great plains electronics
axial
https://www.greatplainselectronics.com/proddetail.asp?prod=CEA-100uF-100V-RMD

radial
https://www.greatplainselectronics.com/proddetail.asp?prod=CER-100uF-100V

edit:
doh! just re-read your post about not wanting to use 100uf - 100v caps

axial 100uf - 160v
https://www.greatplainselectronics.com/proddetail.asp?prod=CEA-100uF-160V-RMD

dammit! these are polarized

have you thought to give ed a call and ask him?

You can use two 200uF 250v polar caps in series. connect the terminals + - - + or - + + -. The end result is a 100uF bipolar cap, im not sure if extra ceiling on the voltage rating is needed in that setup. The capacitance is halved though.

Not sure how physically big this is, but this should work well.

http://www.amazon.com/100MFD-100uF-Motor-Starting-Capacitor/dp/B0087ZB0LI

Need to check the caps on mine!

Even better - $3.19 with free shipping:

http://www.amazon.com/TOOGOO-100MFD-100uF-Starting-Capacitor/dp/B00YOKWYZQ/ref=pd_sim_sbs_328_14

I have been asked several times regarding this cap and have not had luck finding them.

I have seen lots of posts regarding the back to back caps and there is a lot that can happen if you don't do this right (yes, there is a lot of room for human error in this one especially in the future when somebody else looks at this solution and "simplifies" it). I have decided not to suggest that one. But it does seem that as long as you go much higher in voltage with the replacements (as barakandI said) then this seems to work ok. It is somewhat space limited which makes it difficult to fit in even more caps.

I am working on a large capacitor order right now. I'll see if they still make axial non-polarized caps (not many companies make any axials anylonger). If they are available, will probably add in some 100uF, 160V caps. I do know they make these in radial style so we may need to go there.

Can you take a quick measurement of lead spacing for the C1/C2 caps so I can determine the max length? And measure C3/C4 if you can.

Ed

I used Pinbits parts in mine...

https://www.pinbits.com/index.php?main_page=index&cPath=13_11&zenid=d609015aab9555c9ae91fbe935f3e965

Yeah, I see they have the enhanced strobe board back in stock. At $35 it really is a no brainer. Built to last and the strobe is much brighter.

I had pinbits rebuild my AFM strobe board. Works great and I know they changed some parts so it won't fail again. You might want to check with Martin.

But that is just the strobe assembly -- not the driving board.

OK, I have some really good news for us AFM owners: You can repair/rebuild your strobe board with normal, inexpensive electrolytic capacitors instead of impossible-to-find non-polarized caps. Each 100uF/100V NP capacitor can be replaced by a commodity 100uF/250V capacitor and a 1N4004 diode.

Here's the link to the how-to: https://pinside.com/pinball/forum/topic/replace-caps-on-afm-strobe-board#post-3184588

The rest of this post is background on this modification and proof-that-this-is-not-a-crazy-thing-to-do:

We know there was a big mistake in the original design: C1 is rated at 100V, but C1 sees about 145V in normal operation. But there was another "mistake" (really more a missed opportunity) in the design that will eliminate the need for these impossible-to-find non-polarized caps.

The problematic circuit is this:
Original Schematic

C1-C4 and D1-D4 form what is known as a "voltage doubler" (in this case a voltage quadrupler since they employ the technique twice in series). This circuit takes the 50Vac input and turns it into about 290Vdc. I'm not going to explain how voltage doublers work, but there's plenty of info here: http://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/voltage-multipliers/ and on google.

Here's a simulation of that circuit (ignore diodes D5 and D6 for now):
Simulation of voltage quadrupler

I believe the designer of the original circuit used non-polarized capacitors because it seemed like the right thing to do in a doubler, particularly if you don't fully think-through the design. In the original design, C2 does need to be NP, because when the circuit is starting up (when power is first applied) the voltage across C2 (which is the voltage at point B minus the voltage at point A, or "V(b)-V(a)", the gray trace) does go to -30V before it goes positive and settles at about +70V:
Startup of original design

But note that except for the bottom of the first 60Hz (or 50Hz) cycle, the voltage across C2 is always positive. That means we could use a normal, polarized electrolytic capacitor for C2 if only we could get rid of that one-time -30V dip. Well it turns out that's not going to be hard to do. If we put a diode (D5) across C2 as shown, it will allow as much positive voltage as we need, but will clamp any negative voltage to about -0.7V. And -0.7V for 1/60th of a second every time you power on your AFM is not going to hurt a polarized cap (electrolytic caps are designed to withstand -1V to -1.5V specifically so they can be protected by a diode).

