Have you actually measured the EXACT voltage that is arriving at the main MPU board?

You need to stop guessing things and get methodical about it. Grab a meter, look at the schematic and measure away.

The Kahr board certainly works well to correct this type of problem BUT (IMO) it is a TEMPORARY fix and if it's needed to make your machine work, your problems almost certainly go much deeper.

Also please READ UP on Pin-Wiki. There is an excellent tutorial there on exactly WHY these machines reset and it isn't one single issue but many different ones that can (and do) cause these issues.

Bottom line is you need to dig A LOT deeper than you have.

Cracked solder joints on the header pins, maybe? Perhaps the add-on board shifted the pins slightly in one direction, and removing the board shifted them in another?

Have you put the newer Kahr board on and actually looked at what is happening when it resets? Versus when it is not resetting?

Might want to check the 12V supply. The board makes 5V from the 12V supply and it's pretty unlikely that both boards are flawed.

It's the low 5V on the MPU that causes resets. It's not really connected to the 12V - the 12V 'could' go down to 9V and still the MPU probably wouldn't reset. YES, other things will get upset but don't confuse what the two different voltages are for.

From the factory, these games worked pretty well without any additional gimmick boards. And they can always be restored to original condition by replacing aged capacitors or rectifiers.

Resets from low 5v is a common issue on WPC games. It’s a design weakness. A Kahr board rectifies that design weakness.
https://www.kahr.us/daughterboard.html

Temporarily.

Caps and rectifiers are RARELY the draw down causing the reset. That is a myth. Please read the PinWiki that was scientifically pieced together to provide a flow chart for the possible causes and fixes for a 5V watch dog reset.

I have replaced enough BR2's and C5's on WPC driver boards, to know they really are the main cause of resets.

Rectifier eventually fails, because the large filter capacitor takes huge spikes of charging current at input waveform peaks. You can't measure that with DMM, but with an oscilloscope it is easy to see that one half of bridge is failing a little under load, causing LM323 regulator input voltage to momentarily drop below minimum.

85C electrolytic capacitors have a lifetime of only a few thousand hours. They operate in quite warm environment in WPC games. When the capacitor ages, its ESR rises and capacitance drops. It can't charge fast enough because of the ESR, and reduced capacitance makes more ripple in voltage, again causing the LM323 input voltage drop.

Is this a design fault then? No, components simply have some lifetime, and will not fill all specifications when it is exceeded. If this were because of bad design, WPC games would have suffered from reset problem since the beginning. But we only started to see those problems when the games were nearly 10 years old. Of course, using a little bit higher voltage on the transformer, there would have been some more time before the problems. But then the LM323 would have been dissipating more power, get hotter and maybe cause different problems.

The bottom line is, WPC +5V power supply is a solid design, and easily repairable should problems occur.

Not disagreeing, but there are definitely other causes. Sometimes you get someone saying I replaced the BR and C5 and now it resets even more (because they ripped out the plated through holes yanking the cap out.)

Last one I had issues with was my AFM and it was actually the connector on the CPU board causing the drop so replaced both ends and fine (still with the old cap on the board, too.... as I really didn't want to deal with it at the time so I did the other things first.)

Figured it out by reseating the connector a couple times, seemed fine, then came back. So replaced and smooth sailing since.

Previous one was a twilight zone, and that one was the Z connector... ditto with STTNG, Z connector. This was after replacing the bridges and caps, too on those machines (without ripping out the through holes....)

I suppose the Kahr board has its place in an operators toolbag to get the bar game back up and running, but just like replacing the original boards with new (sometimes inferior) ones just "because", I wouldn't use it unless I NEEDED that machine up for something.... like at the beginning of a party in the basement.

Tuukka was concise and to the point:
As caps age - ESR goes up, capacitance goes down. Due to increased ESR, cap can no longer handle ripple current so the filtered voltage dips resulting in brief periods where voltage into LM323K goes below drop out voltage. Will the reset circuits catch this? Most probably yes. Will users ever catch this brief drop out with a meter? Not a chance.

The bridge rectifier replacement was what had me confused for awhile. Why would so many be going bad. Well my guess is they probably weren't. I think people were pulling old bridges out where the forward voltage drop had increased due to prolonged periods of high ripple current but were still functional. New bridges had lower forward voltage drops which appeared to "fix" the issue...for now.

But as Slochar also says - there are also ther issues such as aging or bad connections. Add in damage to boards caused by ripped out vias and broken traces and things get even worse.

In my opinion, if the problem is unknown then always suspect the cheapest and often the problem parts first --> connectors.

