How to test the power supply tip is centre positive?

lukedlb · November 2021

I have to change the tip (jack? what is it called?) on a replacement 12v power supply for my keyboard with the original tip. I'll cut the wires on the old and new power supply and solder and tape them together. Not the best solution, I know, but bear with me (yes, I ordered a replacement from acadaptors-r-us and of course it was the wrong tip; did they reply to my complaint? Of course not).

How do I test with a multi-meter that I have wired it properly to centre-negative?

The tip looks like a regular 9-volt tip you find going into a guitar pedal.

Danny1969 · November 2021

Is it a digital meter ? If so then the red cable will tell you +12V or -12V .... for Boss / Roland you want the red lead on the centre pin, black lead on the outer barrel and the meter to report -12V

lukedlb · November 2021

Danny1969 said:

Is it a digital meter ? If so then the red cable will tell you +12V or -12V .... for Boss / Roland you want the red lead on the centre pin, black lead on the outer barrel and the meter to report -12V

Thank you @Danny1969 . Yes, it is a digital meter, a GBC KDM-120 (https://www.gbconline.it/it/prodotti/schedaprodotto?codice=50.1055.04), which has a manual in Italian. It looks like I'm measuring (and please correct me) the CC Tension, so the Red cable goes in the VmA and the black goes in the COM. I turn the dial to 10 o'clock for 20V.
Do I then simply insert the red cable into the centre of the barrel (and put the black cable on the outer barrel)?
I want to measure +12v.
Before I start cutting, I will perform the test with the functioning unit, get the desired 12v+, and then cut and wire up a different sized barrel.

Danny1969 · November 2021

Yes looks like it.
You can test if you have this right by putting the meter on a battery and see'ing if it identifies the voltage and polarity of the battery.

If you cut and reverse the polarity of the cable then here's a neater way of doing it that's also immune to shorting

Image: http://thesuperheroesonline.co.uk/b3.jpeg

lukedlb · November 2021

Danny1969 said:

Yes looks like it.
You can test if you have this right by putting the meter on a battery and see'ing if it identifies the voltage and polarity of the battery.

If you cut and reverse the polarity of the cable then here's a neater way of doing it that's also immune to shorting

That's smart: staggering the cable cuts.
Now that you mentioned it, my 9 year old son and I had fun measuring old batteries to discover that a good 40% are good to go. My son now has greedy eyes on the battery recycling bucket at our local market.
Thank you, @Danny1969 ;

lukedlb · November 2021

Putting my new found multi-meter skill to hand before I made the snip, I made sure I was doing it right by testing a few different power supplies and making sure the new one didn't fit. One power supply that fitted and had the necessary volts was registering at 3 v instead of 9. Hang on... I gave the voltage selector a good twist and got a correct reading, plugged it into the piano and voila! the keyboard was working again. No snipping required. It turns out 1mA was enough. Rhodes sound and acoustic piano sound restored (it only has two sounds; how I love simplicity), I can't wait to start playing again.

Once again, thank you, @Danny1969 ;

Danny1969 · November 2021

Good work @lukedlb ;

prowla · November 2021

Danny1969 said:

Yes looks like it.
You can test if you have this right by putting the meter on a battery and see'ing if it identifies the voltage and polarity of the battery.

If you cut and reverse the polarity of the cable then here's a neater way of doing it that's also immune to shorting

You should also put heatshrink sleeve on before you solder it.

Push it away from the join whilst soldering and then slip it into place when cooled down.

If you get clever, you can do a further length of heatshrink over the top, to neaten up the appearance.

Danny1969 · November 2021

prowla said:

Danny1969 said:

Yes looks like it.
You can test if you have this right by putting the meter on a battery and see'ing if it identifies the voltage and polarity of the battery.

If you cut and reverse the polarity of the cable then here's a neater way of doing it that's also immune to shorting

You should also put heatshrink sleeve on before you solder it.
Push it away from the join whilst soldering and then slip it into place when cooled down.
If you get clever, you can do a further length of heatshrink over the top, to neaten up the appearance.

The heatsink is on it to the left, ready to slide over in my picture but I don't expect everybody to have it

lukedlb · November 2021

Thank you @prowla and @Danny1969 for the heat shrink advice. I've always wondered if there was a better solution than wrapping it with tape.

Flip · December 2021

Luke, in case the quantities heatshrink's available in (I inherited my stock from days when we did bulk orders from RS) You might find self-amalgamating tape useful. I used it to weatherproof all the camera/power feeds to our CCTV system.

lukedlb · February 2022

Flip said:

Luke, in case the quantities heatshrink's available in (I inherited my stock from days when we did bulk orders from RS) You might find self-amalgamating tape useful. I used it to weatherproof all the camera/power feeds to our CCTV system.

Thanks. Amazon provided for a tenner. I'm set for life.

Howdy, Stranger!

Categories

In this Discussion

Become a Subscriber!

How to test the power supply tip is centre positive?

Comments