Overhauling the Sega Astro City – Part 8, building a control box

Today I’m going to talk about the control box I built as part of the Astro City overhaul.

The purpose of this was to achieve the following:

  • Switch between stereo and mono output, with the mono input spliced internally to be two-channel mono audio, all via RCA sockets
  • House a -5v output produced by a Negatron with a master switch to enable/disable the function
  • Distribute +12v to a fan with a master switch to turn on/off the fan
  • Distribute +12v to power the audio amplifier
  • Route a GND connection to the JAMMA harness, which ensures GND runs across all connected GND loops on the harness regardless of which power connector is hooked up to the power supply (e.g. ensures GND on the JAMMA harness even if I’m powering a NAOMI board direct from the PSU)

The other aim was for the connections to be modular and terminated with molex connections. This way I could build the control box on the bench before fitting it into the Astro City.

The first step once I gathered all the components was to mark out the holes to drill for the RCA sockets and switches:

Once that was done, I drilled out all the holes and fitted all the sockets and switches.

Next up, I went about wiring the audio components together using a 3PDT (3 pole, double throw) switch to go between the two sources, and brought in a master voltage cable to bring GND, +5v and +12v from one of the extra connections on the Sun PSU that weren’t used for supplying the main voltage to the PCB or harness. From there, using hot glue and some cut-down wall plugs, I created some points to screw in some project board that acted as a voltage distributor and hooked up +12v to the two switches (one for the fan, one for the Negatron), and a second piece of project board to house the Negatron. I also wired in an extra GND point to be distributed to the JAMMA harness with the +5v from the Negatron, and added those along with a GND and the +12v for the fan to a standard 4-pin molex connection as used on disk drives in a PC.

In addition, I also wired a +12v and GND to a plug to fit the power socket for the audio amplifier.

The end result came out like this:

Inside view of the box:

Inside view of the top half:

Inside view of the base:

With this completed, I can start gutting the existing cab and modifying the JAMMA harness to suit the new setup and the various molex connections as part of the project.

As noted before, posts on the refurb are being done ad-hoc, so to keep track of the whole project, just use the Sega Astro City Overhaul tag, as the whole series will be added to it over time.

Adding a switch to the 60hz colour mod

Last year I wrote a guide to get colour video out of your Mega Drive via s-video, composite and RF cables, but I’ve noticed that since using the RGB to component transcoder, the colour was flickering on my CRT I play my Mega Drive on. I tested it on our flat panel TV, and sure enough there’s some extra noise in the picture when running the SMD with the external oscillator. So, I figured I’d make a switch to go between the original feed from the Mega Drive and the oscillator, which gives the best flexibility if ever I use the SMD via s-video or composite if I’m not hooking it up via RGB/component.

The theory is simple – grab a SPDT (single pole, double-throw) switch, wire the middle (output) pin to where we used to put the oscillator’s output, wire the oscillator to one of the remaining sides and grab the existing feed off the mainboard and wire that to the other. To help out, here’s what I did:

Equipment needed:

  • Remember to read this one in conjunction with the previous mod and have the oscillator ready to go!
  • Two wires of a length suitable for mounting the switch (remember you’ll only need two as the existing output wire off your oscillator will be fine for adding to the switch, unless you need to lengthen it, then you should replace it)
  • SPDT (single pole, double throw) switch
  • Solder, soldering iron, screw driver and drill/drill bits for mounting the switch


You mod your machine at your own risk. Myself nor anyone else is responsible for YOU modding YOUR Mega Drive/Genesis. If your machine doesn’t work as a result of this, don’t blame me – you do this mod at your own risk.

Step one:

Obviously, discharge and disassemble your Mega Drive and remove the mainboard and have it facing up. Be careful you don’t damage the wires hooking the oscillator you fitted from the previous tutorial.

Step two:

As per the previous mod, here’s the input signal being fed from the external oscillator into the CXA1145 IC. We’ll need to remove this to replace it with the switch’s output so we can select between the original and oscillated frequencies.

