The early Space Invaders boardsets, including the Midway L shaped boards only output to a black and white monitor. The monitor that came with my cabinet is not working. I have not really done any troubleshooting for it, however the chassis was very dirty, and I decided my efforts would be better spent looking for a modern (ish) alternative.
The first thing I tried was modifying the output to connect to a CRT TV via composite. This will give a decent black and white image. This is achieved by connecting pin 18 from the parts side motherboard edge connector to composite cable, along with a corresponding ground. Unfortunately, out of the six 48cm TVs I tried, the image was too wide on the horizontal width. As the space Invaders monitor is mounted vertically, this was the vertical view of the screen. None of the the TV chassis had horizontal width adjustment, and none of the TVs I tried had a “close enough” picture. As 48CM CRT TVs are becoming harder to find, the odds of finding one with horizontal width control was low. I should also note I didn’t want to muck around with the horizontal output circuits of the TV chassis and start changing components.
Next idea was to match a universal chassis to a 48CM tube. Many of the universal chassis have decent screen adjustment controls, including horizontal width.
Now using a universal chassis outputs RGB. Some people have reported that if you tie the output signal before it is combined with sync to R, G & B lines you can get a B+W picture. However while investigating this, I came across this Midway L Shape Space Invaders JAMMA adapter. This adapter converts the B+W signal out to RGB colour. It has several modes to simulate the cellophane strips as well, including:
- Black and white original pass through
- Original colour overlay with red and green bands
- Deluxe colour overlay with red, yellow and green bands
- Taito Colour RGB output
- Home console version
Finally the adapter has a Power on Reset (POR) circuit built into it. This was enough to encourage me to buy an adapter from PCBJunkie on eBay.
So before connecting anything, I needed to convert the wiring to JAMMA and I also converted to using a switching power supply. So with the aid the the pin out and manually tracing the old power supply wires, I have converted to a JAMMA connector, switching power supply and RGB output with the adapter.
The universal chassis came from https://www.jomac.net.au/ and was matched to a TV tube I had available. Unfortunately after getting a good picture, the deflection coils smoked. This can happen due to the rubber yoke stops corroding over the years, and causing the yoke coils to short out. Sigh…. Luckily a good friend had a tube I could use that was compatible with the chassis so all was not lost.
After setting up the new tube, there was quite a bit of discoloration in the picture. Degaussing helped somewhat, but there were large patches which were stubborn to remove.
The cause of some of this was the unshielded speaker. I made my own shield which consisted of a steel post cap, the perfect size to cover the speaker magnet. For added support I held in place with cable ties.
The shielding actually worked well. Now there were a less stubborn dull areas. Shout out to Mark, who also provided the tube and advised how to use magnets to assist with the purity issues. After placing magnets in a few places, the colour was pretty good, considering this will only be used for Space Invaders.
As you can see the image is reversed – which is needed for the upright cabinet, as the screen is viewed via a mirror.
Next major task besides putting more cabinet parts back together is to fix the Space Invaders PCB. There are several sounds missing, so that will require some investigation and fixing.