I have an old CASIO CT-636, which works with 9V adapter, OR 6 D-size batteries. It does not come with an adapter, you have to provide one, and make sure it can switch to negative-inside, positive-outside - which is the old standard of the barrel jack.

I thought it would be nice to make it work with a normal power bank. It will be portable, have plenty of play time using the advantage of the Li-Ion cells, and you can recharge it any time. Batteries are expensive.

You also will be able to plug it in an USB power adapter.

Step 1: How to Make It

I don't want to modify the keyboard itself!

So I need a way to boost the 5V from the power bank to 9V for the keyboard.

But will it have enough power? The keyboard requires MAX 4.5W. A power bank has 5V*2A=10W, so it will be enough, but we need a 2A power bank just to be on the safe side. On the other hand the keyboard will unlikely use 4.5W, only if you pull the volume to the max!

To boost the voltage to 9V I will use U3V12F9 Step-Up Voltage Regulator from Pololu.

It has output current up to 1.4A - so 1.4*9=12,6W which is more than enough to cover the 4.5W keyboard requirement.

Step 2: Parts

You need:

1. Pololu 9V Step-Up Voltage Regulator U3V12F9 - https://www.pololu.com/product/2116

2. USB to DC barrel jack 5.5mm / 2.1mm - https://www.ebay.co.uk/itm/5x-USB-to-5-5mm-2-1mm-B...

3. Soldering iron, and solder

Price ~6$ ?

P.S. You can use separate cable and jacks, because wires inside this one are very thin, and hard to work with, but they fit perfectly the small booster!

Step 3: Cut the Wire and Measure

Cut the wire, and very gently remove the insulation.

Make measurements:

1. From the USB side - plugged into power bank, using voltmeter

- red wire was positive

- blue wire was negative

2. From DC side - using Ohmmeter

- red wire was inside

- blue wire was outside

SO - it complies with positive-in, negative-out, and I have to switch.

Step 4: Soldering

Solder using both metal helping hands :)

I soldered the blue wire from the USB (-), common ground, with the red wire from the DC which goes IN the barrel. And then soldered them both inside the little board on the ground pin.

The others are obvious - red USB to VIN, and blue DC to VOUT.

Be careful, do not pull much!

Step 5: Add Hot Glue

As mentioned above - the wires are thin. You have to strengthen the connection between the cables and the board with hot glue. Before applying gently bend them to form a continuous cable with a bended board in the middle.

They are well secured now!

Step 6: Add Heat Shrink

Use heat shrink to secure the whole thing, and have a nice finish.

I think mine was 14mm, but it was hard to put it. No signs on it.

I used additional black tape on the ends.

Step 7: Time to Play

The cable is ready, time to test it. Looks nice!

The keyboard pulls constant 0.15A from the USB power bank, which is good, because some power banks turn off if very small current is drawn (i.e. when you do not play). After playing for a while it did not exceed 0.2A which is 5*0.2 = 1W power consumption.

So if a power bank has a real 18650 cell with i.e. 2000mAh , it is 3.7*2=7.4Wh, you should be able to play for 7 hours, or may be 4-5 in the real world - per cell !

My power bank has 4 cells - I can play all day long!

Step 8: Conclusion

I really enjoyed this project.

I made it for a friend of mine.

I hope you like it too!

Some more ideas:

- you can put the booster inside and mount a mini usb jack. This way you will be able to use a standard USB cable to power it!

- you can put 18650 cells inside and use TP4056 to charge them. Handy!

Some advise:

- If you can, put 16V capacitors on the input and output of the booster, as big as you can (as Pololu recommends). They will smooth voltage peaks, and protect your booster and equipment.

P.S.

I've just found out that such a cable exist for sale and it is not expensive -

https://www.ebay.co.uk/itm/DC-DC-Converter-Cable-U...

DC-DC Converter Cable USB 5V to 9V/12V DC Jack 5.5mm*2.1mm Step-up Power Module

I don't know what is inside, but seem legit.

In my case though it will not work, because I needed reverse polarity :) Enjoy!