Hello, wanted to share a repair tip. If you are handy with a soldering iron. I have a MM502 I suspect was struck by lightning and the rev counter in the slave ram went bad (also the control board charging circuit burned but that's another repair story). The symptom is as described it the TS guide, 4 beeps pause 4 beeps, 3 beeps for master rev counter failure. I diagnosed/verified it by swapping them, BTW the operation failure mode was the gates would open/close 3-4 inches and you would have to cycle power between the open or close. What turned out to be was the optical switch that counts the revs of the ram. You can buy a new one for about $40 but they were on back order so I bought an optical switch from Amazon (uxcell 13/64 optical coupling photoelectric switch) that was not the right form factor. You pry off the plastic housing on the one to be repaired pull off the transmitter (->|- this is marking on plastic) and rec LEDS, pull the corresponding LEDS out of the new one and replace accordingly, reinstall the original plastic housing and your good to go. Cost me $6.
I registered on this site just to give props to Blakepa! I've been repairing various models of Mighty Mule gate openers for 20 years and this is hands down the best tip I've come across. I'll expand on what he has done.
If your opener acts erratic (seems like it doesn't remember the close point, will only open or close partially, etc.), this has been the culprit most of the time. Instead of purchasing a new rev counter board for $20-40 this is an easy thing to try.
Buy these opto isolators from Amazon or wherever:
uxcell 5Pcs 13/64 inches DIP-4 Groove Type Optical Coupling Reflective Infrared Photoelectric Sensor
You have search because I don't have the post count to post a link. Try Amazon first then Ebay.
As of Dec 2020 it is $8.62 shipped for five.
Desolder/remove the existing opto isolator from the rev counter board. Be careful to not damage the board, pads or traces. Use a solder sucker or whatever technique works to clear solder out of the through holes.
Don't be concerned that the plastic housing is in the wrong configuration, we'll take care of that later.
Note the markings on the original part. The markings on the new part are exactly the same. Insert the new one into the board in the same orientation and solder the four leads. Using small diagonal cutters trim the legs.
Using the same diagonals, cut the plastic housing so that it opens the slot up. Use a needle file to clean it up so nothing sticks out and touches the timing wheel.
Using this method I repaired 2 out of 2 bad counter boards.
Hope this helps someone else!
I've had the gate hit by lightning also and learned about replacing diodes at the motor and adding MOV's to solar panels.
tags: mighty mule, fm350, fm500, rev counter board, RVCTBD50
