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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Author: NES4Life

Just a regular retro gamer and collector with too many projects!

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