Here's the simulation with the diode:
Startup with Schottky diode clamp

I put a diode across C1 as well (D6), but it's really not necessary because the way the circuit works, C1 will always be positively charged.

When you're analyzing circuits with large capacitors, it's important to see what happens when you turn OFF the power as well, because unexpected things can happen. Fortunately in this circuit, everything just gradually discharges to 0V:
Power-On and Power-Off

So my recommendation is to replace C1 and C2 with 100uF / 250V polarized capacitors, add D5 (and D6 if you want), and enjoy a very robust Strobe Board.

Here's a good cap choice:
http://www.newark.com/illinois-capacitor/107tta250m/aluminum-electrolytic-capacitor/dp/28M9179

Use a standard 1N4004 diode for D5 and D6.

Hope this helps. If there are any questions let me know and I'll answer/update the post.

Sure...I've got a question.

WTF does this all mean? My head hurts.

So this is the reason the Pinbits strobe doesn't work without strobe board modifications?

SPICE for simulator? Looks like a much different version than what I last used.
I saw the negative on C2 originally but never even considered putting a shunting diode across him to take care of the negative voltage. Excellent post, Would be nice to somehow work this into pinwiki.

To simplify what he said:
For both C1, C2 caps -- use 100uF, 200V axial electroltyic caps. Could also use 100uF, 160V caps but that is cutting it close.
In order to use these more common caps - you need to protect capacitor C2. Install a protection diode directly across C2. Anode of diode to negative lead of C2, Cathode (banded end) of diode to positive lead of C2. SB140 is a good choice.

I couldn't have said it better myself!

I don't have time right now but will work on the TL;DR summary soon.

Yep - ltspice. http://www.linear.com/designtools/software/ Free and fairly easy to use. I believe it's really nice on Windows - the Mac version I used has a very painful GUI (you can memorize all the keyboard shortcuts to avoid).

Is pinwiki editable by anyone? I'll try to clean up the explanation here first...

Or just send it to Martin at Pinbits - he will fix it with better Caps that will improve the board and he doesn't charge much money. It made a world of difference in mine. My strobe was still working but it wasn't flashing very much before I sent it to him.

Can anyone post or send me a highish-res photo of both the front and the back of the strobe board? That would speed up my how-to write-up (I won't have to pull mine from my machine).

I haven't had a chance to get back there and look at it...and I'm leaving on a trip Thursday. I have a laundry list of post-restoration things to tweak, and I've decided (for the time being) to just ignore it all and make the wife happy with yard work. That AFM was one problem away from fueling a dumpster fire, so I needed a break. Worst part of a restoration is the troubleshooting, IMO.

Two caps "back to back" work perfectly well. I did a lot of research when making these replacement boards and found a very good article by a capacitor manufacturer that basically said it is a valid way to use polarised electros *OF THE SAME VALUES AND VOLTAGE RATINGS*.

It has already been mentioned that, connected in series you HALVE the value of capacitance but ADD the voltage. In the case of the board we make below, we use 220mfd @ 250V so, with two in series, we end up with 110mfd @ 500V.

AFM_Plate_&_Repro_PCB_(resized).jpg

No question, two caps back-to-back can work.

It has already been mentioned that, connected in series you HALVE the value of capacitance but ADD the voltage. In the case of the board we make below, we use 220mfd @ 250V so, with two in series, we end up with 110mfd @ 500V.

Agree on the halving of the capacitance, but not on the doubling of the voltage. Look at it this way: if you had two 250V caps back-to-back in series and you put 500V across the string, you could have 250V across each cap (in which case one cap would have +250V across its + to - terminals, but the other would have +250V across its *-* to *+* terminals, which is verbotten), or you could have +500V across one cap and 0V across the other (to prevent reverse biasing the cap), but then you'd obviously be exceeding the max 250V spec of the cap.

Adding voltages works when they are non-polarized caps (and can therefore split the voltage evenly), but not for polarized. At best you'll end up with a 110uF @ 250V cap.

