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Determining plug polarity of a broken power supply

E

epo1

pfm Member
P/N Musical Fidelity X-Cans F34ADT-8

I've managed to break off the plastic earth pin from this power supply and now can't plug it into a socket. There are no markings as to whether it is centre +ve or -ve so I don't know how to spec a replacement

Looking at the front of the plug, the LH pin (goes into RH hole in mains socket) is live? If I connect a multimeter probe to the LH pin and the centre of the round plug shows a voltage and the outer does not then it is centre +ve; if centre does not show a voltage but the outer does then it is centre -ve. Is this correct?

Also a mm ruler shows the centre hole to be close to 3mm and certainly bigger than 2mm so I am guessing 2.9mm rather than 2.1mm.

I apologise for dragging the level of discussion down to baby talk but I am hopelessly out of my depth with this stuff.
 
Buqqer……I pressed the wrong key and deleted my post.

OK, from further looking around t’interweb, it appears that the output from the wall wart is AC volts, rather than the more usual DC volts from similar wall-warts. So, a bit of an oddity.

Unfortunately those things are not usually repairable, as the plastic earth pin is part of the moulded casing. Therefore I think your best best is to ask MF.
 
Power requirement, therefore adapter output, is 12VAC 500mA (higher mA would be fine, but no lower).


There is a V1 schematic floating about the interweb that indicates that centre pin is live at PSU board input. This may not be from MF though. Easy enough to check by having a look at the PSU board, although I doubt it would be legal to have the outer ring be live on an AC to AC adapter.
 
Are we talking about a cylindrical x-can I.e. V1 / v2 if so 12 v ac . Ideally find a minimum 1amp psu. Rapid online used to do a suitable version.
 
Yes I have seen compatible online, RS and Farnell do them. I do not know if it is centre +ve or -ve. MF say ask the distributor so I have. I will also reach out to JSAudio as it may also benefit from recapping seeing as it is approaching 24 years old.

I take it the multimeter idea is silly.
 
The multimeter idea isn't necessarily silly, only with AC to AC it really doesn't matter which way round AC +ve vs. -ve is at output. Regardless, your adapter mains ground pin is (or, rather was) a plastic 'dummy', i.e. no ground connection within the adapter, nor to X-Can, only 12VAC +ve and -ve.

You can set you meter to AC volts and insert one probe into the barrel, with the other probe held to the side, either way round, and should see a bit higher than 12VAC (typically a bit higher, as it will be being operating with no load, 13V and change isn't unusual).
 
Craig B said:
it really doesn't matter which way round AC +ve vs. -ve is at output.
Click to expand...
So my broken PSU says it was 12V/500ma, as long as I get something with the correct barrel size that is AC output 12V / 500ma (or possibly 1A is said to be better?) I should be good to go?
 
I’ve checked as well it’s definitely ac yeah.

You just need
AC
12V
Equal to or more than 0.5amp rating
Correct plug size.

Might be 5.5 mm by 2.1 mm but check that and you’re good, I’ve no idea.

Many for sale online. Double check it’s AC you’re going sold as that’s less common.
 
epo1 said:
So my broken PSU says it was 12V/500ma, as long as I get something with the correct barrel size that is AC output 12V / 500ma (or possibly 1A is said to be better?) I should be good to go?
Click to expand...
Yes.
 
Last edited:
The simple fix is just jam it into a 2 way extension block and force the earth cover down with a small screwdriver, then plug into wall. Obviously....
 
Daft question, is it not possible to just put the broken earth pin, into a socket, then plug the power supply in? or am I missing something
 
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