Digital Piano repair - completely lost

[jlake]

Hi there,

I'm an EE student at University in the UK, so when my digital piano stopped functioning, I thought I'd take a look at it before letting somebody else do the work.

OK, digital piano works fine for five years, and then, blows the UK->EU socket fuse. I replace the fuse, the piano works fine for two minutes, then blows the fuse again. I replace the fuse, the piano doesn't turn on AT ALL.

I take the piano apart and my first area of diagnosis is the power supply board. I'm looking for dodgy capacitors here, five years is a long time and is about inline with the expected lifetime of some (cheaper) and even more expensive electrolytic caps.

.

[VOLTAGE SELECTOR ABOVE NOT PRESENT IN MY MODEL]

The board looks completely fine, there is no charring and the capacitors are very healthy looking. I take to the board with a cheap DMM to check for continuity etc, and everything is fine. The resistors have the correct values, and so on.

I add power to the board and go probing to check for voltages and then FZ3 blows - secondary side -- I'm unsure what I was probing at the time. It must be said that before FZ3 blew, the voltage from +18V to PGND was not 18 volts; it was significantly less.  I continue probing and I get to checking the diodes. D1 and D2 are perfectly fine and R1 holds 1kOhms. The diode bridges were a funny one.
DB3 had continuity across its two middle pins - which from what I know, it shouldn't have by looking at the schematic.
DB1 worked fine.
So, I take the capacitor out (C5) and test it and it is open in both directions - so, it's fine(?). After taking C5 out, there is still continuity across the two middle pins of DB3.
I take DB3 out from the PCB and test it, its two middle pins are now open, no matter the direction - so, it's fine(?).

I will get back to this with some real voltage measurements when I can. Any ideas so far?

Kind regards

[andybarrett1]

Hi

The short you are seeing across the AC I/P to the bridge is the secondary  side of the TX.... Meters tend to see TX's, coils etc as a short circuit.

From what else you have mentioned the problem is after Fuse 3. If it isnt DB1 (doubtful) it is something dragging the 18v down... You said the 18v was low before the fuse died?

Ohms law says if current goes up Voltage comes down... Check the 18v supply further down stream where we cant see

BR
Andy

[klr5205]

Were you measuring the bridge rectifiers in-circuit?
If so, the middle two pins are connected to a winding on the transformer (aka "a wire") and therefore should have continuity between them.

Since your 18V rail was "significantly" less than that, I'd guess that it is shorted to ground somewhere. Once you replace FZ3 (and possibly DB1) I'd disconnect the power supply board and make sure it's operating correctly by itself.

Then, you can investigate the rest of your piano to see what is loading down the 18V rail.

[jlake]

Were you measuring the bridge rectifiers in-circuit?
If so, the middle two pins are connected to a winding on the transformer (aka "a wire") and therefore should have continuity between them.

Since your 18V rail was "significantly" less than that, I'd guess that it is shorted to ground somewhere. Once you replace FZ3 (and possibly DB1) I'd disconnect the power supply board and make sure it's operating correctly by itself.

Then, you can investigate the rest of your piano to see what is loading down the 18V rail.

I did measure the BRs in the circuit. The first BR's middle pins had no continuity, but then the other one did. So, I desoldered and had a look out of circuit and it turns out it's fine. I'll pop it back in soon and check the voltages again.

[jlake]

Hi

The short you are seeing across the AC I/P to the bridge is the secondary  side of the TX.... Meters tend to see TX's, coils etc as a short circuit.

From what else you have mentioned the problem is after Fuse 3. If it isnt DB1 (doubtful) it is something dragging the 18v down... You said the 18v was low before the fuse died?

Ohms law says if current goes up Voltage comes down... Check the 18v supply further down stream where we cant see

BR
Andy

Hi Andy,

I'll get back to you with some specific values soon. The 18V and the 6V were lower than labelled.

I was thinking that it may be because there is no load across either of those rails, so I'll attach a 1k resistor and check again - maybe that will yield different results.

When you say further down the line, do you mean the next PCB?

I am actually just waiting on a new DMM, I tried to measure mains AC with my cheap one and got a reading of 24V - I assume it's bust! 

[andybarrett1]

Hi

The voltage on 18v being lower suggests there is to much loading...Maybe a Reg, Zener, Cap has gone down.
 Yes sorry further down line... wherever 18v unregulated goes to

Your DMM not helping....

Let us know how you get on .... Have you got circuit of next bit ?

[jlake]

Hi

The voltage on 18v being lower suggests there is to much loading...Maybe a Reg, Zener, Cap has gone down.
 Yes sorry further down line... wherever 18v unregulated goes to

Your DMM not helping....

