Tech Forum

No! This is wrong!! The isolation transformer powering the monitor in an arcade game is there because the older monitors use a "hot chassis" design meaning that they contain no internal isolation from the AC line. The ground that the signal input is referenced to is not ground at all without an isolation transformer! Connecting a monitor to the logic board in the game without isolation is going to result in a flash and bang that usually causes serious destruction to both the monitor and the logic board. It is not there to protect the player from electric shock as the published reply states. All exposed metal parts must be bonded to the ground wire in the power cord for that.

Arcade monitor isolation transformers are readily available and inexpensive. I hope this answer reaches the person who originally asked before they take the advice that was printed. Note that some newer monitors do have built in isolation but unless this is known with certainty *do not* attempt to bypass the transformer.

I'm worried that the monitor discharging advice given here could potentially kill someone. While the alligator clip/long screwdriver method is sound, touching the corners of the CRT is not. To discharge the monitor the screwdriver has to be inserted under the anode cap. Just touching the CRT or frame will not discharge the monitor, and could result in 20,000-30,000V being discharged through the tech. CRTs contain very high voltages. Please make sure you know what you are doing before attempting to service one.

I read the published answer to this in N&V, and the arcade preservationist in me cringed a bit. If the cabinet had an isolation transformer, in the bottom, you definitely want to run it with one. The transformer is actually there for the monitor, not the game PCB. The older CRT monitors used in arcade games don't have an internal isolation transformer on the chassis, unlike newer CRTs with an internal switching power supply that do. Running without an isolation transformer will most likely result in a dead monitor. Most arcade monitors run without an isolation transformer will end up blowing their Horizontal Output Transistor (HOT).

The transformer, if it really is a 1:1 isolation transformer, is probably for your protection not the user’s protection. Second it may also be to insure that it doesn’t matter how the plug (I’m assuming it is non-polarized) is plugged into the wall.

On the other hand if this is an older unit with a picture tube the transformer might be the horizontal flyback transformer that generates anything from 10,000 to 30,000 volts to be placed onto the picture tube. However, a flyback does not look like an isolation transformer, so I have to assume it is not one of those and that you know the difference.

Thus we’re back to my first two possibilities, it is for service safety and using a non-polarized plug - again service safety. Just be sure to unplug the power cord, then discharge the picture tube, wait about 15 - 20 seconds and discharge the picture tube again, repeat until no spark is generated. The old standard way to do this is to use an alligator jumper from ground to a big/long screwdriver with an insulated handle & bring it close to some part of the picture tube, then do it again to a different part of the tube. The picture tube has four corners so I would go round & round till no more spark jumped.

The isolation transformer is for protecting the MAIN METAL PARTS of the arcade machine from electric shock - specifically the coin-handling mechanism. If you’re not going to use the coin machinery (i.e., replace it with a switch or similar), then you could probably get away with not using the line isolation transformer. HOWEVER, consider integrating EMI/RFI/surge protection on the AC power input to help the machine live a little bit longer.