Sunday, October 5, 2014

Headphone Jack Modification

In preparation for one of my most anticipated projects of all time, the bicycle stereo system, I needed an adequate source for the music to come from. My default choice was the "my first Sony" cassette player, as I've already been using it as a stand alone bicycle stereo (it's actually mono).  This device features a built in 4W speaker and has been working well so far, especially on trails, but if I'm going to compete with traffic, I'm going to need more power, which is where the bicycle stereo will come in.  More on that later.  The first problem was that this cassette player mysteriously didn't come with a headphone jack, I'm going to need one in order to connect to the amplifier later.  I suppose the manufacturers wanted kids to annoy the crap out of their parents by restricting them to using the speaker.

Anyhow, this isn't the first time I tried to add a headphone jack to this thing.  My previous attempt was a failure as I tried cutting the opening with a soldering iron X-acto knife attachment which was really messy and smelled bad.  The aftermath of this is visible in the picture below.  Also in the picture is the new tool that will do the job right, my new Dremel Craftsman hobby tool with grinding disc attachment.
My first sony with botched headphone jack aftermath and Craftsman hobby tool Dremel knock off.

Here it is after cutting out the yellow section, much cleaner (the red section was cut later).  Instead of trying to install the headphone jack and switch directly into the plastic like last time, I decided to make a sub-plate out of some thin galvanized steel which will be hot glued in place.
My first Sony after cutting away yellow plastic and piece of galvanized steel.
The switch is to enable muting the speaker.
Here I am using the hand Whitney punch to make easy work of the holes.  The outside contour was cut with aviation snips and then de burred.
Using a hand Whitney punch to punch holes in metal for headphone jack and switch.
Then using a square file to make the hole for the switch rectangular.  This technique might look familiar to anybody who has seen the blower post.
Using a square file to make a square hole from punched holes.
Here is the plate completed with the switch and headphone jack.  I pop riveted the switch on because I don't think it needs to come out any time soon.  The hole for the jack was punched first, then drilled to size since the punches don't go that big.  The additional holes on top and bottom were for my original plan to screw the plate into the plastic.  Circumstances didn't allow for this, so I just decided to hot glue it on.
Headphone jack and switch assembled in sub plate.
Here is how the plate fits in position, much cleaner than last time.
Headphone jack assembly in position.
The inside of the cassette player intrigued me from a toolmaking standpoint as the tool marks from the injection mold are visible.  I guess at some moldmakers in Thailand, Japan or elsewhere decided that since nobody was ever going to see the inside, they didn't bother polishing the inside half of the mold.  Who knows, if I measure the diameter of the tool marks and find that they used an inch size end mill, would that imply the mold was made in America?  Another tool making mystery...
Visible tool marks from injection mold on inside of cassette player.
Where was I? Oh yes here is the inside after I soldered all the stuff together.  Good thing this cassette player is so huge, there was plenty of empty space inside for all the extra wires to fit.
Headphone jack and switched soldered together with speaker.
From this poorly drawn electrical diagram, you can see how the wiring works.  The (+) wire that originally went to the speaker was rerouted onto the (+) terminal of the headphone jack and also to the switch, and then the speaker.  This means the headphone jack is always connected and all the switch does is cut the sound to the speaker.  The ground wire is shared since it isn't doing any harm.
Electrical diagram for headphone jack mod.
Oh look, it's written on my to-do list.  A preview of some other potential projects.
Here it is after hot gluing.  I think there's enough surface area to not worry about it coming undone from the force of plugging and unplugging.  Also hot gluing was a good idea to seal all the gaps so debris won't get inside the cassette player.
Headphone jack and switch in place after hot glue.
All done, now I can listen on the bus and not get kicked off.  But really, once I have a bike stereo, I won't want to take the bus.
My first Sony, headphone jack, and headphones.

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