Yamaha PSR530 Keyboard, No Display

Hi there and welcome to Electronics Point

Synth problems that progressively worsen may be due to liquid getting between the keys and dripping on the circuit board below. The liquid may be sweat from your furrowed brow as you rock out, or drool if you let the drummer use it, or some other bodily fluid perhaps, or just beer. This can eat away tracks and damage vias, and cause partly conductive paths between points in the circuit that are not supposed to have any conductive paths between them.

So I would start by removing the key assembly and going over the whole board carefully under a strong light with a magnifier and checking for damage. Cotton swabs with isopropyl alcohol are good for cleaning any crud away and showing you what's really there. Any track you suspect is broken can be checked using a multimeter on continuity range, with one probe at each end of the track.

While you're there, check for cracks as well, especially around heavy components that are mounted directly onto the board, if there are any.

Backlights, especially older ones, can deteriorate and this may be a separate problem.

 

Yes, but the circuit board under the keyboard assembly doesn't just do the keyboard functions. Usually there's just one main board that does a lot more than just the keyboard interfacing.

 

If the keyboard has a separate circuit board, and there's no way for liquid to get to the main circuit board, it's not very likely. I'm not familiar with that model. Synths that I've seen do have a separate circuit board for the keyboard but it doesn't completely protect the main board below it from liquids that can drip between the keys.

 

Why do you suspect capacitors? Synths are pretty low-power devices and usually have a small mains transformer, not a switching power supply. Switching supplies are used when higher power is needed and noise is not a problem.

No one is going to be able to say "it's this component here", unless it's a common fault. Even then, with the fault description of "no sound", it may not be the common fault. You need to get in there and have a good look. Take some photos too while you're at it.

 

That unit has a completely separate power supply. Problems with capacitors in the synth are very unlikely.

Did you give the main board a very careful thorough inspection?

What service equipment do you have? Multimeter? Oscilloscope? Logic probe?

 

OK, photos are probably not that important. But you're pretty limited with just a multimeter.

Here's a good place to start: the power supply.



Voltage comes in on CN701 at the right and is "regurated" by four devices. I have marked their output pins A, B, C and D in the diagram above. Here are the pin assignments for these devices.



You should check the voltages at each of the marked points using your multimeter with the black probe connected to the 0V rail which is shown along the bottom of the diagram. You can find the 0V rail on pins 4 and 5 of CN701 at the right side.

When you measure these voltages, be very careful to hold your meter probe firmly and don't push too hard, because if it slips and shorts the input and output of the "regurator" together, you can probably say goodbye to most of the ICs on the board. They don't handle high power supply voltage very well!

The voltages you should measure are:

• "A": IC702 pin 3 should be +5V
• "B": IC704 pin 2 should be +9V
• "C": IC701 pin 2 should be +5V
• "D": IC703 pin 3 should be +3.3V.

 

OK, obviously there's a problem with the 5V rail. The low voltage on the 3.3V rail will be a result of the low voltage on the 5V rail so don't worry about that yet.

The problem with the 5V rails could be IC701 faulty, or it could be an overload on the rail.

How hot is IC701 getting?

 

OK, it's possible that IC701 is faulty, but most likely something is loading the 5V rail down.

I can't give you a step-by-step because the service manual doesn't have images of the circuit boards so I can't trace the 5V rail, but you can try a few things.

See if you can find any tantalum capacitors. These are common causes of problems. There are two types - through-hole ones (with wires that go through holes in the circuit board) and surface-mount ones (that are soldered onto pads on the top of the board).

Through-hole ones have a distinctive shape a bit like a teardrop. They're usually mustard-coloured but sometimes green, red or orange. They are marked with a capacitance value, often indicated as three digits where the first two digits are often 10, 22, or 47, and the third digit is 4, 5, 6 or 7, and a voltage marking (e.g. "16V"). Polarity is also indicated somehow, usually by a line on one side or perhaps a "+" symbol.


Surface-mount tantalums are rectangular, usually mustard-coloured but sometimes black, have a polarity indication at one end (often just a line), and capacitance and voltage markings like the through-hole ones.



If you can find any of these, check each one to see if it's hot. Test them carefully at first; you don't want to burn your finger!

Also, check every IC (in fact, any black plastic component with three or more pins) to see if it's hot. Discharge any static electricity on your body first, by touching the heatsink tab of one of the regulators.

 

I don't see any tantalums in those photos. Did you find any? Have you checked the underside? Check all the semiconductors on the underside as well.

Also try unplugging all the connectors (the two that go to the keyboard, and any others) and see whether IC701 still gets hot.

Edit: Except CN701. Don't unplug that. That's where the supply voltage comes into the main board.

 

OK, that's interesting. So unplugging some of the connectors has caused the IC701 voltage to increase from about 3V to about 4.7V? Can you plug all the connectors back in, check the IC701 voltage has gone back to about 3V, then unplug the connectors one by one and measure the IC701 voltage after each one? Hopefully there will be one specific connector that, when you unplug it, causes the IC701 voltage to jump up to about 4.7V. Be REALLY careful when you measure the IC701 voltage to make sure the multimeter probe doesn't slip and short onto anything.

That isn't the only problem though, because 4.7V is too low, and IC701 is still getting hot, so there's another overload somewhere.

You will need to take the board out and check the underside for tantalum capacitors and ICs that are getting hot.