Addendum to Asteroids -> Asteroids Deluxe conversion

Ray Ghanbari
ray@mayo.edu
4/9/95

NOTE: you are responsible for what you do, not the author or anyone else. Do not attempt the following conversion unless you know what you're doing and are willing to take responsibility for the results. 'Nuff said.

Once you rig up your adapter and start playing, you may notice that the sound is heavily distorted. Fret not, this does not mean that the game is melting down! Asteroids (and other Atari vector games) use an auxillarly board to provide regulated power, and to provide amplification for sound. The Audio/Regulator board in Asteroids Deluxe was the Audio/Regulator I board (A/R I), while Asteroids had a plain old Audio/Regulator board (A/R)

If you look at the schematics, the only difference seems to be that the A/R board had additional circuitry to support an Audio Disable signal. Since this pin is always grounded in the traditional Asteroids harness, this part of the circuit is never used. I guess the engineers at Atari decided Audio Disable was no longer necessary, so they removed it from the A/R I board, and subsequently, the Audio/Regulator II (A/R II) board that was used in Battlezone and Red Baron (among others)

(As an aside, does anybody know if Lunar Lander used the Audio Disable signal?)

Unfortunately, when the Audio Disable circuitry was replaced, the resistor network between the two audio inputs and the audio amplifiers changed, changing the signal amplitude seen by the audio amps. As a result, using the A/R board with Asteroids Deluxe results in the amplified signal being clipped, leading to the heavy distortion that you hear.

There are two ways to fix the problem. The first (and easiest) is to replace your A/R board with a A/R I board, or preferably, an A/R II board. The reason to opt for the A/R II board is that the A/R II board added support for additional voltage levels, which you will need if you want to play Battlezone and Red Baron in the same cabinet. Apparently, there are different versions of the A/R II board available, which are configured to supply different voltages. Again, if you have a choice, try to find one that provides -5V (7905 voltage regulator), +12V (7812 voltage regulator), and the standard (?) +22V and -22V (unregulated). These are the additional voltages that are needed by Battlezone. These voltages are segregated on a seperate connector on the A/R II board, and hence should not require any changes to you wiring harness (the rest of the board seems to be pin compatible with the A/R board, but you should double ckeck this)

NOTE: Before you risk your electronics, _ALWAYS_ check the line levels on the reegulator board, make sure they match the schematics and your wiring harness. When you finish, repeat the procedure just to be sure.

If you do not have access to a ready supply of A/R II boards, then it is fairly simple to modify the audio amplification portion of the A/R board. You will need the following:

2 1k Ohm 1/4W resistors
2 0.22 uF ceramic disc capacitors (25V)

First, locate C6 and C15. These are 10uF electrolytic capacitors. If you do not have the schematics handy, follow the traces on the board. Audio 1 comes into pin 9 on connector J7, goes through a 10k Ohm resistor, then connects to the "+" side of C6. Audio 2 comes into pin 8, then into a different 10k Ohm resistor, then goes into the "+" side of C15.

Once you locate C6 and C15, replace them with the 0.22uF disc capacitors (orientation doesn't matter with ceramic caps)

Next find transistor Q4 and Q6. These are 2N3904's, and are used in the audio disable circuitry. Again, if you don't have schematics, keep following the traces. One leg of Q4 is connected to the "-" side of C6, and one leg of Q6 is connected to the "-" side of C15. Remove these transistors (note that this will make the audio disable circuitry ineffective)

You now need to add the 1k resistors. Here is what the final circuit should look like (Audio 2 has the same topology as Audio 1):

                            0.22 uF
               10k        + |  |
 Audio 1 ---\/\/\/\/---|----|  |---------[...rest of the circuit...]
(J7 pin 9)             |    |  |
	               |
	               |
	               /
	               \
	               /  1k
	               \
	               /
	               |
	               |
	               |
	             -----
	              ---
	               -

What I did was solder one leg of the 1k resistor to the ground terminal of Q4 (it has a square pad instead of a round pad) and soldered the other end to the lead on the "+" side of the 0.22uF cap. Do this for both audio inputs. For one of the audio inputs, the resistor and cap are close enough that you can connect them directly. For the other, I had to use a length of insulated wire to bridge the components together (double check that nothing is shorting!)

Reconnect the board, and the distortion should be gone...

(Note, as a side effect, the volume level on Asteroids will be lower than the volume level on Asteroids Deluxe, but that's what the volume control is for ;-)