Fitting the GBexx buffer board

The GBexx board was designed a fraction of a millimeter too big. So it could be sanded to size. At least, that's what I say now. This kind of small PCB is hard to design accurate to the millimeter. The PCB as made by Bellmann had a little ecxess width, but still fits inside the GBC slot.
As a result, the contact fingers are not perfectly symmetrical on the board. There is an offset of 0.5 mm towards the side of pin 32 (GND). This topic is about fitting the GBexx board to better fit the GBC cart slot.

In this chapter, all the pictures were made by an Olympus Mju 720 camera with a resolution of 7 MP, mainly in macro mode. The pictures used on this site are downscaled images, but if you visit the HiRes picture site (see navigator frame) you get access to the full 3072 x 2304 pixel images to show you all the details you ever wanted to see.

Ready built GBexx card

To the right you see the GBexx printed circuit board (PCB) with all the components on it. From left to right we see:

The status LED was not in the circuit. I could have placed it in a few other positions as well and I might still change it that way. When routing the board, I poured a lot of copper in a Ground plane around the 34 pin IDC connector until it barely touches the +5 Volt power grid (also a copper plane). The gap was meant to be bridges by a capacitor.


The status LED

In the picture on the right, you can easily see the 'gap' between the two copper planes. I scraped off the solder resist with a sharp knife (careful!) and tinned the bare copper. Initially I mounted a 16 uF capacitor there but I encountered strange phenomena when doing multiple insert/extract cycles with the cart. So I decided to remove the capacitor again and mount an LED for a fast and reliable indicator about the presence of power to the cart. Call it an optical indicator.

I used an SMD LED plus a normal thru hole resistor (Red LED plus 470 Ohm resistor). The LED was connected to the GND plane and the resistor was connected to the rightmost wire bridge (+5V) as you undoubtly already have seen in the picture.

The LED works perfectly. Now I can see immediately if the cart mates with the connector. No twiddling with a multimeter. Just look and see. Simple. In the HiRes image is another picture of the resistor/LED pair. Or just download it here: http://www.mijnalbum.nl/Foto=NGAXB7TR. It will open in a new window. If you want the HiRes image for some silly reason, you need to visit the OnLine album though. The link is in the navigator.

Right now, I'm adding some 47 nF caps to the board as well. As you can see below


Improved GBexx

Here you see the improvements I made to GBexx. These improvements were intended to be done (in the design phase already) on the component side since the bottom side is in close proximity to a metal plate. A short is easy to make. So all caps (and the newly decided LED) are on the topside, i.e. solderside, i.e. component side.

Changes:

Now the circuit is up to scratch. The capacitors are needed for stable operation of the busbuffers since these can draw quite an amount of power. The LED is still visible when the cart is inserted.


Sanding down the GBexx

My GBexx was slightly offset. The fingers did not perfectly match with the connector pins. The difference was very small: 0.4 mm. Much to less for using a saw or a powertool. So I reverted to the trusted piece of sandpaper.

The sandpaper in the picture is for an excentric sander (Bosch PEX series). This one was a grain #240. Put it flat on a table and move the GBexx PCB over the sandpaper. Make sure the correct side is sanded! In my case, I had to loose some width on the side closest to the GND traces.

Method:

  1. Put the sandpaper on a flat surface
  2. Take the GBexx in your hand (correct side down)
  3. Put the PCB on the sandpaper and move it forward with one gentle stroke
  4. Lift the PCB up and move it back
  5. Check with a caliper every 5 strokes
  6. Repeat steps 3 - 5 until you have taken off enough material
Take care: Do NOT move the PCB back and forth over the sandpaper. It will take off more material in a shorter time, but you have almost no control over the place WHERE you take off the material. In most cases, you will end up with a rounded edge. Stroke it. And in ONE direction ONLY!

But then again: you're a big boy (are there any girls reading this?) and you should know what you're doing all together. Suit yourself. I warned you if things go wrong.

As far as the women is concerned: I only get fanmail from men. So if you are the odd woman out there, send me a small line via E-mail. Just for fun.


Make the GBexx somewhat thicker

When you measure the thickness of a PCB inside it's cart, you will find that is slightly over 2.3 mm thick. This PCB was made with normal thickness: 1.6 mm. So it is not thick enough. So we need to add some fat.

In this case, I decided to take a small piece of aluminium sheet metal, which happened to be 0.75 thick. I cut it to size and glued it to the FR4 substrate with double sided tape (the kind used for glueing carpets to the floor). Now the thickness is OK. Take care not to insert the GBexx upside down though, since it will short out all pins of the connector. It happened to me a few times and the GBC is still alive, but I do not recommend doing this on purpose.

If, for some reason, the PCB is slightly too narrow, or needs an offset to the left or right (only possible off course when it IS too narrow), then you can glue the riser material with a negative offset as well, to make the card fit snugly in the cart-bay of the GBC.

If you have several materials in your junkbox (or none at all), try to find a thickener that is not conductive. It does have some advantages over a piece of metal.


The proof of the pudding

The picture shows more that what I can type in a 1000 words. The contacts of the GBC connector leave scratch marks in the (soft) tin coating of the contact fingers of the PCB. As you can see, the scratch marks are quite well aligned on the edge connector. Our modifications seem to have worked out.


Houston: we don't have a problem!

The new GBexx cart now has correct thickness and correct width, so it can be inserted. See what's going to happen. And you already saw it: the LED is on! So the power and ground pins line up in a decent way with the connector.

A small test with the DMM shows that the pins closest to the +5 Volt fingers do not have +5 Volts on them, so there is no overlap of contacts (i.e. no shorts). Same on the other side (GND contacts).

I only made pictures in the old layout, with just the LED added. This evening I rechecked the circuit after I added the capacitors and moved the LED. It still works. As was expected.


GBexx: the hole

I neede to cut a hole in the back of my GBC in order to be bale to see how good or bad the contacts mated with the edge card fingers. If you look carefully through the hole yoiu can see that the black 'thingies' are right between the tinned contact fingers.
That's good, since these black thingies are the retainers for the contact springs in the GBC connector.

That's it! The electric part is working. Still, when you power up and watch the screen, pay attention to the black bar underneath the word 'Gameboy'. It should be a perfectly black bar. But if you insert the GBexx, the bar takes on all kinds of fill styles. It took me some time to find out why this was.

But the answer is easy: there is no memory attached to the system and when the GBC tries to load from non existent storage it will take in random data.
You can check this, by putting a piece of paper over the databus contacts: the bar will be perfectly solid black again. Also when you hold your finger on the 34 pin connector such that all pins are covered, tha bar is black and solid again.


Page created 29 November 2006,