OK, here's the *practical* how-to guide to replacing the NP (Non-Polarized) 100uF caps on your AFM strobe board with readily-available Polarized 100uF/250V electrolytics.
- Order 2 (or 4) 100uF/250V axial capacitors. You probably only really need replacements for the 2 NP caps, but I replaced all 4 since the game's 20 years old and I don't want to have to go back in the strobe box anytime soon. I bought these: https://www.digikey.com/product-detail/en/illinois-capacitor/107TTA250M/1572-1310-ND/5410816, but any 100uF/250V axial cap from a reputable manufacturer should work fine. Also you'll need two 1N4004 diodes (https://www.digikey.com/product-detail/en/fairchild-semiconductor/1N4004/1N4004FSCT-ND/1626118 if you don't already have them on hand.
- Make sure your AFM is powered off. The high voltages in the strobe board will discharge in less than a minute after it's turned off.
- Remove the board from your AFM. Remove the cover around the board first (4 corner screws), unplug the connectors from either side of the board, then unscrew the remaining screws (2 on top and 1 on the bottom) to remove the PCB and the backing plate.
- You should now have this on your workbench (minus the cable that I left on for a few photos):
- Pry the PCB off of the backing board by twisting a large flathead screwdriver against each mounting pin between the backing plate and the PCB so that all the pressure on the PCB is applied right next to the mounting post. If there is any distance between the mounting post and the pressure on the PCB, you could damage the PCB. Put the backing board aside. Now we're here (front and back):
- If you look carefully, your C1 may also be bulging from being overstressed:
- Next, desolder C1 and C2 (and C3 and C4 if you're replacing all 4 caps):
C3 and C4 have "+" marks to identify the positive side of the capacitor. C1 and C2 do not, because the caps were non-polarized. Since we're converting to polarized, we should add "+" marks, and since we're also adding a diode to each cap, let's draw that on as well:
Note that the positive end for C1 and C2 is on the bottom of the board while C3 and C4's positive is on top. MAKE SURE YOU GET THAT RIGHT! Otherwise the caps will explode when you power it on. (I'm not saying there's any *extra* risk with this technique, just that any electrolytic cap installed backwards can explode when power is applied.)
- Install the caps (and the diodes) for C1 and C2 (and C3 and C4 if you're replacing them as well). It's easiest to do C1 and C2 one-at-a-time because we're attaching the diode on the back at the same time. So first, install C2 by sliding the leads through the holes (and triple-checking the polarity). Then turn the board over and wrap the ends of the diodes around C2's pins (triple-checking the polarity of the diode as well - the diode band should be on the bottom of the diode, closest to the big copper rectangle (which is a tiny heatsink for the power resistor R9). It should look like this before you solder:
Before you solder, make sure:
- The capacitors are flush with (or a few mm higher than) the PCB
- Neither the capacitor or diode lead are touching the copper rectangle)
- The diode's body should be in-between the two PCB traces that connect to the diode (in the example above, if the diode was closer to the top instead of closer to the bottom, the bottom lead of the diode MIGHT short to the trace connected to the top of the diode - you want to avoid that).
- The diode's leads are wrapped around the capacitor's leads (I use needle-nose pliers to make a good mechanical connection), and
- The diode's leads are flush with the PCB.
- The band on the diode is on the bottom.
- Solder the leads to the pads. I'm not going to teach soldering here but if you're new to it, make sure the solder wicks around the cap leads, the diode leads and into the hole on the PCB. When you're done, you should see a nice solder joint on the other side of the board as well (compare it to the other solder joints on the board if you're not sure).
- Clip the capacitor leads and repeat for C1. The back of the board should look like this:
- Install C3 and C4 if you're replacing them.
Front and back should now look like this:
You're basically done - reverse the removal steps and fire it up. The only other tip I have is that when you mount the PCB back onto the backing plate, I recommend you push the PCB down over each mounting point with a 1/4" socket (or similar) - that will apply equal pressure around the PCB while allowing the mount to pop through. It's a tight fit and you don't want to risk damaging the PCB.
My board fired right up strobed away as expected. I played for about 30 minutes (including one Strobe Multiball where I made exactly 9 hits before I drained ), and everything seemed fine. If there were any serious issues with the modified design or the board work I did, they would have shown up (in the form of loud pops and smoke) within the first minute or two.
Hope this helps (and was less cryptic than my first contribution to this thread: https://pinside.com/pinball/forum/topic/replace-caps-on-afm-strobe-board#post-3178085 )!