Sunday, November 8, 2015

CoCo Conversion for SMS Joypad

I've never been a huge fan of gaming with a joypad, but many people are. With that said, I do agree that a digital gaming input can certainly be more effective than the CoCo's analog joysticks for any number of games. My preference for digital joysticks on the CoCo was strong enough to lead me to build a CoCo joystick from arcade components for use when showing Fahrfall to the general public. I even published the schematic I used for building the internals of that joystick in an earlier post on this blog.

Using an arcade stick for playing (most) CoCo games is great! But an arcade stick can be a bit large and unwieldy, and it certainly does not travel well. Having a joypad is a nice alternative, but making one from scratch is a lot to manage. Using a donor joypad is an attractive option, but which one? The NES controller fits OK, but it has more buttons than the CoCo can use. It turns-out that the SEGA Master System controller is also a good fit, and it has the same number of buttons as are supported by the CoCo3.

Part of me hates destroying one vintage device to enhance another. However, there are a lot of SMS controllers out in the wild. So, I made the call to sacrifice an SMS controller in order to make the world better for CoCo users! If you think this was a bad choice, well, then I hope you'll forgive me and move on... :-)

Peeking Inside

As a first step, we must open the SMS controller. Inside (at least for the ones I have seen), there are a couple of small PCBs with conductive pads for all the buttons.

The wires from the joystick cable connect to the edge of the PCB holding each button, and a small wire connects the two PCBs for a shared ground signal. This is about as simple as things get..

Cut and Splice

Removing the wires on the direction pad made room for installing the resitors needed for the circuit that turns the button inputs into the analog signals needed by the CoCo. The existing PCB had most of the connections needed, but one problem is that the original SMS controller circuit had each button designed to pull a signal to ground when pressed. As a result, there was one trace that connected to all four direction buttons.
Direction PCB w/ central ground connection
The CoCo circuit needs two of the buttons to connect to Ground, but it needs the other two buttons to connect to +5V. As a results, the PCB needed to be modified to accommodate that connection. A hobby knife was useful here both for breaking the connections and for scraping away enough solder mask to allow for new connections to be made.
Direction PCB w/ modification
Once the direction PCB was modified, it was a trivial (if somewhat tedious) operation to attach the eight resistors need to implement the switched voltage divider circuit used to feed the CoCo's analog joystick inputs.

"Big" resistors connected
One axis completed

Both axes completed
Twist and Tuck

With the resistors added to the direction PCB, all that is left are the finishing touches. The existing cable was retained, but the connector was removed and replaced with the CoCo's 6-pin DIN joystick connector. The wires were connected to the existing PCB holes for the two buttons and ground, and directly to the resistor leads for power and the direction axes. A little creative twisting and some nip and tuck made everything fit neatly back into the original SMS housing.

With everything sealed-up, I used a standard label maker to indicate what I had done. Maybe you can dress yours up more nicely?

That is pretty much it! Please refer to the joystick schematic from my earlier posting for more information about the switched voltage divider circuit. Thanks for reading, and happy CoCo-ing!

Saturday, November 7, 2015

Vectrex CRT Replacement

NOTE: Handling a CRT of any size is a potentially dangerous proposition. Not only are CRTs driven by very high voltages and capable of storing a high voltage charge for long periods of time (even after being turned-off and/or disconnected), but they are also glass tubes containing a powerful vacuum and therefore pose the risk of serious injury associated with implosion. Any attempt to replicate the actions described below is done completely at your own riskTHIS MEANS YOU!

Twenty years or so ago I was fairly deep into arcade collecting, including the "elite" world of collecting vector arcade games. Vector games are a breed apart, using a display technology that is in many ways more closely related to oscilloscopes than to televisions. When I first started arcade collecting, I had never heard of the Vectrex. But it wasn't long before I heard talk from the other vector arcade collectors regarding the Vectrex. Soon I was not only aware of the Vectrex but I was desperate to have one...

In the days before eBay, it was often very difficult (or just extremely lucky) to find any sort of rare item. This was even more true for an item with any age on it. While this was a long time ago, the Vectrex still was more than a dozen years old! Plus, those that I knew who had a Vectrex just weren't interested in parting with it. Finally I came across someone who had two non-working Vectrex machines, which I acquired in hopes of making at least one working machine.

Some of my friends may have seen that back in December of 2013 (after a long hiatus) I succeeded at that goal by swapping the logic boards between the two broken Vectrex machines. Around that time, I also determined that the CRT in the (still) non-working machine had a broken neck. The resulting lack of vacuum in the CRT rendered it completely useless. I had done some research into possible donor CRTs that might be available, but I mostly put the project aside until one day not too long ago I stumbled upon an interesting video on YouTube...


I had long ago heard that no one was making CRTs any longer, and it had not occurred to me to search for a NOS supplier. In my excitement, I ordered a tube and waited for it to arrive. In less than a week I was in possession of a brand new (or at least NOS) CRT that was a perfect match for my Vectrex. Or was it?

Original CRT w/ Mounting Ears

Replacement CRT -- No Ears!

(Not) All Ears

A closer look at the video might have prevented my dilemma. You see, the original CRT is constructed in such a way that the steel band around the edge (which is supposed to limit the possible effects of an implosion) holds in place four mounting lugs for securing the CRT in place. The new CRT has the anti-implosion band but it lacks the mounting lugs. In retrospect, had I realized that this would be the situation then I might not have ordered the replacement CRT at all. In this case, I am glad my excitement was enough to put me in this predicament. :-)

Strap Up!

Inspriation struck when I thought of the strapping material that plumber's use for hanging pipes and a number of other utilitarian purposes. Although I considered using the traditional metal variety, the modern plastic version seemed like it would be easier to fit to the purpose. Plus, I thought that it might be a bit less likely to present an electric shock hazard to anyone that might be poking around inside the Vectrex (e.g. me)...

The Vectrex in question still has the logic board problems that led me to the original logic board swap. So, in order to test the CRT replacement I connected a working logic board to the newly serviced monitor section of the Vectrex in question and I was able to play a rousing game of Minestorm. Overall, the machine is still in the non-working category, but at least the vector monitor is saved!

So, the point is that if you have a Vectrex and the CRT is busted or just worn-out, there is no need to despair. At least for now there are still workable NOS replacements that can save the day. With that said, do make sure that you handle the CRT and other high voltage electronics properly. Otherwise, the dead Vectrex will be the least of your family's worries!