MIDI Keyboard works on battery but not power supply - Page 1
Forum section: electronics / repair

author: optonox
author_rank: Newbie
timestamp: April 08, 2021, 11:57:33 pm
content: Hi,I have a MIDI keyboard. I have had this issue "fixed" by a "professional" several times, but it never really worked.The keyboard has two options for power, a battery pack made of 6 AA batteries in the back or a barrel jack power supply.The issue is that the keyboard works on battery power but as soon as a power adapter is plugged in to the device - it will not turn on.This happens even when the power adapter is not plugged in to the wall and if the battery pack is plugged in or not. I have tried all combination.I am assuming the purple "air wire rework" was done as part of one of the repairs, which, I think initially seemed to help but now do not work.I attached photos that I think are helpful but I can take more. I am appreciative of any help and how I can fix this.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 09, 2021, 01:20:11 am
content: Reflow the barrel jack. Get your iron as hot as it goes with a big tip. From the last photo there is likely a crack in the solder.

author: Nusa
author_rank: Super Contributor
timestamp: April 09, 2021, 02:15:04 am
content: Quote from: optonox on April 08, 2021, 11:57:33 pmThere is a switch inside the jack that opens when the plug is inserted, which disconnects the battery. The battery is not supposed to work unless the plug is physically removed. That symptom is by design.If you have a meter, test the plug of the power adapter to be sure it's actually producing voltage of at least 9V DC. If not, that would also explain your symptoms.

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2021, 02:39:33 am
content: Are you sure you're using the correct power adapter? Most musical gear uses center-negative power, not sure why, but most AC adapters are center-positive. There is no standard though unfortunately so you have to look at the rating plates. A lot of gear gets damaged because people assume that if the plug fits it will work.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 09, 2021, 07:02:54 am
content: Replace the DC socket, they are very cheap, this should have been done by whoever fitted that wire if this has been a problem more than once.https://uk.farnell.com/pro-power/ppw01010/2-1mm-dc-socket-locking/dp/3498419?st=dc%20power%20socketThe outer connector can get sprained or tarnished and fail to contact the barrel, but the switch that enables battery operation may still work.There are different sized centre pins in the plugs and sockets, the most common being 2.1mm. A 2.5mm plug will fit in a 2.1mm socket but the undersized pin may not make good reliable contact.Get a 2.1mm socket and try it on the power supply before fitting it.Assuming, of course, that the power supply is the correct voltage and polarity and working.

author: optonox
author_rank: Newbie
timestamp: April 09, 2021, 09:10:49 am
content: Quote from: james_s on April 09, 2021, 02:39:33 amThis fixed my issue. Unbelievable but I have never seen the "center-negative power supply" and can't believe it mattered.Thanks james_s (and everyone else)

author: Nusa
author_rank: Super Contributor
timestamp: April 09, 2021, 02:31:53 pm
content: Your keyboard must have a protection diode for reverse polarity power...be glad it still works!

author: highpower
author_rank: Regular Contributor
timestamp: April 09, 2021, 06:11:41 pm
content: Quote

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2021, 06:16:52 pm
content: Quote from: optonox on April 09, 2021, 09:10:49 amPolarity absolutely matters with DC, most devices will let the smoke out if they are connected to reversed polarity, you got very lucky that whoever designed this one thought to include reverse polarity protection and that the bean counters didn't nix it. *ALWAYS* inspect the data plate next to the power jack and on the AC adapter you intend to use to make sure the polarity and voltage matches. Just because it fits doesn't mean they are in any way compatible.