People have been replacing failing linear power supplies with switchers for years, at least as far back as the RGVAC days, so this is nothing new. People would also do this to the 6809-based Williams hardware (Stargate, Robotron, etc.) with success but would complain about occasional loss/corruption of CMOS memory (settings and bookkeeping information). Someone eventually figured out that the reason was because of how the old linear supply worked alongside the board's CMOS write protection circuit. The linear had a larger capacitor on its +5v line that was there for more than just regulation - it kept the +5v line up longer than the other voltages. The board's CMOS protection circuit monitored the +12v line, and when it started to go down, it would kick in the CMOS write protection, before the +5v got low enough for the CPU to go nuts.

But when you moved to a switcher, all of the voltages came down at about the same time. This left no time for the CMOS protection circuit to work, and you would get occasional stomping of your CMOS memory. People suggested that adding a large cap to the +5v line should allow the +12v line to drop before the +5v line, allowing the CMOS protection circuit to kick in.

Back in '09 I figured that since a voltage detection circuit was already possible, I wondered if there were any simplified implementations of them floating around. Enter Microprocessor Supervisor circuits like the Maxim MAX820 and MAX792 circuits. Those were a little heavyweight and could actually also implement the CMOS protection circuit that was already on the board, but did not want to have to do any mods to the board. I wanted something that could just drop in.

I never went any further with it because I was as confident in my abilities back then and did not even want to think about PCB fabrication. Recently, when I got back into the Crowbar

Years ago, back when RGVAC was still fresh in people's memories I was reading about the woes caused by AR-IIs having their regulators go bad and doing really bad things like putting ~7 volts on the 5v line, killing most of what was connected to it. I wondered if it was possible to have a voltage detector that would do something, like open a relay, if the voltage got too high.  Sounded simple enough but wondered why no one ever did it.

Looking further into it, I learned about crowbar circuits which are designed to short out a circuit (generally resulting in a blown fuse, tripped breaker, or power supply shutdown) and discovered that companies made voltage sensing chips that could be used to implement a crowbar circuit.  The Motorola (later On Semiconductor) MC3423 was of particular interest to me.

Back in '09 while between jobs, I decided to try to implement something.  It ended up being a small tangle of wires and parts on perf board, but it worked and did what it was supposed to do - I turned up the voltage and it blew the fuse.  I started a new job soon thereafter and never thought about it again.

However, I recently came across a post in one of the arcade repair related groups I frequent which was discussing “bulletproofing” an AR-II.  It turned out that not much had changed in the past 10 years or so - people would mention “bulletproofing,” and then only talk about doing the Sense Mod with the occasional parts replacement.  I decided to dust off the old idea and take a stab at designing my first PCB.  As I was working with the schematic I realized that it could have applications beyond an AR-II, so I generalized it a little more and made it easy to connect to any power supply.

The result is the JRTwine Crowbar Circuit device. And while it does not technically bulletproof an AR-II, it can help prevent things from going too badly if the voltage regulator fails. It also has other applications - anywhere you want to help protect a low voltage circuit from being damaged by overvoltage conditions.

I am currently testing out different applications with different people right now. The goal is to sell the device as either a bare PCB board, or as a partial kit containing the following components:

• MC3423 IC
• 8-Pin Socket
• 2N6505 SCR
• Fuse holder or fuse clips (depending on supply, maybe optional?)

(Customers will have to provide their own resistors and LED.)

I figure a target of around $10 works for the kit, and maybe $3-4 for just the PCB? Dunno if I want to be selling assembled units.