Negatron Blog

Installing a New Battery in a Yamaha DX7

I recently had the chance to replace the battery in a Yamaha DX7 Synthesizer. The manual for the DX7 states that the battery should last for 3 to 5 years and it was well past that time. I first thought that I could just run down to Walgreens and pick up a “button” style battery and pop it in but after poking around on the Web, I found out that the DX7 uses a special kind of CR2032 battery with solder leads mounted on it. I found a suitable battery through Mouser (part number 614-CR2032FH-MFR). To replace the battery you basically have to take the whole synth apart, but Yamaha tries to make the procedure as painless as possible.

I found some very helpful instructions through Dave Benson’s DX7 Page (also archived here) for replacing the battery. These instructions were spot-on!

There were a couple of things I noticed while disassembling the synth.

1. The connector labeled C6 on the main board is not connected to anything.
2. The main board was a little difficult to remove from the chassis after all screws and connectors were out. There are two tabs that are used to mount keyboard. You have to angle the main board between these tabs and the back of the chassis to lift the board out. Be careful when doing this!
3. To remove the old battery, I used a solder sucker to get rid of most of the old solder which makes it easier for the old battery to pry up and out.
4. The battery evidently had been replaced before. There was a service sticker from some repair shop on the back of the unit. The trace between the battery’s positive lead and the next component (which is a diode) had been severed. I had to solder in a short jumper over the trace to fix it.

Overall, replacing the battery was not a difficult task but somewhat time consuming. Have fun!

You can click on the images below to get a larger image:

Yamaha DX7 open with keyboard in place

Yamaha DX7 close-up of connector C6

Yamaha DX7 open with keyboard removed

Yamaha DX7 main board

Yamaha DX7 solder side of main board near battery

Circuit bending a Mix Me DJ

The “Mix Me DJ” was another great find! It was not that difficult to bend and the results were great. I basically added a pitch control which can be toggled between being controlled by a knob or by a theremin-like photo eye (a CdS photocell). Also added 1/4 inch jacks for input and output and gave it a nifty paint job making the plain old boring scratch wheel into a super-satanic-scratch wheel! I also added a volume control as this thing was just way too loud and there was no way to control that.

When the internal clock speed goes too fast, the circuit really starts glitching and making all sorts of mean noises. When first playing around with the device, I was sort of avoiding the sounds generated by the pop-out keyboard but as it turned out, the little keyboard produced some of the sickest sounds!

There were two fake knobs above the scratch wheel on either side. I cut off the tops of the faux knobs with a hack saw just leaving a ring to mount the real pots into. Nice.

Probably the hardest thing about modifying this beast was putting everything back in it’s place after the bends were made. If you want to bend one of these, it might be a good idea to jot down some notes on where all the screws, springs, etc. go. You might want to put sets of screws and stuff in their own baggie or envelope or something.

Watch this neat video:

Here is a very large image with the bends annotated on the image.

Enjoy!!

How neat are on-board pots?

I build a lot of pedals. Probably my least favorite thing to do when assembling pedals is wiring potentiometers. I found a great way to cut down wire cutting, stripping and soldering time greatly by just using the right angle PCB mounted pots as opposed to ones with solder lugs. Of course, I’ve had to reconfigure PCBs to accept the new-fangled pots but the pay-off is well worth it!

To make mounting easier when using the PCB mounted pots, I got a small cardboard box and made a cut-out in it a little smaller than the circuit board. Then I taped on strips of cardboard right on the edges of where the circuit board will actually fit in… the height of the cardboard strips keep the pots pretty close to a right angle when soldering everything down.

Since the pots are usually a bit mis-aligned when you start soldering, I typically solder one of the legs in place with just a small amount of solder while holding the pot in place with my fingers. I make sure that the pot’s leg is flush with the pads on the PCB and everything is close to being centered and straight. After the solder cools in a second or two, I remove my fingers. Now technically this would be considered a “bad” solder connection because I might have moved my hand while the solder was cooling. Not to worry. That’s why you need to use a minimal amount of solder here. Just enough to “tack” the pot to the PCB. I then go to the unsoldered leg on the opposite side and “properly” solder each of the legs… and then the last one with the presumably “bad” solder connection. Now all three of the legs should be soldered on there properly!

Sometimes, the back of the pots hang over part of the circuit board with pads and traces. Just to make sure nothing shorts out on the back of those pots, I usually squeeze a bit of hot glue between the back of the pot and the PCB just to be safe. Just a little dab will do ya!

I also recently reconfigured a pedal to take PCB mounted pots with straight (i.e. not right angle) connectors. I’m still working on a more streamlined way to solder these, but so far it’s been pretty smooth sailing!

The only thing left to tend to is to make sure that your enclosure has holes that match up almost exactly with the pots. Make sure to measure, double-check and measure again! I usually make the holes in the enclosure a little bit bigger than normal (but not too big, you don’t want the holes to be exposed under knobs!). It’s super nice not having to wire a bunch of pots!

Circuit Bending a Talk ‘N Learn Alphabet

I picked up a Talk ‘N Learn Alphabet at my local rubbish store. After opening it up and prodding around for a bit, I found 2 really good bends. I also added a 1/4 inch output jack to plug directly into other effects, an amp, etc.

One of the bends is a pitch control. I used a 250k pot with a 470k resistor inline. I also decided to put an inline switch to defeat the bend if the user so chooses. The second bend really glitches and distorts the output sound. This was obtained with a 1M pot with a 220k resistor inline. I also added a DPDT switch which gives the user the ability to route the output to a 1/4 inch jack as opposed to the internal speaker. I had to put a 10 ohm resistor across the output jack leads to dampen the otherwise hot output.

These two bends alone are worth hours of fun. The vocal sounds sound as if there is a high pitched voice and a lower voice layered on top of each other. With the knob nearest the speaker, you can make the voices really sputter and cough. In “music” mode, the bends make the “songs” just pump out what sounds like random notes.

Yeah! here’s a video of the Talk ‘N Learn Alphabet in action:

And here is a BIG photo of the guts with the bends explained:

Have fun!!

Replacing a non-operational motor on a Tascam Portastudio 424 MKII

I was fortunate enough to pick up a used Tascam Portastudio 424 MKII about 4 years ago on Craigslist. It worked flawlessly when I first got it. Recorded many a Xome tracks on the beast and eventually set it up so that the output of the 4-track was plugged into the soundcard on my computer. It was so handy having this setup to try out new gear and record that the Tascam almost never was powered down.

I decided to check out some old 4-track tapes recently and noticed that when I pressed play, rewind or any of the other tape transport buttons that the appropriate light would come on for a few seconds, some clicking was heard but it pooped-out.

I decided to open her up and poke around a bit. I noticed that the motor underneath the cassette tape bracket seemed like it wasn’t moving when perhaps it should. After taking out the bracket and looking at the motor, I did a quick Google search on the part number printed on the motor’s label (which proved not as fruitful as I thought it might be). But I finally found a place in Indiana called Studio Sound Electronics that carries the exact replacement. I found from the information on their site that it was a 12-volt Mabuchi motor. I placed 12-volts on the motor and yup, no movement.

I decided to order a replacement from Studio Sound Electronics. The motor was $9.95
And shipping to California was just under $5. I must say that Studio Sound Electronics’ service is wonderful. The motor arrived just a few days later.

I popped in the new motor, soldered the 4 wires into place and put everything back together. Bam! Works like new again!

Here are some hints if you’re looking to do this repair yourself:

** There are about 15 screws on the bottom of the unit that need to be removed including one that is on the little “ledge” on the back of the 4-track.

** After carefully opening up the unit, you have to disconnect 5 connectors - 4 from the big circuit board and one two-position connector from behind the transformer. Since they are all different positions and there are slots so you don’t put them in backwards, it’s not really necessary to mark them or anything like that.

** Remove the connector from the back of the cassette tape bracket.

** Remove the entire bracket assembly with the screws on the top of the bracket.

** Remove two screws on each side of the bracket to access the bottom bracket that holds the motor. One on each side is screwed through a plastic clip thing. You might want to put these away carefully so that you get the plastic clips on the correct sides when you put it back together.

** Move the motor’s belt over to the side and remove the 3 flat-headed screws that mount the motor. Gently pull off the belt guide.

** Once the motor is free, cut the 4 wires connected to the motor. You may want to memo where each of the wires go. The wires go as follows: A: blue, B: yellow, +: red, -: black.

** Strip each of the wires (about 3mm will do) and solder the wires on to the new motor.

** Putting the 4-track back together is the reverse of taking it apart. Just make sure you have all your electrical connectors connected!

** The motor in the Tascam 4-track is a Mabuchi Motor EG-530KD-2B. It’s a 12VDC 1600/3200 RPM CCW (counter clock-wise rotation) motor. It’s available from Studio Sound Electronics at http://www.studiosoundelectronics.com/cassette.htm

What? Another new blog?

Go figure… everything’s all changed-up all over again. Call me ADD? Well… I wanted to enter the blogosphere to share with other some stuff I’m in to. Actually, I started this blog just so I could use the word “blogosphere”.