4.96 is actually ok. Many of the WPC89/-S games work fine with 4.90v when you measure the 5V on the C31 of the MPU board.

I do not use the Kahr board.

Have you had your meter/scope calibrated recently?

If you really want to adjust it to be over 5v, install this PSU5 LM323K Replacement part. Just don't go over 5.25v. https://www.ezsbc.com/product/psu5/

The Kahr board is a good stop-gap tool, but IMO should not be a permanent fix. every pinball toolbox should have one in it!

Yes. They are adjustable. Again, just do not adjust past 5.25v. That is the highest voltage that 74xxx parts can handle.

As mentioned above, 4.96 is fine. Play it and stop worrying about it.

Some people are putting the Kahr board in now as a preventative measure. You shouldn't assume the game has issues just because the board is installed. Measure your 5V on the driver board. If it's good, remove the Kahr board and test the 5V on the CPU board. Over 4.90V? Then play it until it resets.

Yes, a highly recommended. The fact that they run so much cooler is a huge improvement, plus they are more stable, so win-win! Have installed 2 in different games

If you use that EzSBC voltage regulator, just be aware that your machine will make a loud farting noise when you turn it off. Just an FYI and it's actually normal.

That said, I do have some of the Kahr boards on all my WPC route games. They are there just for insurance purposes so I don't have to worry about them randomly resetting while on location. Once I bring the game home I pull the board and use it on the next WPC heading out on route. I do not use any here at home. But if I'm hosting a party or tournament and one starts to act up, I'll throw a spare on it to get me through the day.

Don't forget you can adjust the LM323 that's in there by severing the ground and adding a resistor.
https://www.pinwiki.com/wiki/index.php?title=Williams_WPC#The_Absolute_Last_Resort

Although pinwiki calls this a hack, if you read the datasheet for the 323, floating the ground up through the resistor in the application notes. It's a hack in that the WPC driverboard doesn't have the pads correct for this, but it's not a hack as far as the 323 is concerned.

Think of other makers (gottlieb) that had a pot to adjust the supply - same idea, here, just with a fixed resistor.

They probably should have incorporated use of the UVLO (under voltage lock out) on the EzSBC to get rid of this. Water under the bridge and doesn't really hurt anything anyways. But I wonder if a clamping diode from output to input would fix this. Anode to vout, cathode to vin. This way the output would be shunted back to the input (and filter cap) at power down. You will sometimes find these already installed in LM323K circuits to prevent the output cap from discharging into the LM323K at power down.

Not a lot of love for the Kahr board on the forum, but after repairing several driver boards, new Z connectors, and even trying brand new boards in my TZ I still got resets. Not one single reset since I put in the Kahr board. So I have no problem saying go ahead and use one if you want to.

A friend had a Road Show on location that kept resetting. I did everything to it to stop the resetting (including new capacitors and bridge rectifiers and connectors), but it still was doing it. Finally put a kahr board on it and presto! No more resets. It had been on location for three years with that kahr board and was running perfectly. The location closed due to coronavirus lockdowns so my friend sold the machine to a home user.

So I disagree with the 'no love' comment - when pin sound copied my product the outpouring of support expressed in the forums sure felt like love and I still appreciate it. In Nicolas I lost somebody who I thought was a friend but I was reminded of the many, many others I've accumulated through pinball.

A little history on the ezSBC PSU5 - Daniel created it because I asked him to do it. I've had a long, productive business relationship with him and he still does all the surface mount work for my daughterboard here in the USA (in Arizona) (the copycats are just selling crap they have assembled in cheap chinese fab shops). Here's a thread from six years ago where I called out the PSU5 here on Pinside: https://pinside.com/pinball/forum/topic/lm323-replacement-released-at-ezsbccom. He also does the surface mount assembly for my Skee Ball Display boards.

The PSU5 is great because it eliminates the heat burn-off in the linear regulator, but installing one doesn't re-balance the electrical loads - the only way to rebalance 5v consumption across the available supplies is to install my daughterboard. I walked through this in detail on the stage at the Northwest show in 2018 but unfortunately the video of that presentation no longer exists. The short version of my explanation centered around the usable life span of the power supply - specifically the MPU under voltage sensor drives power supply repair to happen more frequently than would otherwise be needed.

Still short, but not quite as short version follows (skip to the end if you don't care): Components tend to degrade as they age - the caps, the bridges, the headers & pins, the solder joints, etc. Borrowed from my Tacoma presentation, here's a representation for how a electrolytic cap can degrade over time: representation of component degradation

The power supply is a system assembled of such components and thus degrades over time as well - sort of like a summation of the degradation curves for all the individual parts. The under voltage sensor triggers (i.e., causes a reset) when the power supply system briefly cannot deliver sufficient energy. Move the MPU off the "goes everywhere" 5v line and you can take advantage of significant additional lifespan from the now "goes everywhere except the MPU" power supply.

Screen Shot 2020-12-17 at 2.36.09 PM (resized).png

The soldering iron can remain cradled until you approach the second green line. When is that? Check for the flashing red indication on the Pro version of the daughterboard. The chinese-made copycats didn't copy that capability.

More of my discussions from a couple years ago were in a thread similar to this one: https://pinside.com/pinball/forum/topic/kahr-daughterboard-question-is-it-a-permanent-fix#post-4324443 .

-Rob
-visit https://www.kahr.us to get my daughterboard that helps fix WPC resets or my Pinball 2000 H+V Video Sync Combiner kit

No offense intended. I was just saying that I've had good luck with your board.

Yeah I used to be a Kahr hater until I actually used one now I'm fine with it.

Cheap product, saves a couple hours pulling a board and changing BRs, and it works great...what's not to like?

Weren’t this this like $20 a pop at the shows years ago? Used my last one but now at $32 plus shipping, might as well just fix the board right.

Kahr board is a no brainer improvement over an initially flawed design.

Well, the first version was $25 in 2014 and you paid shipping unless we met at a show. You are correct today's price of the Classic is $32, note however I also offer a "self assembly kit" for $27 - that's $1 less than the $28 price step that happened in 2015. Back then I could ship one nationwide for $1.93. Now it costs $4.25 to ship. I do my best to control the cost side and I'd say my track record is better than the USPS record for transporting small padded envelopes.

-Rob
-visit https://www.kahr.us to get my daughterboard that helps fix WPC pinball resets or my replacement LED display boards for model H & model S Skee Ball

I just got my first karh daughterboard in the mail today. As usual, I found the real problem while the fix was in the mail. In my TAF's case, the thermistor in the power box was the culprit. SOOOO.... I had a perfect test case to test the kahr board. It has proven to eliminate the resets! In fact, I have the thermistor on the way and will store my deluxe daughterboard in my tool kit for future testing or use.

The one thing that concerns me is that my AF has flashing green.. Is this a problem I need be concerned about?

I'm thankful for this solution to transient resets. I notice not a single hater mentions the thermistor.