Repairing a reconditioned Asteroids motherboard
By, William Stephens
(clicking any picture in this article will bring up an enlarged version)

The procedures outlined in this article are very dangerous for the clumsy and inept among us. If you think this applies to you then read but don't do, you might kill yourself.

This one is actually a continuation of the Electrohome G05 article . To recap, we got a good looking Asteroids for a customer upstate and the monitor would not work. Well.. neither did the motherboard or the power regulator. I got the monitor to fix because vector XY monitors are easy and I have a lot of the parts necessary for quick repair. The only problem with XY's is having a test rig which would be a couple of strange parts... let's see... An isolated 24 volt center tapped supply, a vector generator of correct parameters. You could probably use a DC power supply and some potentiometers to move the beam around, anyway it's a messy proposition. One thing that would make this all real easy is to actually use the real game and power system... the vidoegame itself!

My shop tech fortunately had ordered a re-conditioned working Asteroids motherboard from Quarter Arcade . They have a great reputation and our experience with them has been flawless, at least up until now. Using this motherboard and other reconditioned modules made the monitor repair quick and easy. During the testing however, we noticed that the motherboard was not quite reconditioned enough. The thrust sound was missing and the other sounds were there but a bit weak and crackley.

I moved the entire game up to the bench and put the mother board on the counter-top still connected to the game and decided to find this sound problem and fix it myself even though we paid a "Buy it now" on eBay of $175.00 to avoid problems just like this. Our problem is we have a delivery date to fulfill and we can't wait around for back shipping and re-shipping, phone calls, emaill and the like right now. I really don't like working on motherboards because they are fiddley and touchy, it seems once you dig in more problems pop-up and before you know it, your worse off that if you never looked at it in the first place. This problem however should be easy to fix in theory at least.

You may notice a yellow paper pad on the repair bench in the photo, this is what's on that paper:

I tend to diagram out circuits I need to understand and it's nice to have a place to make quick calculations and notes or just scribble if I get bored.

The circuit works by using U10, R10, P5 and N3 as a noise generator. This circuit makes noise all the time. The 4016B switch (R12) controls when the rest of the audio circuit hears this noise. The thrust function of the CPU determines when to switch R12 on and off. The thrust function of the CPU obeys the pressing of the Thrust button on the control panel, so when you press the thrust button, R12 passes the noise.

From there, it passes through 2 OP Amps to "condition the noise", a bit of waveshaping: These two OP Amps are on a chip containing 4 OP Amps, the other 2 amps sum and invert all the sounds just before they are injected into the main audio amplifier. This leads me to believe that this OP amp chip, an LM324 has one or two bad amps and the others are slightly damaged causing the crummy sound reproduction.

I powered up the game and hooked up a logic probe and probed the circuits starting with the noise generator parts (U10, R10, P5 and N3). Sure enough I saw changing logic levels so I turned on the Oscilloscope and looked for "nosie" at the output of this part of the circuit, and sure enough, I saw noise. I moved further through the circuit to the 4016B switch and probing with a logic "1" to the gate of the switch, I observed that the noise now appeared at the input to these OP amps in P12. This is where the noise trail ends because no noise is coming out the output... thus, no thrust sound. It looks like P12 is bad, now, to replace it.

I don't have any LM324's but NTE makes a replacement (NTE987) so I hopped out to the electronics store, picked one up, and did the installation. When I replace IC's on old circuit boards I always socket them. It went bad once, it can go bad again sometime in the future and if you solder and unsolder things too much on these old boards, they come to pieces.

The 6 pictures above roughly show how I do this. The first three pictures show that I clip the leads off the chip leaving the leads still soldered into the board. The practicality of trying to unsolder a 14 legged device from a double sided board is virtually null. Cutting the pins now turns the situation into 14 separate solder connections making things much easier. Now as in the 4th picture, I can heat the pin up from the solder side and with some needle nose pliers pull the lead stub out of the hole from the component side. This makes short work of clearing the holes of debris. Next I solder suck the holes to completely clear them. Now I have a fresh clean hole pattern on the board to make my installation.

For sockets I use some .100" headers, I clip off the amount of pins I need, for this installation that's 2- 7pin strips. I gently push them into the holes, check and correct any lifted pads and solder them in place. After I clip off the excess leads I use an ohmmeter and check every pin/trace connected for proper continuity. Now I carefully press the new chip into the socket and all should be done.

I hooked the board back up to the game, turned it on, pressed "player 1" and the game started up. I pressed the thrust button and sure enough the thrust noise happened as the ship thrusted and all the other sounds sounded brighter and louder... like a real correctly working video game. This one is now fixed. As far as Quarter Arcade goes... whoops!!! they really do sell quality stuff, this one just seemed to slip through the cracks, it happens.

--William Stephens

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