Yamaha cp4 power supply problem

Hi everyone,

I have a problem with my yamaha cp4 stage piano. Every time I turn it on it makes a short buzzing sound and it shuts down after a second. I took it to a tech who said he didnt experience any problems, made me pay for ‘resoldering’.
Still having the same problem, I took it apart and found out that the buzzing sound is coming from a small 5V relay sitting near the power supply. Took the relay out and tested it, it works fine on its own. I have only basic electronics knowledge, so still no idea what could be wrong.

The easiest solution I think would be replacing the board where the relay is on, I did not find it available anywhere. The power supply board sitting right next to it is available on the syntaur webshop though.

Otherwise I could test some more with my multimeter, and hopefully find a defective component that I can replace with my half decent soldering skills. But with my limited electronics knowledge I dont think its very likely I can locate the problem on my own.

I can upload some photos if that can help anyone come up with suggestions.


CP4 is a little weird beast (for YAMAHA anyway). The little relay sits in what they call the SWERP board. The SWERP board is the on/off switching board - it contains a small SMPS that supplies +5VDC (STANDBY power always on) to the on/off circuitry which controls the little relay. When tuned on, the SWERP turns on that relay which in turn provides power to the main SMPS for the system. Here comes the “weird” part. The main SMPS board is NOT a YAMAHA Part which also means there is no circuit diagram available for it from YAMAHA. Once main SMPS turns on the main CPU starts running and then sends a signal back to the SWERP board to keep the little relay energized until the on/off button is pressed again. That signal comes from the main CPU board (DM) via the AJK board (where all the I/O connectors are to the SWERP board. Here comes the issue, problem could be absolutely anywhere - from bad main SMPS to bad 3.3V regulators on the main CPU board to bad contacts somewhere in the chain of connections.
The tech may well have not been able to experience the problem if it happens to be caused by a bad electrolytic capacitor some place in the DM board or that main SMPS board. What he’s been “soldering” only he knows.
Thing is - you may need a good tech for that - DO NOT FIDDLE with the main SMPS because there are lethal voltages in there - and may even be there after mains power is removed.

Thanks for the quick reply. I’m aware of the potential lethal voltages. Too bad hearing that diagnosing the problem isnt nearly as simple as simple as i had hoped. I found a couple of ‘Yamaha service centers’ in my country, guess I’ll take the piano to one of them.

As a long shot - when you “tested” the relay … did you also check resistance across the contacts when relay is active?
I had a case once where a relay was working perfectly but resistance across the engaged terminals varied from few ohms to sometimes 300 … 500 ohms - albeit it was in the audio circuit not power.
If you have access to a relay maybe just replace it anyway - just in case !?

I took the relay out and fiddled with it some more. Resistance is 100 ohms both for on/off state, it matches the datasheet: https://nl.mouser.com/datasheet/2/315/PANA_S_A0000329307_1-2522295.pdf . One odd thing I found is that it turns on at around 5.25 V, a little more than the nominal 5V. I guess this is normal, and what the ‘pick-up voltage’ and ‘drop-out voltage’ in the datasheet refer too.
I can order the same relay for 5 euros but with 20 euros shipping, do you think it is worth the shot?

No no … I meant the resistance of the contacts … not the relay coil. When you apply the voltage to the relay that makes it active … check the resistance between the two contacts that are active.

As an aside - relay should pull at 80%max = 4V

Whoops I read your post wrong, the resistance of the contacts is simply 0 ohms when the relay is active. I measured the voltage over the coil again, I am not sure at what voltage it turns on but it seems to turn off at around 4.7 V.

Well - something you can try if you dare … could you attach a DVM across the diode (D4 in yellow) that is parallel to the relay, turn on the unit and measure the voltage.

Anything to the left of the dotted line in the circuit diagram has lethal voltages up to 350VDC

Relay buzzing in this case is caused by unstable voltage going to the relay.
I would use an analog volt meter as it can show much better any oscillation in voltage. A digital volt meter is auto leveling may or may not show this voltage oscillation, or auto regulate it giving a false lower reading.

My first suspect would be a capacitor in the relay turn on circuit is faulty, which is causing the relay to rapidly cycle, causing the buzzing.

I measured the voltage across the D4 diode with a digital multimeter.
When turning on the power, the relay was buzzing for a short time and the voltage shortly read 5.45 V. Then after some stuttering the relay stayed on as well as the piano itself. The D4 voltage stabilized at 4.52 V, but slowly decreased to 4.2 V in 10 minutes or so. At around 4.2 V, the relay repeatedly switched on/off, the LCD screen stayed on. Then for 15 minutes or so the voltage stayed within 4.20-4.35 V, the relay and power stayed on. Very weird.

I tried it two more times, the relay would initially buzz a little but again, it would stay on as well as the LCD. These times the voltage was 4 - 4.10 V.

Now I’ve reassembled the piano, and it seems to work just fine? No buzzing sounds when powering on and I can play it normally. I’m hoping it stays like this, I don’t know with the random voltage measurements. Could they indicate any problem?

I don’t have access to an analog voltmeter. Also my multimeter doesn’t have an ESR meter, so testing capacitors is difficult.

That is definitively too low voltage - if you look at relay specs that is as close as you can get to the 80% pick-up voltage.
As a start I would replace the two capacitors C13 and C14 - that circuit is designed to draw just a small current to keep the relay pulled. If one or both of the capacitors draw too much current the secondary +5VDC ripple becomes too large causing the DC component to drop considerably.

You could of course attach the DVM to the same diode across the relay and set the DVM to measure AC - that will show you the AC component (ripple) of the voltage.

I don’t know, the past few days the piano worked like normal. Not any buzzing or sudden power out. Did the same test again today and the voltmeter indicated 4.5 V consistently. The indicated AC was always 0.005 V, but my meter is only reasonably accurate from 2 volts AC. (according to the manual)
I think I will leave things as is until the problem might arise again.

I still think it’s somehow marginal … at least vindicates your techie :slight_smile: