Sunday, April 21, 2013

Building a Modified EA Tremolo Pedal on Veroboard

Good morning, everyone. Who wants to build a tremolo pedal today? Well I know I do.

The original EA Tremolo was published in “Electronics Australia”, an Australian (you don’t say!) electronics magazine, in November 1968 and later proved extremely popular among the DIY community. Since then, several modifications/improvements have been made.

Here’s the one I’ve decided to try:

A description of the circuit can be found here:

Helpfully, a Veroboard layout is already available. This one was laid out by Andrew Carrell, who asks that it not be reposted, so instead I will simply provide a link:

[Note that the flashing LED appears to be wired in the wrong way around in that veroboard layout, so swap its polarity. See comments below for a bit more info.]

As you may be aware, I’m a bit of a fan of Veroboard circuits, since they can be quite easy to build (assuming you choose a fairly basic circuit) and don’t require you to buy or make a specific circuit board for each project (here’s one I did earlier, for example:

I decided to try a few new techniques this time around, which I’ll mention as we go along.

Let’s start with the Veroboard itself. We need a 26 x 9 grid (26 holes x 9 strips), so we cut that out first.

Then we need to cut the tracks in 17 specified places, as shown in Andrew Carrell’s layout. Previously I’ve used a number of different tools to do this, all of which have been a little more time-consuming than I would like, so this time I decided just to drill holes through the board at those specific points:

I was hoping this would help avoid having to check each track to make sure it wasn’t touching the next one near the break, but I found I had to do just as much tidying up as with previous tools. Also, if you drill too many holes close to each other, you’re going to weaken the board (not really a problem in this case, but worth thinking about in future), so I don’t think I’ll be using this method again.

I'd previously laid out all of the components I’m going to be using on a piece of paper. I haven’t really ever done this before, but found it a great way to stay organised and reduce errors.

Note that due to a lack of local availability (story of my life) I had to substitute all three of the transistors for alternatives, as follows:

BS170 - > 2N7000
J201 -> 2N5457
2N5088 -> 2N5089

The components are soldered to the board:

And the wires are added:

Now let’s deal with the enclosure.

I decided to just mark the holes on top of the plastic protector that came on the box.
These are the hole sizes I needed (careful – you might require a different size):

Pots – 7mm
Audio jacks – 9mm
DC-in jack – 12.5mm (I drilled to 12mm then used a taper to enlarge it just a little more)
LEDs – 8mm
Switch – 12mm

Then I punch dents to centre the holes:

The holes are drilled using a stepping drill bit (of course standard drill bits will work too):

And here are the finished holes (I had to remove the back cover to finish the switch hole, since the stepping drill bit was hitting off the back cover):

You’ll notice that I haven’t drilled the LED hole(s) yet, as I wanted to check on a few things before committing to a position.

OK, now we put everything we can in place. You may notice that I’ve not put the DC-in jack in (I decided to move its position and you’ll see it making an appearance later in this post).

I’m laying everything out to get an idea how long I need to make the wires. Some are cut a bit shorter and one or two might even be replaced with a longer wire if necessary.

NOTE: If you need a hand wiring up the switch, jacks, DC-in, etc., have a look here:

Once all the wires are cut and soldered in place, we have this (note the new DC-in jack position and the LEDs):

And here’s how it looks from the top. I haven’t decided how I’m going to finish the enclosure, so it’ll stay like this for the time being.

So what about the hole left by the original DC-in jack position? Well, it turns out I had an old coin lying around (a Spanish one peseta coin) that fitted exactly in the hole. Here it is:

A couple of things I should point out before letting you hear the pedal. Firstly, in the Veroboard layout, there is a note that says: “Vol 2 goes to output”. This is basically the output of the CIRCUIT and should go to the appropriate pin on the stomp switch, not straight to the output jack.

Secondly, the value of the 100k trim pot (TR1) is critical and it must be adjusted slowly and carefully while using the pedal in order to find the sweet spot (my pedal had NO OUTPUT AT ALL until I found the right position, at which point the output was suddenly perfect). Alternatively, you can measure the voltage at Q1 drain, and adjust until it reads between 4.5v and 5v.

There's nothing else to add now, except that here is a demo. You will notice that the yellow pulsing LED (which indicates the speed/rate) is always on once the jack in plugged in. Apart from letting you know the speed/rate, this also serves as a good reminder that you are always using the battery, even though the effect isn't switched on. The steady green LED indicates that the pedal is switched on. The pots are, from left to right, Volume, Speed/Rate and Depth.


Tuesday, April 16, 2013

The $10 “Broken” MXR Phase 90

You know sometimes just being willing to try to fix stuff allows you the freedom to buy things that you otherwise would’ve steered clear of, even though they’re not really broken.

I was in one of the local guitar shops not long ago; one that sells second-hand pedals. In their pile they had this:

It’s an MXR Phase 90

with the back cover missing (and replaced with some electrician’s tape):

I asked how much they wanted for it and they told me that it was broken. I said I was still interested, figuring it might be worth my while to try and repair it if the price was right. After some discussion with the pedal owner (one of the guys who works in the shop), they asked me for NT$300 (about US$10), reminding me again that it was broken.

I figured for ten bucks it would be worth it for parts alone, so I readily handed over the cash. The guys in the shop were amazed that I was willing to buy a broken pedal. Meanwhile I was amazed that that seemed so strange to them.

So I got it home and peeled off the PVC that was stuck on the back. I expected it to be all dirty and possibly corroded inside, but this is what I found:

Surprised by just how clean it was, I decided to plug it in and see what appeared to be the issue. First I tried it with a power supply, and then with a battery. In both cases it worked fine. I mean, it was perfect; not even any dodgy jacks or a sandy-sounding pot or anything.

So I set about making a back cover for it.

I actually have a few project boxes that are the same size as this, but I’d feel bad wasting one of those just for a back cover, so I found another back cover from a box that had been cut up and used for something else. It was a bigger box, so the outline of the box is traced onto the back cover first:

Then the back cover is cut down to size, as follows:

And filed flat:

The holes were drilled for the screws:

And then those holes were countersunk:

The corners were rounded:

And then the edges were smoothed:

Here I'm giving the base a good scrub with a wire brush. This'll help prepare it to receive paint.

At first I wasn't even going to paint this, but I had this orange primer lying around that seemed to match pretty well, so on it went:

After a couple of coats:

Then some clear coat is added:

For the second (and final) clear coat, the can is held at a distance while spraying in order to get a bit of an orange peel effect:

The reason for that is to try as best as possible to match the finish on the pedal itself, which you can see here:

After the clear coat dries, the base is screwed on:

Then some rubber feet are fitted:

And there we have it. A perfectly functional MXR Phase 90 for ten bucks. How’s THEM apples?

Sunday, April 7, 2013

Repairing Broken AKG K240 Studio Headphones

Christian over at Black Sunshine Media (Wait, you don’t know about Black Sunshine Media? Get over there and check the site out immediately. It’s awesome.)

Anyway, what was I saying? Oh yes, Christian over at Black Sunshine Media asked if I could possibly fix his headphones, since they were his favourites and an essential part of his kit. He showed me this photo:

I think the technical term for what happened here is that they “assploded” (through no fault of their own, I might add). According to Christian, they got a bit squashed at some stage.

Anyway, I took one look at the wires and said something entirely too self-confident like “piece of cake”. In fact, the wiring was the easy part. Figuring out how to put them back together took a bit more, well, figuring out.

Before continuing, the other wire is disconnected to make things easier to work with:

It didn’t even dawn on me to take a photo at this stage, but the next step here was to put the back of the pad back on. This wasn’t rocket science and no tools were required. Here’s how it looked after doing that:

Next was to figure out how the whole thing was going to go back together. Actually it’s a very clever mechanism. Basically (excuse me for going all engineer on you again), it works on a universal joint principle. This is what a universal joint looks like. Your car has several. This joint basically allows for freedom of movement along two axes, while maintaining a positive connection along the third.

(Image taken from

And that is what allows the pads on each side of these headphones to move a little bit to fit the angle of your head, without actually falling off (yes, I appreciate the irony that these ones actually HAVE fallen off, but that wasn’t technically supposed to happen).

Anyway, the little joint itself, which just to clarify, is this:

Was still connected to one part, but not to the other. The problem is that there was no way to connect it to the other part while trying to fit the headphones together, because you just can’t get your fingers (or any tools) in there to position it right, while simultaneously closing everything up.

But if you look closely, you can see that the part it is already connected to is specifically designed to assist with this exact thing, i.e. it helps correctly position the joint as you are closing things up.

Wait, that's a bit hard to make out. Let me remove it slightly so you can see a bit better. Imagine you need to push that circular bit down. You can see that there is a bit of a slope guiding it into place. See it?

So what we do here is take the joint out completely and connect it to the OTHER part (the one that is just too difficult to connect to while we’re trying to close everything up). Just squeeze the joint together a little, which will make it narrower while you move it into position.

Note the little tapered edges pointed out below. Make sure they are facing out the way, so that they can interface with the sloping guide mentioned previously when it comes time to put it all together.

Then we quickly add a couple of little extensions to the wires, since the red one in particular is dangerously short and I’d hate for it to snap again.

I’d usually put heat shrink tubing over this, but I don’t want to apply heat anywhere near the pad covers, so I’m just using PVC tape instead.

And we solder the wires back in place:

Now we carefully position things so that everything is lined up right and the joint will clip nicely into place (on both sides) as we close this up.

"Persuade" it into place with a flat-head screwdriver:

A nice click is heard, the world rejoices and we’re done.

Piece of cake (kind of).