The Kahr module is a good aid to determine if the problem is power supply based but the source of the problem should still be fixed. Continue to use the Kahr module after the problem is fixed? Although I don't see a reason for it if main power supply has been fixed, I also don't see a reason not to use it other than cost. It does have added benefits: It partially offloads the LM323K which is always a good thing. And since it is a switching type power supply - the input current through the big caps and bridge rectifiers is also reduced which is also a good thing.

Let's face it, WPCs are a bad design. Far fewer DE & Gottlieb machines from 90s have reset problems than WPCs. There isn't enough margin between the watchdog voltage and voltage regulator output.

Also, power supply technology has advanced since the 90s and a switching regulator (as used on the Kahr board) is superior to the factory LDO. That's why I generally replace the regulators on WPC PDBs with the EZSBC equivalents. I completely agree that a Kahr board won't solve all your power supply woes and it's best to do a formal diagnosis, but the board itself is not going to hurt anything and might work fine for quite a while in some situations.

I agree connectors are another main cause for resets. Remember, if removing and reinserting a connector makes the problem disappear, it is only a temporary fix. But clearly shows that the connector must be replaced. Both sides. Sometimes the connector solder joints vibrate and break. Resoldering helps.

And yes, power supply technology has advanced quite a bit in 30 years. And since genuine LM323K's are getting scarce, a drop-in compatible switcher module is a good fix.

I am not totally against Kahr boards. They are a good and quick fix, but I still like to keep my games in original condition, even if better technology exists.

The root cause is not the margin between undervoltage detection threshold and voltage regulator output, but the margin between regulator minimum input voltage and what is available at C5 filter capacitor. And I still wouldn't call it a bad design, only that there is not much tolerance for ageing parts getting out of spec.

I don't know how the undervoltage detect works in DE or Premier boards, maybe the threshold is lower, but then proper CPU operation might be at risk. Williams designers might have been more concerned about CPU working 100% correctly than taking a chance with erratic operation. On location, a game that resets when both flipper buttons are pushed is more likely to get fixed sooner than one that might give you free credits or otherwise glitches.

Gottlieb System 1 games power down CMOS protect circuit was revised a few times, but they never got it working properly. Repeatedly fast plugging and unplugging the game sooner or later corrupted CMOS RAM, which was not checksummed by the game OS, resulting you could get 50 free credits.

LM323 is a linear regulator with a ~2V dropout. If the input voltage drops below 7V, the output voltage would drop to input - 2V (although there is a capacitor on the output side as well), but it would not go to zero and does not cause a reset by itself. The reset is caused by the watchdog detecting when the MPU 5V line drops below 4.7V.

And yes, given how pervasive this problem seems to be, it is unquestionably a design flaw. TTL logic typically has +/-10% margin on VCC and designers could have used a regulator with an adjustable pot set slightly above 5V to compensate for IR drop.

Not sure id call it a design “flaw”. But rather we are well beyond the intended lifespan. They were designed for a couple years of flawless service and beyond that wasnt considered. The fact we have boards running thirty years later is a testament to their engineering in the day.
Although I understand your argument. In hindsight, yes, knowing what we know now and with modern components we would do it differently.

As others have mentioned, you really shouldn't be asking about resets here without measuring some voltages first. Secondly, when asking about an electrical issue here, you need to mention any mods that have been added. You mentioned two or three above, but it appears you left out a whole lot more.

Your profile shows feedback from the seller of your game. A couple more clicks led to the ad for the game.

'Many upgrades including ColorDMD, magic wands, spinning hat, working tiger saw, trunk locks and more.'

https://pinside.com/pinball/market/classifieds/archive/113284

Your 12 volt circuit was badly overloaded. That's why it was resetting.

The resets stopped because you took the CPU board current draw off the overloaded 12 volt supply. Your 12 volts will likely still read low due to all the mods, but the CPU doesn't run off the 12 volt circuit any more. Back to the 5 volt circuit with daughter board removed.

People install the daughter board for different reasons. It's not always a driver board/ connector issue. House may have low voltage, game might be plugged into a power strip with 5 other games, game may use a long extension cord. Some people install the daughter board as a preventive measures, when the game isn't resetting. A cheap and easy to install device that assures the owner his or her game will never suffer from the dreaded resets. If the mods were installed in your game before the daughter board, whomever installed it didn't really know what they were doing (by adding the CPU board load to the already overloaded 12 volt circuit).

You need to test voltages. You may also want to message the person you got it from and ask him why he installed the daughter board.

Use a separate power supply to run the mods.

Fix the 12v circuit. Usually the cap is the culprit and needs replacement. Hopefully it hasnt leaked and taken out a bunch of components.

