaro89]

Dear 73's de Edd,

thanks for the reply.
The Jumper 17 is quoted in @StevenVerlinden post.

...

2. If all supply voltage are ok and still no good sound
- check MUTE: you can measure this signal on the R62 terminal closest to the edge of the board. It should be close to 0V (certainly less then 0.5V). If the voltage is higher then about 0.6V (most likely it will be above 5V in that case), the signal from the synthesizer board is blocked.
- check AMP_MUTE: you can measure this signal n the R10 terminal closest to the middle of the board. It should be above 2.5V.

If any of these 2 signals is not ok I would strongly suspect your headphones plug. It has an internal switch that may be faulty. Try to plug in and unplug the jack (several times). you should measure 12V at JMP17 when the headphones are unplugged. If that is not the case (even after a few trials), you should replace the headphone jacks.

If both signals are ok, then go ahead and check the signal paths decribed above by 73's de Edd.

However, I have some additional info.
Also the outputs are very low and the right channel is lower than the left one.
Same thing for the headphones. So the issue is likely upstream of the three different output pathways.

I'll do as you suggested.

Thanks!

 

[manuagle]

Hi,

I am having kinda the same issuewith my SP250, no sound from right speaker with the piano in front of me, super soft from left one, checked with phones the exact same thing.

Found out my powersupply (which said 10V) was actually giving me 16V :S ouch...

Got the right one 12V 3.5A, checked voltages, everything is fine there.

Jumped the Yellow spots, same issue.
Jumped the red spots, Eureka, sound on both sides,but, I guess obviously, soft.

So, what could I do next? Caps looks fine, soldering job looks fine, could my IC1 been blown?

Any other test would be very much appreciated.

Before it went bad, whenever I turned it on it made a loud thump, and volume slider made a raspy sound, I checked it and at 3/4 the run it goes OL so before finding thisforum I was about to change it.

SEND HELP!!!

and thank you.