You are partially correct. One cap will have the full voltage BUT only at the moment of power application in an AC circuit (50/60hz). In any case, the caps we use at 250V are each more than double the originals at 100V. See the attached info from the cap manufacturer, a short excerpt is also here:

caps_(resized).jpg
Caps-appguide.pdf

I've had Mikes's (Homepin) strobe and strobe board in my AFM since building it in 2014. Works great!
https://pinside.com/pinball/forum/topic/theres-one-more-afm
http://www.johns-arcade.com/#!product-page/c1cs/6e4d3d6a-895c-fa32-47aa-42170ddb50b9
Wally

What was I partially incorrect about? My only point was that a 250V electrolytic cap back-to-back with another 250V cap doesn't give you a 500V cap - it becomes a non-polarized 250V cap. I guess you could argue that it's 500Vp-p, but that's not how max capacitor voltages are spec'd.

In any case, the caps we use at 250V are each more than double the originals at 100V.

I agree. Though I'd rather use silicon diodes to control the current flow than rely on the characteristics of a reverse-biased electrolytic. If I *was* using the two caps back-to-back solution, I'd still put a 1N4004 across each cap to guarantee no reverse biasing and have a high-current path that doesn't go through a reverse-biased capacitor.

If you break it down, the back-to-back solution only ever (except for the bottom of the first 50/60Hz cycle on C2) uses 1 of the 2 caps used to form C1 and/or C2. The other capacitor in each pair is basically just acting as a diode, as well as cutting the total capacitance in half. The back-to-back solution apparently works, but it's certainly not as good from a cost, physical size, and design elegance perspective as this new modified design. I suspect the modified design (by not using electrolytic caps as diodes) is additionally also more reliable as the caps age over time, but I don't know enough about reverse-biased electrolytics to know for sure.

I'm not saying anyone should worry about their boards if they are currently working. But if you're repairing/restoring a board you might want to consider this modified design.

If you read the application notes I attached they describe the resultant max voltage of the back to back pair as equal to one of the caps BUT only for the first half cycle - after that the total voltage is much higher than a single cap although not the sum of the two.

We are nitpicking anyway as the application notes clearly suggest that it is a perfectly sound method of creating a non polarised part and that is the point I was making.

I just reread both notes for the fourth time and still can't find anything mentioning the total voltage being any higher than that of one cap. But I may have a blind spot - can you quote just the part where they say that? The closest I can find is:

Article #1: "When voltage is applied, the correct polarity capacitor gets the full voltage." That's exactly what I've been saying: with one polarity voltage, the cap in the "right" direction gets the full voltage (250V max in our example), then when the polarity is reversed, the voltage across that cap goes to zero (actually more like -1V) and the max of 250V is applied to the other (now the correctly-biased) capacitor. At no point do you ever get more than 251V across the two capacitors (the additional 1V comes from the reverse biased cap acting as a diode).

Article #2: the closest I can find to a relevant passage here is the exact same sentence that's in article #1: "When voltage is applied, the correct polarity capacitor gets the full voltage." (Almost makes me wonder if there's a little plagiarism going on.). Again, the "full voltage" is the max voltage of one capacitor, and it applies not only for the first half cycle but for any amount of time the voltage is that polarity.

BTW, what the last sentence of article #1 is saying is that the only reason the back-to-back capacitors don't overheat and explode in our AFM application is because they are seeing DC most of the time - one is always reverse-biased and acting like a diode, and the other just sees a large DC voltage. They only see any AC during startup, before they settle to their DC values, and after the strobe fires, where the voltage across C3 and C4 is depleted causing the C1/C2 voltage doubler to have to supply current again to charge them up. The modified circuit I'm proposing never subjects the capacitors to any of the excess heating and corresponding short life the article mentions, which is one of the reasons I consider it a better design.

We are nitpicking anyway as the application notes clearly suggest that it is a perfectly sound method of creating a non polarised part and that is the point I was making.

Sorry if I seem nitpicky - I just genuinely want to understand your point because if what you're saying is correct, it contradicts a lot of what I know about electronics/physics/mathematics and I'd like to correct my understanding if it's wrong.

This is just not true. I bought the Pinbits enhanced strobe(the small board with the u-shaped tube) and it has worked perfectly with my stock strobe driver board. Zero problems and much brighter than the original board.

Thanks for the clarification. I was just under the impression that more than a few people were having problems getting it to work without strobe board modifications.

I have restored two afm's using pinbits enhanced strobe with original board combo with zero issues. That's why I though you may have crossed up the plugs. If it was working before, it in theory should have worked after.

Okay...haven't spent any time looking into it. Waiting for my Pinbits shipment and trying to keep wifey happy with some yard work in the interim. I'll report back here and on my restoration thread when I find out what my issue was. (But I'm leaving tomorrow on a trip, too!)

