Electronics question

[seedubya]

Not an electronics guy so riddle me this if you can please

A friend brought the transformer for her Roland piano into a local electronics place for repair. The output on the transformer should be 12v DC \ 2A. When she brought it home it wouldn't power the piano so she asked me if I could test it. My Fluke meter tell's me it's outputting between 15.9 and 16.3 volts every time I test it. I've tested half a dozen other transformers and they all test within 5% of the expected value so I'm assuming my meter is good.

They replaced the cable entirely on the transformer. It's an older unit and has screw to take off the cover so is serviceable by a professional. Can anyone think of any reasons why the unit might now be outputting the wrong voltage so I can send her back to these guys armed with the right questions?

 

[Mike McCall]

Do you have the schematic which includes the transformer specs? If so, check the input voltage spec. Sometimes on older gear they specify 110V instead of 120V which could cause the variance.

 

[Moltuae]

Most likely the output is unregulated, either by design or because of fault. The 16 volts you're seeing is typical of what you might see pre-regulator in an analog (non-switched mode) PSU. Sometimes the voltage regulation is done in the equipment itself, rather than the PSU. And sometimes the rated output voltage is the under-load voltage.

I would try a known-good 12v PSU (rated at 2A or higher) to see if it works. Also, it would be worthwhile checking the original PSU voltage while its under load (piano connected and switched on).

 

[Diggs]

sometimes the rated output voltage is the under-load voltage.

^ This - Those types of transformers are never regulated.

 

[Markverhyden]

Basic trouble shooting. First step, determine what the original problem was and see if it can be replicated. So was the piano not powering up before?

The current measured voltage is a bit high. But as @Moltuae mentioned it may be due to design. The next step would be to hook it up and make measurements inside. That can get complex. The piano may not have service access points to make measurements so you may have to make then on the PS side.

Of course another power supply would be good. Since 12vdc is automotive there should be plenty of options. But you have to be careful in what you do. I'd, after checking the specs of the piano, might look at using a tickle charger.

 

[Mercenary Roadie]

You said they replaced the cable, so verify that the polarity is correct on the plug.