TLDR Summary: If you...
- Are not sure how / why the WMS A-C relay works
- Are fighting suspected relay issues but confused because "it seems to be working wtf"
- Are not sure how to replace the relay on a WMS power board
- Are wondering if a faulty relay can be “hacked” into working again for troubleshooting
I brought in a Diner (Sys 11 from 1990) to overhaul for a customer. Among its many issues were several coils not working. I eventually reduced the nonfunctional coils to a stubborn set of four that all seemed to share a connection through the A/C power relay - except, that relay was working! Or was it? Searching here on Pinside revealed several threads and questions about this mysterious relay, but little in troubleshooting analysis beyond “It either works or it doesn’t”. Huh.
In this Diner’s case the relay seemed to work because it switched from A to C… yet while all the “A” side (flashers) worked, some of the “C” side (coils) did not. Ah, and the “C” sides that weren’t working had fuses… but the fuses were good! So I tested the affected coils separately, and they worked! So I tested their transistors and related components: also good! So everything still pointed to the common failure point being a relay, which verified “good” through testing...?! Arrgh! Just what was going on?
Let’s look at the schematic:
The A/C relay is the dashed-outlined box just right of center. A relay is a sealed module consisting of a small solenoid coil and a few switches. As a “coil mech” of its own, it’s triggered like any other solenoid in the game; that signal feeds from the MPU J6 (the schematic line going straight up & down from the relay). The power inputs are left of the relay, and there are two: a 25V supply, and a 50V supply. The outputs are highlighted on the right, and switched: the 25V line switches from “A” to “C”, and the 50V line switches from “A” to “C”. The relay’s internal coil operates both switch stacks simultaneously and the default path is “A” as shown. Note, each output on the 25V side is fused, while the 50V lines have no fuses.
The key takeaways, which I never fully understood myself until studying this schematic, are:
The simultaneous output switching gives this relay “only two apparent options”:
- both 25 & 50 V going to “A” at the same time
- both 25 & 50 V going to “C” at the same time
Yet, the relay does not affect merely two discreet circuits (labeled “A” and “C”), but FOUR:
- 25V “A”
- 25V “C”
- 50V “A”
- 50V “C”
So armed with this information: testing the relay itself (via the game's “A/C solenoid self-test") only verifies that the relay’s MPU driver works, and that the relay's internal coil is working: that’s what makes the clicking noise.
But what happens if the relay’s internal switch stacks - or just one stack... or even just one side of one switch - goes bad?! [Hint edit: see https://pinside.com/pinball/forum/topic/williams-a-c-power-relay-explained-how-to-replace-and-maybe-fix#post-5762662 for symptoms...]
While I’ve dealt with bad relays before and never knew one to fail this way, eventually I was almost sure "one side of one switch" was defective... but it still seemed hard to fathom and I wanted to be absolutely sure. Unfortunately, while many relays have clear and/or removable covers, for some reason WMS went with sealed opaque units in this application. Arrgh, no choice other than to remove the relay and crack it open!
The yellow box on the Auxiliary power board is the culprit in this case. Sys11 games often use similar relays elsewhere on small daughter boards (usually for GI switching). The same concepts will apply, though the specific circuits & applications may be different.
^ A WMS daughter relay can be easily swapped by replacing the entire module, but be warned: removing the A/C relay from the power board is not as simple as an IC, connector pin, or transistor. You absolutely need to use a desoldering braid and/or a large desoldering tip. The relay prongs are jammed in tight. Be careful not to overheat the board!
^ Only when the holes look like this (or better), can you even begin to think of removing the relay. This board looks nasty because I actually had to do this twice: once to “test hack-fix the bad relay” and again once a new replacement arrived. Thankfully, it looks nice again after the relay is replaced with fresh solder applied. At any rate, once the holes are as empty as you can make them, you can begin gently prying the relay from the top of the board. You may need to use a small screwdriver - go slow, be gentle, and careful not to gouge the traces!
^ Here’s the relay. Completely sealed with epoxy! Why TF was this necessary? To verify a suspected failure, we need to cut it open.
^ Using a Dremel with reinforced cut-off wheel (ALWAYS use reinforced, the standard ones are eye-seeking garbage), carefully cut the perimeter of the case about 1/16" deep, from about 1/8” up from the base, as shown here. This is also why you have to remove the relay before you can open it, duh
(You may notice clear tape laminate on this example: that's because I'd done all this before deciding it merited this write-up. Since the material removed in the cut-off process won't allow the cover to reseat flush or remain safely secure, I taped it on).
^ Hey look: a solenoid coil with a switch stack! The spring on the pawl above the coil, puts tension on the center leaf to favor the “A” side as normally closed. When the coil is energized, it pulls the center leaf to close the “C” side instead...
^... and while the switches on the 50V side look new, ugh... the 25V side looks pretty jacked…
^ and by removing the spring pawl, we can see the leaf is jacked too…
^ and even MORE incredibly, *somehow* (I’ve no idea how?!), the corners on the leaf were bent, interfering with the contact point on the “C” side! Whatever caused it - 3 decades' of use, or a manufacturing defect, or a "stuck circuit" fault sending melty current through the relay too long - it was clearly no longer capable of making good reliable contact (note this photo was taken after I cleaned it)! So the 25V “A” and 50V "A" sides would work, but when the relay pulled in to “C”, the 50V switch could close, but not the 25V switch. BINGO!
Without a spare relay on hand, to test my theory I cleaned and adjusted the 25V switch much like you would do with any other leaf switch. I then replaced the cover, soldered the relay back on the board, installed in the game and… IT WORKED! Probably not for indefinitely, but enough to buy me time for troubleshooting and testing. And also to confidently conclude there were no weird issues with PIAs, drivers, diodes, wiring, etc - always a huge relief.
Given the difficulty in removing/installing the relay I wouldn't necessarily recommend doing a hack fix: you should replace with new in the first place, if possible. But if you're in a bind, it is indeed possible to fix one to limp along. It might even be easy if your relay has a removable "snap" cover!
^ New relays are clear and have removable covers. Date the replacement like you would anything else. Marco sells nice quality units from Tyco Electronics but if you go for an alternate source, make sure the voltage and duty cycles are comparable to the original.
I hope this post is helpful to your troubleshooting efforts.