Not going to argue about theory, just report my experience. You've linked to my site, I've done the modification I detailed on three different AFM strobe boards. Two of the games have been on location, turned on 12+ hours a day. One is on location now. The method I detailed works fine and is still working.

Nice to know adding a diode is also an easy solution.

Homepin: I just realized that the Homepin I was having the capacitor discussion with was *probably* the Homepin that made that beautiful replacement strobe board shown in https://pinside.com/pinball/forum/topic/replace-caps-on-afm-strobe-board#post-3178339 with the www.homepin.com URL printed on it. (I really need to start paying more attention to handles. )

I want to make sure everyone reading this thread knows that I agree that your board is a solid design and will work fine in their AFM. It's a much better board/better design than the original. It remains the best solution to the original problem: how to replace an unavailable NP capacitor with an even higher voltage (and even less available) NP capacitor.

When I analyzed the issue, I realized that the design didn't actually need NP capacitors, so I was able to suggest an alternative solution. I did not mean to suggest that your solution/board was inferior - we just had a little back-and-forth about how to make NP caps out of P ones. The design needs 160V caps, you believe you created 500V caps, I believe you created 250V caps, but whichever one of us is right you have at least 56% voltage margin (which is very safe) vs the -37% of the original design (which is really inexcusable).

TL;DR (which is what I should probably change my handle to): No one should have any qualms about purchasing the Homepin strobe board, and it would be what I recommend to people who don't do board work or just want to buy a well-engineered solution that works.

Hello Yonkiman. I appreciate your technical input in an area that I certainly do not claim to have intimate knowledge of. All discussions, lively or not, are perfectly OK IMO until they become personal attacks and I don't believe there is/was any of that in our discussions.

I researched an area of electronics and decided to use a solution that I was taught to use 30++ years ago. At that time there was no technical justification for using electros in this way but I knew it worked, was safe and a valid way to create an NP cap.

I appreciate your very valid input to the discussion and your suggested repair method is certainly the way to go for repairs to existing boards. I ALWAYS advocate repairing original boards where possible rather than simply replacing. Interestingly, almost all of the replacements I have sold have gone into machines where the original board is missing or into new builds such as Wally's brilliant "ground up" AFM build (I wouldn't have the patience for a project like that).

Hey Mike (Homepin)
Thanks for the nice compliment.
Big thanks to you from me and all the others that have scratch built AFM's and use your Strobe Board. Sweet design!
Wally

Tried my second Pinbits strobe and that didn't work either, even though the stock one still does. I'm guessing I need the bigger caps in my strobe board? Would it be the lack of a charge strong enough to light the strobe?

I'm not versed enough in electronics theory to understand this, but I know it's not a cable or strobe problem since the original works.

Sometimes the trigger needs replacing. Little blue rectangular box with super-thin leads.

Can you provide a link to that part? I'm assuming I can get it at GPE? And if someone can post a link to the correct upgraded caps and/or diodes I'd appreciate it greatly.

Just did this board mod. Fixed my strobe issue, but now every time the drop targets is hit the sounds cuts out and seems to reset. Any ideas?

No and no. When we unplug the strobe board, the sound behaves normally. Got a PM in to Yonkiman, hoping he can tell me what would cause that.

Okay...the strobe...and game...are finally working properly.

This was a tough one. My friend and I went through the following permutations:

Stock strobe board + Pinbits strobe=not working
Stock strobe board + stock strobe=working
Capacitor upgrade + Pinbits strobe=strobe working, sound cuts out on drop target hit
Pinbits rebuild kit + Pinbits strobe=strobe working, sound cuts out

Now, here's the incredible part: I'd been having opto issues (top left popper would go off during attract mode, Return to Battle would trigger for no reason). When my buddy was rebuilding the high voltage board, he found that diode D7 had been reversed! And it was a stock board...it didn't look like anybody had ever replaced that diode. Go figure.

Diode flipped, board rebuilt and caps upgraded with diodes, strobe working, sound working.

Thanks to all in this thread for your input!

Great catch!!

Remember what I said in my PM about pinball electronics flagrantly ignoring the laws of physics? I just looked at the schematic, and if you'd asked me "What would happen if D7 was reversed?", I would have said "That's easy: *nothing* would happen. The board would simply never fire the strobe because a reversed D7 would just block the trigger signal for the opto isolator, so as far as the board could tell, players just never made it to Strobe Multiball. Very basic diode theory.

But you saw it working fine and shutting down the sound board.

My head just exploded. Again. From freakin pinball electronics.