Vector arcade test bench – pt 2

This is a continuation of the project to build a working vector arcade test bench. In part 1 I introduced my 1979 Asteroids arcade machine, covered the Atari powerbrick that would power such a machine, the fact I’d picked up a  dilapidated spare, how I’d replaced some of the major components that were dead or dying, and the tasty paint job I gave it.

If you have little knowledge of the internals of classic Atari arcade machines then you might reasonably think that the next stage is to attach a game PCB to the powerbrick but there’s (un)fortunately more to it then that!

Introducing the Atari Audio Regulator PCB (AR and ARII in this case)IMG_1007.JPG

The AR is essentially a board dedicated to amplifying the game sound (as the name might suggest) but additionally it regulates a number of voltages for the main game PCB. It’s another part of the power system that Atari decided to separate out and which seems, for the most part, to have been a pretty good idea. If a fault developed on one of these relatively cheaper boards than an engineer could simply swap it out and keep the game running without touching the game board that included more expensive components such as ICs. Atari repurposed the AR board throughout their early games and, as the different games required different sets of voltages, made different revisions of it as they went along. There’s 1 or 2 versions of the AR1, about 6 or 7 AR2 variants and a handful of revisions of the AR3. The AR2 is the type that supports most number of classic cabs, being backward compatible with game PCBs that required the AR1, with, from memory, version ‘-02’ of the AR2 being the most compatible (on the left in the picture above) as it’s fully populated with components.

The main issue with these boards is that Atari tried to make them too clever by sticking in a ‘sense’ circuit. This circuit is meant to monitor the voltage level on the game PCB and, if it changes, take action by raising the voltage to the board. In the early days I’m sure it helped make the games more reliable but, over 35 years later, it’s not really doing what was intended. As the edge connector on the game PCB ages its contacts become less perfect and the power transfer therefore diminishes resulting in a lower voltage getting to the board. As the sense circuit attempts to compensate for the lower voltage by increasing the voltage it sends to the game pcb, it exasperates the situation, heating up the edge connector which then decreases the electrical contact. A death-loop ensues and the end result is high voltage on the 5V line (probably 9V+) and a couple of components on the AR board breaking and / or bursting into flame. The solution is to keep the game edge connector properly maintained, fit a set of ‘Guddler-approved’ jump leads between the game PCB test points and the AR’s test points (to bypass the aging edge connector), and / or, for a more permanent solution, perform the ‘sense mod’.

I’ve replaced the burned transistor at J6, patched the broken track and put the sense mod in by joining pin 1 & 2 and 3 & 6 at J7:

J6 shows high-voltage damage and J7 is sense modded

The sense mod is pretty simple in that you just solder two pairs of points together on the AR board so it ends up analysing the voltage it produces instead of the voltage coming back from the game PCB. By reading the AR’s own generated voltage it makes no further adjustments to the output voltage as it reads it as spot on. I decided to go down the sense mod route for my own AR2 and will also be acquiring the ‘Guddler-approved’ jump leads in the near future.

Some excellent links about AR sense mod:

 

 

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Vector arcade test bench – pt 1

I’ve not long been the owner of a classic Atari Asteroids arcade machine and, like all things old, it needs proper maintenance. I’ve taken it upon myself to learn the machine inside out starting with the PCB and how to manage its upkeep.

 

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Asteroids 1979

 

The awesome thing about classic Atari games is that they are amazingly well documented and nearly every part is replaceable if it fails. The challenge comes in finding out what has failed. After a week I lost the ‘thump-thump-thump’ heartbeat that sounds throughout play. I checked the schematics, read a few posts on UKVac and then located the IC that commonly fails. A quick eBay purchase of the (now socketed) IC followed by some rusty soldering resulted in the sound coming back again. A couple of months later though the machine has developed another fault and is constantly resetting (watch-dogging as it’s known in arcade repair land). So what next?

I was fortunate enough to come across a sale of classic arcade parts and now I have everything I need to make a vector test bench.

Stage 1: prep the power supply

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Atari vector power brick

Early Atari vector games required a series of different AC and DC voltages so Atari created a beefy power supply which is now fondly known as a vector power brick. They’re usually pretty rusty, have a number of blown fuses and one dilapidated large capacitor (known as a Big Blue). My brick has had all the rust removed, been primed and sprayed, had all 3 x 3A and 3 x 7A fuses replaced and the Big Blue swapped for a modern replacement. I took the time to replace all the spade connectors, fitted a cherry door switch and replaced the dated power cable. A quick follow along with a YouTube tutorial and I was happy the voltages were correct.

Part 2 should show me testing the AR2 (Audio Regulator) PCB which further controls and maintains the vital voltages to run the main game PCB. Now if only I could find another good YouTube tutorial