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Transformer is going bad. Best options?

post #1 of 9
Thread Starter 
My aunt obtained a Roland JX-10 for free because it had 5 dead keys. I simply cleaned the PCB with alcohol and cleaned the keys and now everything is fine except for one thing. The transformer is making a loud screeching noise so I plan to replace it. The transformer says something like 100V/31V and 107V/37VA so I have no idea what that is so can anyone help with that? I might just have to measure the output to make sure of what it is. Anyway, once I figure out the correct DC voltage, can I just buy any transformer that is rated at that VDC and replace it?

I know it is an old keyboard but it was free and still serves its purpose. Nothing wrong with it so far. I am trying to play it and mess with the output jacks to see if there are any shorts. Thanks a lot guys.
post #2 of 9
Thread Starter 
Anybody?
post #3 of 9
Well if it's working it's not the trnasformer. I'd bet some of the filter caps in the power supply are going south. Change them on spec.
post #4 of 9
Quote:
Originally Posted by mminutel View Post
The transformer is making a loud screeching noise so I plan to replace it. The transformer says something like 100V/31V and 107V/37VA so I have no idea what that is so can anyone help with that? I might just have to measure the output to make sure of what it is.
Is that a wall wart? Transformers by themselves are AC devices. AC in and AC out. Wallwarts can be either AC or DC. -- A DC supply has additional parts besides the transformer.

+ It is unusual for a transformer to fail without something else failing first.

In any case something spec'd in VA is usually AC output. Do you live in Japan? 100V input is not right for the US. In fact that could be cause of the problem.
post #5 of 9
but is optimus prime alright?
post #6 of 9
sure it's the transformer? they shouldn't screech and they output AC not DC. So if its a wall plug jobbie and says DC it's more probobly a failed filter component after the bridge rectifier. Wall packs are usualy double insulated though so it may not be easily fixable.
post #7 of 9
Thread Starter 
I was thinking that transformers make AC go to DC but I was wrong. It looks like this, but it is not the correct one: I just wanted a picture to compare it to. http://www.hammondmfg.com/pdf/5c0018-19.pdf

If you do not mind, what makes the voltage change from AC to DC because I always thought that it was the transformer.

Should I replace all of the caps as well while I am at it?

I live in the United States, and I am 95% positive that it is the transformer making the noise. I will google around to try to find out the exact specs on the transformer.
post #8 of 9
Transformers only work with AC currents; the principle of operation relies on changing magnetic fields generating electric currents - static fields don't generate a current, so AC must be used. They're used to change one voltage to another (used in power supplies), or to change one impedance to another (often used in amplifier output stages).

Normally electronics require DC current, so this AC current must be rectified. There are many ways to perform this, the most common is with a device called a bridge rectifier. It uses four diodes to move the 'bottom' part of the AC wave into the 'top'. You still don't have DC, but the voltage is 0V or greater at all times. Various degrees of filtering would be applied to smooth this out into something close to DC. The simplest approach is to use a large filter capacitor directly across the output of the transformer. Complexity goes up from there, with the next step up being an LC filter, then a pi (CLC) filter, and so on, with a regulator of some type often added afterwards if line regulation is required.

The article on rectification at Wikipedia has some good diagrams that might help.

Replacing electrolytic caps might be a good idea if you're up for the effort. If the keyboard is working fine though, you probably don't really need to. Try to avoid mucking with parts in the audio section of the circuitry; I don't know what vintage this keyboard is, but many early electronic keyboards have significant analog signal processing that might be affected.

If you have a multimeter, you can measure the output of the transformer (remember to use ACV mode), and select a transformer that is around that range. 37VA seems a reasonable size for a keyboard, so you probably won't have too hard a time finding an appropriate 40VA model, if not even a drop-in replacement. The manufacturer might have substituted a 100V primary transformer because they needed an increased secondary voltage, or it could just be an imported unit. If you can't be sure and it's been working fine for 20 years, you're probably okay just finding a 120V primary unit with a similar output rating to what you measure (given 120V, the 100V primary unit will have a proportionally increased output).
post #9 of 9
100V AC is probably from Japan. I heard screeching sound before when I pump 230V to a 120V transformer.
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