Modern PSUs can push -5V too Low: Check your 4116 Arcade Boards under Load

[Dionea]

I recently repaired a Missile Command board, which uses -5V for its 4116 RAMs. That rail is critical. If -5V is missing or too far out of spec, 4116s can die quickly.

While troubleshooting, I measured around -7V at the RAM chips. I first assumed the board had a -5V fault, but after repair the actual issues turned out to be two bad logic chips and a faulty ROM. The board worked again properly, but... the -5V rail was still sitting around -7V.

My setup is an Axunworks PSU (using a Mean Well unit) and a HAS 5.0 supergun. When not connected to the board, all values were fine. It seems some modern PSUs do not regulate the -5V rail properly unless there is enough load on it. Most folks check +5V and +12V, but often forget to check -5V separately, unless troubleshooting (so did I...).

I solved it by making a small helper JAMMA adapter with 3 x 100 ohm, 3W resistors as switchable dummy loads on the -5V rail. With the extra load, -5V settles around -5.2V to -5.3V. Each resistor draws about 50 mA, so all three together add about 150 mA load, while each resistor only dissipates around 0.25W, safely below its 3W rating. (I can post the Gerber, if anyone interested)

So for boards like Missile Command, Space Invaders, Tutankham, or anything using 4116 & other -5V mandatory RAMs: check your -5V rail under load at the board, not just +5V and +12V. Otherwise you may be slowly cooking your RAMs.

 

[Ape Cosplay]

Something I do in all my cabs is installing LCD voltmeters after calibrating them (there's a small pot for that at the back) but they're usually pretty damn close out of the box. See pics below (red is +5V, green +12V, blue -5V). You could install those on your setup.

 

[Dionea]

Something I do in all my cabs is installing LCD voltmeters after calibrating them (there's a small pot for that at the back) but they're usually pretty damn close out of the box. See pics below (red is +5V, green +12V, blue -5V). You could install those on your setup.

Nice set-up!

Your PSU probably has a dedicated linear regulator so all the individual rails can be more directly controlled.

Modern Mean Well-style multi-output switchers usually regulate +5V as the main rail (with that being also the only one that can be adjusted with a pot), while -5V is a small auxiliary/cross-regulated output. Because of that, the -5V rail can drift too negative under light load, so adding a resistor load helps pull it back toward -5V.

See a picture of my helper board with a voltage display for -5V. The 3 resistors can be individually switched.

 

[Ape Cosplay]

Only +5V is adjustable. You're right some PSUs need as minimum load to operate properly, even on the +5V rail for some of them.

People had issues with Recalbox in Astro Cities: