When restoring video games or pinball machines some of the work you do will be to repair problems, others are preventative maintenance. Either way you almost always start with the power supply.
A weak or malfunctioning power supply will cause all kinds of problems, especially intermittent ones. In servicing a dead Tekken 4 I noticed it had one of the large power supplies mounted to the middle of the rear of the cabinet. These fail quite often and I’m honestly not sure if an equivalent of the same size and form factor is still available. Its not a big deal since I just replace it with a standard “9 pin” power supply.
I took out the original unit to take it apart to remove the internals and modify it to work with my new unit. This keeps the functionality and location of the power switch as well as to not leave a gaping hole in the back of the cabinet.
With the original power supply modified I can re-install it back into the cabinet. All I did was modify the wiring to match a plug I installed in the cabinet. This will send 110v to the “9 pin” power supply, the monitor and the marquee light fixture which is tapped off the input side of the isolation transformer.
Before connecting the power supply connector to the game harness I wanted to check the voltage. I make the initial setting 5.1v, this is a safe place to power up the game and make adjustments from there. When the game powered up it worked ok for a few minutes then started to lock up. I rechecked the output from the power supply and observed it was still 5.1v. Also when attempting to play a game it would freeze. Tekken 4 uses Sony’s Magic Gate system which is essentially a PlayStation. The game would freeze when attempting to spin the DVD player since it was starving for power.
Unfortunately I didn’t take a picture when initially measuring voltage at the Jamma harness but it was only 4.38v which is way too low. I turned the voltage up on the power supply +5v adjustment knob and brought it to about 5.12v.
In the above pics I’m taking voltage readings from the Jamma connector then from the cable that sends power to the main game board.
After adjusting the power supply voltage so the game was getting at least 5.1v, I powered it down and back up again to verify game operation, I rechecked the voltage at the power supply and saw it was a whopping 5.58v! As we can see there is considerable voltage drop between the power supply and the game boards. Connectors are always a weak point. They should be a starting point when diagnosing problems. Dirty and corroded connectors will always have some voltage drop.
When looking for whats causing voltage drop a simple ohm reading across wires and connectors isn’t enough. As you can see in the picture below I’m only observing .4 ohms which is deceiving.
If you just rely on taking ohm readings you may not pinpoint the exact place the voltage drop is occurring. So then what do you do? You do a voltage drop test. This is extremely simple. You place your meter probes in parallel with the wire or connector you want to test.
Above you can see I have the red lead on the red +5v connector and the black lead is on the red +5v line from the Jamma board to the main game logic board. Here we can see there is almost a quarter of a volt drop. That’s pretty significant. As you can probably guess there’s also considerable voltage drop on the ground side too. See below.
When you are doing a voltage drop test you are actually reading the voltage potential that isn’t getting through the connector. This is useful for locating points of high resistance that may not be so obvious when taking ohm readings. I should point out this is a dynamic test. You are observing readings when the components are doing the work. An ohm test is just a static test. You are taking a reading when the components aren’t doing anything. This is why I always say a part may test good but fail under a load.
This board had some heat damage on the ground side. There is a lot of current flowing through the connector which creates heat. See below.
The above photos are an example of points of high resistance. Always check your connectors!
The voltage drop readings were after the above repairs on the heat damaged Jamma connector. I found this when the game wouldn’t power up at all until I wiggled the Jamma harness. Like I stated before, some things will be to repair obvious problems, others will be for preventive maintenance.
Sometimes there isn’t much you can do to fix places of high resistance. Cleaning dirty and corroded connectors will help but it may not be enough. You may just have to adjust voltage to compensate. Don’t worry, places of high resistance don’t suddenly become low resistance, if anything resistance will only increase with heat and age.
Hopefully you found this article helpful and informative. Let me know if there’s any other topics you would like me to cover or problems that have you stumped in the comments section.