One thing I seldom see mentioned is to make sure the board is screwed in well and ground connection is good. I had a TZ that was resetting. I looked at the 5V with a scope and it didn't look too bad but when I measured the ground, it was coming up 0.3V. Turns out the board wasn't screwed in well. Doh!

OK. Something that all of you people adding tons of mods to the existing circuits (12V) fail to understand is that this 12V rail is supplied by a LM7812 regulator which is actually fed by 2 x IN4004 in series. See the snip of a typical WMS schematic, the 12V regulator part, below.

Let that sink in first - you are taking the power for your string of LEDs/Colour DMD/anything from a 12V supply that is already feeding many different parts of the machine - the MPU for the switch input circuitry, numerous opto boards (varies depending n the exact game) and several other things in the machine.

First, the series diodes are rated at 1A so there is a limitation immediately. Second, LM7812 series 3 terminal regulators are rated as 1A regulators, this is MAXIMUM and with large heatsinks. It certainly isn't recommended to run them at 1A continuously as it is going to end in tears!

Bottom line is STOP hanging all manner of things on this 12V rail in the machine. It was NEVER designed for all of the additional loads people are putting on this circuit. Yes, that includes Colour DMDs.

Use a separate supply for all of your mods and knock yourself out adding what you like but LEAVE the existing 12V circuit in peace - PLEASE!

regulator (resized).png

schematic (resized).png

Did you see the list of mods above? Color dmd, tiger saw blade motor and hat motor are all likely running off 12 volts. Who knows what else is running off the 12 volts.

Needs a separate 12 volt power supply plugged into the utility outlet and a bunch of the mods moved to that supply. After that is done, the 12 volts should be tested on the driver board before any repairs are attempted.

Wrong post! My mistake.

I would say "JUST" - and by the skin of its teeth!

We have 1A available TOTAL and that IS NOT continuous nor is it maximum. The machine will already draw more than 500ma from this 12V for the various opto boards and part of the MPU - that doesn't leave much overhead even for a colour DMD.

If it runs OK with the DMD connected expect the 12V regulator to run much hotter than it was designed to.

This is NOT good for the longevity of the electronics in the machine. Just because "it works OK in my machines" does NOT make adding these things correct or even safe.

Put something like this in your machine and connect ALL mods to it. One issue is you would have to work out how to wire it AFTER the main power switch so it comes on/goes off with the machines power switch.

power supply (resized).png

Not very helpful - the UNREGULATED 12V that you point out measures 14.8V in my machine because it is UNREGULATED. Folks, please DON'T go connecting things here or it could end badly for your add-on. If your add-on (mod) has an onboard regulator all well and good BUT don't expect this to be the case.

The bottom line is don't just start adding things unless you know what you are doing!

Like the one DumbAss makes.

WPC aux2 (resized).JPG

Likewise. Just too busy to respond to most things. I really didn't want to post here but the mis-information drove me over the edge. I think it's great people are trying to help. No!!! Really!!! I think it's better if you bring "receipts" - i.e. proof rather than just stating something as truth. "Receipts" definitely helps those reading who are unsure or who don't know where to look for the information. I don't believe most everything I read until I find corroborating evidence. That's "caveat emptor" at work.

I fail to see how my statements are "Not very helpful" but I guess we can agree to disagree. I can't prove that your statement is neither true nor false. It's opinion.

Unregulated means exactly that - unregulated. It is indeed often measured at ~+15V because it's unregulated. The actual voltage fluctuates depending on the load. ColorDMD almost certainly has either a 5V or 3.3V regulator. It can't perform any form of digital logic without one. LED strips don't need a regulator. Their brightness may fluctuate as the voltage fluctuates.

Maybe it would have been better if Williams called it "+15 V POWER" or "Unregulated 15V" instead. I didn't create their nomenclature. I'm just using what the schematic and manuals use.

My gripe is with statements saying that all the mods added to the system are drawing the "12V" supply low. Those mods are connected to +12VU. That has NO direct effect on +12VR because they are derived from separate transformer windings. The Kahr board uses +12VR as the source for its 5V regulator. It does not use +12VU so the mods added to the system (at J116, J117 or J118) are irrelevant.

This I can agree with.

Is there a link to buy this thing, or do I just PM DumbAss?

And why do I feel like I'll get moderated for a personal attack on a Pinsider when I say I want to PM DumbAss?

Another option? https://www.pinballlife.com/power-tap-and-8-way-power-splitter-board-for-williamsbally-wpc-pinball-machines.html

I was trying to use a separate power source because I've got so much stuff running on my AFM, I think it's causing some issues.

I think people are forgetting that the Kahr board is sold in two versions: a "basic" no frills version and a "deluxe" version. I have had trouble with the deluxe version, but never any troubles with the basic version.