[awright]

Do you meqn q music "keyboard" or a computer keyboard? I assume you mean a musical instrument.

Sounds to me like it is quite tolerant of power supply voltage variations. If it works, go ahead and use the 500 ma. PS, but either measure the current to be sure you are not exceeding its current limit or monitor its temperature (by finger) very carefully, especially on hot days.

If you see a circuit board it was probably originally a regulated supply, but not definitely. If the old PS is non-functional, open it up to see what's there and to see if it was regulated or if you can repair it.

Your interest is in whether the old PS was LIGHTER than normal, not heavier. If it is lighter than you would expect, it was probably an SMPS (Switch Mode Pwer Supply) and, thus, regulated.

Have fun.

awright

[salpha001]

i've also done some test on the keyboard, the original power supply work at 12v 700ma as you know, i've connected a 1000ma ps 12v - worked, then tested at 9v worked, 7.5v - worked, 6v - worked but got a static noise sound coming out of the keyboard.
then tested the 12v and 9v 500ma and it worked.
Is this normal?

[salpha001]

side slots are decorative, but i can see the board if i look from an certain angle to the frontal slots.
it's not more heavy than other power supplies this size.
the case is glued and the screws have no head at all, but i'll try to open it

[awright]

Hard to say. Are those slots merely decorateve and blocked or actually open to the interior of the unit? Can you see a circuit board inside the slots? For a better view, carefully break off a rib. Does it feel heavy for its volume?

Or, you could ask Yamaha or get access on this site to the manual for the instrument it is powering.

The third alternative if this PS has failed is to crack it open and take a look at its guts. No sense keeping it intact because you're not going to fix it that way. Actually, for fun and education, there's a small chance that you can repair the original PS if you open it up carefully enough that you don't do further damage to the guts. If the guts are not fully potted, you may find a failed fuse or diode that can be replaced. If you find a regulated PS inside it may be beyond the beginner to repair and not worth the time except for the challenge and fun.

The PS may be screwed together, but is probably glued together making judicious use of force mandatory. Use a thin chisel, like an already damaged wood chisel, and tap it all around the joint between top and bottom halves of the case. You will be breaking the case, but hopefully in a controlled and moderate manner. Escalate the force until something starts to give, then lead the crack around the entire case. Use a light hammer and anvil, not manual force that can lead to serious gashes in flesh.

When you get it open, show us what you find.

I'm going to bet it is a simple, unregulated PS.

awright

[salpha001]

thanks once again, here's a pic of the power adaptor

[awright]

Well, I was going to launch into a description of how to test the old power supply, but that won't help if it is inoperative. For others looking on to thei thread, if you load the supply down to, say, 50% and then 100% of rated output with power resistors and observe a significant change in voltage, the supply is unregulated. Often an unregulated supply will measure 150% or so of rated voltage when unloaded due to the marginal power transformer and capacitor.

One way to check is to look at the equipment using the power supply. Is there a voltage regulator chip in the power input line or does the power just go directly into the circuitry, without internal regulation? Many devices do not need any voltage regulation because there is no voltage sensitive function in the device. But some that do will have a regulator at the power input. Some will use a regulated power supply to keep the noise and heat away from the device or to keep the device more compact for desktop use.

What does the device being powered do?

If you have the original power supply that came with the equipment feel its weight. If it is fully enclosed in a molded plastic case with no vent holes and is heavy for its volume it is probably an unregulated transformer/rectifier/capacitor supply. A regulated supply contains active circuitry and generally will have ventilation holes for cooling. The more sophisticated modern regulated supplies also do not use a power transformer to step the incoming power down, but rectify the line input directly and pass the power through a much smaller, lighter transformer that operates at 20 KHz or higher. The whole power supply is, thus, lighter for its volume and power rating.

But remember, if in doubt, just get a regulated supply.

Good luck.

awright

[salpha001]

sorry i forgot to log in so it came out as a guest.

[Guest]

thank you for your replies
the original power supply died some years ago, and i have bought a 1000ma 12v to replace it and it worked ok for 3 or 4 years but it's dead now too.
there is no particular reason, its just that i've order a couple of power supplies for a couple of equipments, all of them work at 300ma besides the keyboard that works at 700ma.
how can i tell that the original supply was regulated power supply with active circuitry or a transformer-rectifier-capacitor power supply?
thank you for your help

[awright]

Definitely NOT a good idea to replace an original 700 ma supply with a 500 ma one. Is there someparticular reason that you are considering using a 500 ma replacement supply? There are many higher capacity supplies out there at moderate cost.

IF you measure actual current draw by the equipment and IF it is below 500 ma (which is a possibility), you could substitute the 500 ma supply. But it is unlikely that the manufacturer provided a supply with a capacity higher than is actually needed because it increases his cost with no commensurate benefit.

You should check whether the original supply was a regulated power supply with active circuitry or simply a transformer-rectifier-capacitor power supply, then match the original. Replacing either type with a regulated supply of sufficient capacity is OK, but replacing a regulated supply with an unregulated one is definitely not OK.

awright

[vtech]

In general the rating of the replacement supply should be the same(or more Current--wise) as the originall yet the polarity of the connecting plug MUST be exactly the same.

Depending on the actuall current draw of the equipment, may be able to use a 500ma instead of a 700 ma. While it won't burn up the equipment, may not be adequate & cause the adaptor to overheat & eventually fail.

To know for sure, look a the rating on the equipment itself.---may be listed as 12V, xx watts.