Step three:

Here we have a SPDT (single pole, double throw) switch wired up and ready to go. The middle (yellow) wire goes to the CXA1145 IC’s input, which is where we used to send the oscillator if you followed the previous mod; the green wire is from the external oscillator, and; the brown wire will take the original feed from the Mega Drive’s mainboard for RGB compatibility.

Step four:

This shows the middle (yellow) wire from the switch wired to where we used to put the oscillator. It functions as the output from the switch’s two sources.

Step five:

I’ve highlighted a point on the underside of the mainboard where you can easily solder a wire to connect to carry the Mega Drive’s original frequency to give RGB compatibility.

Step six:

To ensure I only wire the correct signal, I use some electrical tape to isolate the solder point.

Step seven:

Tin the tip, heat some solder and attach one of the wires from your switch’s input poles to the point. I’ve used the brown wire.

Step eight:

If you haven’t done so already, grab the output from the oscillator and wire it to the other side of the switch. If the wire isn’t long enough to reach to where the switch will be mounted, remove it and use another wire – it’s best to avoid joining wires together part-way along the oscillator’s input to ensure the signal remains strong and steady.

Step nine:

And that’s all there is to it – use the output from the oscillator (green) and attach that to the other side of the switch, secure your connections with electrical tape, and get ready to assemble. For those interested, the black wire at the top of the mainboard in this pic is a hardwired composite socket I built so that I could use standard AV cables with my Mega Drive.

Step ten:

I wanted to keep everything on the back of the unit, but placed in a way that would allow for the Power Base Converter to still be hooked up, so I installed the switch next to the 60/60hz and JP/Eng language switches which are next to the AV socket, where the Ext. socket would be if I had an earlier-model Mega Drive 1.

All in – would be great if I’d lined up those holes a little nicer though! Once I had the switch installed, I found I was no longer getting any issues with my Mega Drive when running it in RGB via the transcoder, so if you intend to run the machine in RGB at some stage, fit the switch!

Overhauling the Sega Astro City – Part 2, audio amplification and fans

I’ve already got the ball rolling in part 1 of this series, so let’s move on to part 2 – audio amplification and fans.

In line with the previous post, I want to add a way to have a stereo amp in the cabinet to take care of line-level sources and enhanced stereo sound (e.g. Naomi, CPS-II, Sega Model 3, etc). So, in the spirit of making the cab as universal as possible, I need to look at adding in a small amp that can be switched on or off as required, and be able to direct either amplified sound off the JAMMA cable or amplified stereo output as required. So, this is what I’ve come up with:

Audio wiring

It’s not all the difficult – I’ll need to rewire the 4-pin connector that goes from the speakers at the top of the cab so that they’re separated into stereo channels, then create a little switch to roll between a split dual-mono amplified output (from the JAMMA harness) or stereo output from the stereo amp, which in turn is powered off a 12v source (with switch to control when it’s on) and grabs its input directly from the PCB (Naomi, Model 2, etc).

This then means the audio can be setup in the cab when inserting a new board. All I’ll need are a pair of stereo RCA cables to run form the CPS-II or Naomi boards (and make a stereo RCA adapter for the Model 2/Model 3 boards) and run them off a pair of 12v mono amps I already have around the place, and I should be good to go.

The other addition I’d like to add is a stereo controller I can mount on the cab to adjust the output before it goes to the speakers as required – there’s currently a cheap knob the previous op added to give this kind of functionality, but it’s not a true stereo actuator, and doesn’t do much except distort the audio 😛

Of course, the great thing with this setup is that it won’t cost much at all to add this kind of functionality. To make it clean though, I’ll plonk the lot into a small project box and mount everything inside it. This way, I can use the same project box to run a 12v switch to selectively power the 12v fan I got with my Model 3 kit before. And to make sure I don’t mangle my hand again, I’ll also grab a pair of 120mm fan guards to avoid any more stupidity 🙂

Thus ends part 2 – part 3 will deal with the lovely scope creep and deciding what to do with my monitor 😀

To keep track of the whole project, just use the Sega Astro City Overhaul tag – the whole series will be added to it over time.