Let us know how you get on .... Have you got circuit of next bit ?

Andy, I must tell you that I tested this power supply board out of the system. There are very few components secondary side, so I'm not sure what would cause the low voltage readings.

I've got the rest of the circuitry at hand if anybody has any ideas:
.

[IGNORE THE POWER CIRCUIT IN QUADRANT H6 - THIS IS NOT FOR MY MODEL.]

[andybarrett1]

Hi

If out of circuit (CN1 not connected) and without a LCR Bridge or Variac, I would drop D1, D2, and  C14 to see what TX and bridge does on its own. Watching the 18v DC.

Thinking again maybe leave F3 out and take a reading from the TX AC secondary anyway. Check if still ok when you put F3 Back

Have you checked D1 and D2 for any faults ? leakage etc ?

BR Andy

[jlake]

Hi

If out of circuit (CN1 not connected) and without a LCR Bridge or Variac, I would drop D1, D2, and  C14 to see what TX and bridge does on its own. Watching the 18v DC.

Thinking again maybe leave F3 out and take a reading from the TX AC secondary anyway. Check if still ok when you put F3 Back

Have you checked D1 and D2 for any faults ? leakage etc ?

BR Andy

I have checked D1 and D2, both completely fine - conducting in only one direction... I'll check PDs across them soon.

I'll also get back to you with TX voltages.

Jamie

[Tomorokoshi]

Who is the manufacturer?

What model is it?

[klr5205]

Looks like a Yamaha CLP-320 or a variant thereof, based on the block diagram he posted.

[German_EE]

It's a modular unit so this means that you can attack the short circuit PCB by PCB.

Start off by unplugging EVERYTHING at one end and marking the cables. Replace your fuse again and try the power supply, it should be OK. Now plug in the cable from the Audio Power Amplifier PCB and try again, see if the fuse blows, if it does then the fault will be on that PCB. Keep on plugging stuff back in until another fuse dies and then you have found your faulty PCB.

Now it gets a bit more difficult. Do a visual check on the PCB and look for burnt or blown items, if you can't see any then look for bulging capacitors. After that test any voltage regulators on the PCB and see if any have failed short circuit.

[jlake]

Who is the manufacturer?

What model is it?

Looks like a Yamaha CLP-320 or a variant thereof, based on the block diagram he posted.

klr is correct, Yamaha CLP-320 - it's the German model, imported to UK. 

[jlake]

It's a modular unit so this means that you can attack the short circuit PCB by PCB.

Start off by unplugging EVERYTHING at one end and marking the cables. Replace your fuse again and try the power supply, it should be OK. Now plug in the cable from the Audio Power Amplifier PCB and try again, see if the fuse blows, if it does then the fault will be on that PCB. Keep on plugging stuff back in until another fuse dies and then you have found your faulty PCB.

Now it gets a bit more difficult. Do a visual check on the PCB and look for burnt or blown items, if you can't see any then look for bulging capacitors. After that test any voltage regulators on the PCB and see if any have failed short circuit.

I'd love to do that... If only I could  . After the socket fuse blew, I had a look at the internal fuses and all were intact - fine. So, what is the reason for the piano not to turn on after replacing the socket fuse and inspecting all internal fuses?

I'll do this again with the new fuse and see if the situation has improved, allowing me to actually power the piano.

[helius]

So, what is the reason for the piano not to turn on after replacing the socket fuse and inspecting all internal fuses?

One reason could be a faulty mains input cable. Or a faulty power filter, or another broken component on another board... not all faults are shorts, and components that fail short usually take out other components.

[jlake]

So, what is the reason for the piano not to turn on after replacing the socket fuse and inspecting all internal fuses?

One reason could be a faulty mains input cable. Or a faulty power filter, or another broken component on another board... not all faults are shorts, and components that fail short usually take out other components.

Helius,

I believe the problem is a faulty mains input cable. When I "wiggle" the cable, my AC voltage drops from 240V to 2.4V! Could this blow the socket fuse?

I was thinking that sparking could have been occurring inside the plug, causing a current surge, overloading the fuse and blowing it. This would certainly explain why none of the internal appliance fuses were not blown.

Is this typical, for cables to fail?

[helius]

You have to be somewhat careful because there could just be a loose connection in the instrument socket.
But yes, it's pretty common for the flex to break, they get folded up and creased, pinched under furniture, and gnawed by animals. Some were substandard when they were made.

[helius]

If some part of your circuit has measurable inductance, then it works the other way: increasing resistance causes voltage to increase. That's the reason that you see sparks when the pins make poor contact.
Not sure how that really helps in diagnosis....