Odd Software for Odd Jobs


Adding DC power output to my "boombox"

So, I added an AUX-IN to my old Sony boombox. I can now connect my MiniDisc recorder or iPhone and play my tunes, as well as listening to tapes and CDs … oh, what a time to be alive!

I had been running my MiniDisc recorder off a single 1.5V AA battery but wondered about adding a DC output also to the boombox.

The radio tuner section that I have commandeered for my AUX-IN receives 6V from the main board. The MiniDisc recorder takes 3V from an external supply.

I started by figuring out what type of plug the Sony takes … the "Sony yellow tip barrel connector".

Turns out it's a 0.7mm (inner diameter) x 2.5mm (outer diameter) plug … Jaycar catalogue PP-0503. Also known as the "Nokia plug" apparently. Never knew that!

I'm going to use a 2.1mm plug & socket on the other end. So I made up a short cable.

2017-05-04 21.13.42

The first step was to see how much current the MiniDisc recorder would be drawing. I hooked it up to my lab power supply and gave it 3V … it drew about 300mA maximum (when first spinning up the disc and reading in the 10s of audio for the buffer).

I did muck around with regulators but gave up (I'm not clever enough). It's possible to get a 3V regulator but it needs at least a 3V headroom … with a supply of 6V that might be okay … in an "ideal" world. I didn't have any of those handy or easily available. I do have 3.3V regulators … a LM2936 and LM3940. The LM2936 would've been nice but it can only supply 50mA - not enough. I did fiddle around with an LM317 as well but I could never get it to work.

Besides, regulators are inefficient and I'm not sure how much current I'll be able to draw using the tuner's power rail.

There is a 3.3V rail available for the CD player, but I assumed that it wouldn't be powered if the radio tuner (i.e. AUX-IN) was selected. Maybe it is, dunno.

So I just used a more efficient DC buck converter instead! Simples!

2017-05-04 20.59.45

Stuck 6V in, tuned the trimpot to 3V out.

2017-05-04 20.59.34

And the MiniDisc record is happy!

2017-05-04 21.12.45

Checking the service manual again, I could see that +6V and GND were available here - there's a 0Ω link (JC1) for the +6V and a spot next to capacitor C58 for the GND … these are probably the easiest spots to tap in.

File 5-5-17, 12 25 42

There's no convenient hole like there was for the R + L channels, so I just drilled my own.

File 5-5-17, 12 26 09

Hacking away … new DC power output added to my earlier AUX-IN addition.

Honestly, it'd probably be easier if I just made my own replacement radio tuner board … bonus points. Need to check if I can get a similar flat flex connector.

2017-05-05 10.41.23

New hole drilled for the 2.1mm DC power socket.

2017-05-05 10.47.52

The AUX-IN and DC power sockets mounted.

2017-05-05 10.52.34

Jamming the 6V-to-3V converter in.

2017-05-05 10.58.28

Power out … and audio in!

2017-05-05 11.11.43

Sony never meant for this to happen, but their CDF-S350 is now powering their MZ-NH600 as well as taking audio in from it.

2017-05-05 11.11.52

Drawback - the audio and DC power share ground … so you hear digital noise when the MiniDisc recorder is starting up and when you change the song. This noise is not evident when playing, however, so I'm happy to live with it.