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FC350BJ/110V Controller Transformer and Switch Failure.

Gary Anderson All Messages By This Member #120485 I have the FC350BJ motor controller on my Cummins mini-mill.? It is also used on mini-lathes so this applicable to this forum.? It's a second hand machine so I don't know its history.? ?Motor/controller was running as new.? Machine was unplugged and not used for a few weeks.? Plugged her back in and the motor would not turn-on.? The switch portion of the potentiometer/ switch was faulty.? In the CCW position the switch should be closed.? I had ~1 Meg Ohm of resistance.? The transformer for the low voltage +15V, -15V? circuit was open on the primary side.? Luckily LMS eventually had these parts in stock. LMS Part# 2492(Transformer), #1282 (Potentiometer).? ?I am grateful for their sourcing of replacement parts.? After replacing the two known faulty components, the mill now turns-on with full range speed control.? I have not yet tested under motor load. ? I understand that the potentiometer will eventually wear out. The fact that LMS carries the transformer leads me to believe that this is a somewhat common problem with this controller.? I did not find anything on the web about this particular problem.? Hence one reason why I am posting here.? Maybe help out another person that has experienced this fault.? ? Right now, my only brainstorm is there was a power surge when the pot/switch was in the CCW position.? The Metal Oxide Varistor didn't clamp and protect transformer, but it did clamp and drew more current than the pot/switch could handle. I have not yet replaced the MOV on the PCB.? ?I haven't completely thought through the problem, yet here I write.? ? Does anyone here have advice on other components that may be faulty/compromised that lead to burn-out of the switch and transformer?? Or modifications that can be done to make this controller more robust that are worth the time and money??? ? Resources that I used to assist me during my debugging: John Gerling's write-up https://olduhfguy.com/ ? For now, the mill is up and running.? I am still eyeing the KBMD-240D as a more robust alternative. ? Regards, Gary ?
Mark Kimball All Messages By This Member #120486 The wiring diagram for your controller can be found on LMS: ?.? It shows that both sides of the AC line are switched.? It would require a pretty substantial spike on the power line to take out the switch AND transformer! ? It definitely would be a good idea to check out the MOV
Roy All Messages By This Member #120487 Replace the MOV! They suffer internal damage whenever they clamp. Here's an insomnia curing paper on the subject; ? ?< > ? To minimize future problems, consider powering the lathe from a surge protected power strip instead of plugging it directly into an outlet. ? Roy
Brian All Messages By This Member #120488 RUD failure.? ? For those who might not have heard the term before it's referred to as Rapid Unexplained (Unexpected/Unknown/Unscheduled) Disassembly (Decompression/Disembowlment) and so on.? I don't actually know of its etymology but was once told it came from the military or quite possibly somewhere in the space race industry (or I heard it in a movie). tldr; think of it like a very fast reacting fuse where every time the breakover current is exceeded a micro fissure or 'hole' is created in the substrate and eventually it's weakened enough (swiss cheese) that it either explodes (if you're lucky and all the visual inspection artists point it out rather quickly because all the smoke gets let out) or it just dies; doesn't wave a red flag-just sits there looking pretty.? ? -Me: trying to put the "fun" back in electronics.... because aren't 3 letter words supposed to be exciting??
Tony Smith All Messages By This Member #120490 开云体育 Weird fault.? Very uncommon for a transformer primary to die, they often have thermal fuses which fail.? They’re supposed to fail if the transformer overheats (Hey Dave, sound familiar?) but occasionally die just because they feel like it.? ? You’ll find it if you cut away the insulation tape, you might see a little bump.? If the primary is ok replace (or bypass) the fuse.? If replacing remember to crimp in the new one, don’t solder it.? Trap for young players, everyone does it once. ? The MOV isn’t strictly needed.? They suppress spikes by shorting out (and getting warm).? They eventually fail by staying shorted and then take out the fuse.? Spikes shouldn’t bother the transformer but definitely bad for the electronics.? Repeated spikes might have taken out the thermal fuse. ? In theory bypassing the thermal fuse and removing the MOV would get it running again until you can fix it properly. ? Bad switch on the pot is strange as well, might be coincidence. ? If the machine was plugged in but switched off (via the emergency switch), then all of those components would have been isolated from spikes on the line. ? I’ve no idea what went wrong, but good to see you fixed it easily. ? Tony ? ? ? toggle quoted message Show quoted text From: [email protected] <[email protected]> On Behalf Of Gary Anderson via groups.io Sent: Sunday, 2 March 2025 3:40 am To: [email protected] Subject: [7x12MiniLathe] FC350BJ/110V Controller Transformer and Switch Failure. ? I have the FC350BJ motor controller on my Cummins mini-mill.? It is also used on mini-lathes so this applicable to this forum.? It's a second hand machine so I don't know its history.? ?Motor/controller was running as new.? Machine was unplugged and not used for a few weeks.? Plugged her back in and the motor would not turn-on.? The switch portion of the potentiometer/ switch was faulty.? In the CCW position the switch should be closed.? I had ~1 Meg Ohm of resistance.? The transformer for the low voltage +15V, -15V? circuit was open on the primary side.? Luckily LMS eventually had these parts in stock. LMS Part# 2492(Transformer), #1282 (Potentiometer).? ?I am grateful for their sourcing of replacement parts.? After replacing the two known faulty components, the mill now turns-on with full range speed control.? I have not yet tested under motor load. ? I understand that the potentiometer will eventually wear out. The fact that LMS carries the transformer leads me to believe that this is a somewhat common problem with this controller.? I did not find anything on the web about this particular problem.? Hence one reason why I am posting here.? Maybe help out another person that has experienced this fault.? ? Right now, my only brainstorm is there was a power surge when the pot/switch was in the CCW position.? The Metal Oxide Varistor didn't clamp and protect transformer, but it did clamp and drew more current than the pot/switch could handle. I have not yet replaced the MOV on the PCB.? ?I haven't completely thought through the problem, yet here I write.? ? Does anyone here have advice on other components that may be faulty/compromised that lead to burn-out of the switch and transformer?? Or modifications that can be done to make this controller more robust that are worth the time and money??? ? Resources that I used to assist me during my debugging: John Gerling's write-up ? For now, the mill is up and running.? I am still eyeing the KBMD-240D as a more robust alternative. ? Regards, Gary ?
Gary Anderson All Messages By This Member #120491 Thank you all for the feedback. I haven't devoted any real time to work further on this project, but wanted to acknowledge? ? I didn't notice a thermal fuse bump on the transformer.? But it's on the bench in the office, and I may start to remove the insulation tape.? ? When the transformer was removed from the board, I did check the MOV.? It measured high resistance.? The MOV part number is 471KD10, e.g. 470V typical.? ?Min is 423V, and 423/sqrt(2) is 299V RMS max.? Of the cuff, this seems too high to be of any real benefit in a 120V mains AC application.? ?For added piece of mind, I will use an external surge protector.? (I did use one most of the time)? I have other unvetted speculation ideas too.? The hobby machine's electrics weren't designed with best known practices (>$).? ? ? Maybe my problem is indeed a one-off and not a common problem with the FC350BJ/110V Controller in the MiniLathe community. ? Regards, Gary ?
Gary Anderson All Messages By This Member #120493 I still have no real idea on what caused the failure of the transformer and switch.? I just replaced the parts and hope for the best, with the lurking feeling that it will happen again.? I now do have a better delta connected surge protector installed at the AC mains plug.? No issues so far with very limited use. ? Before power-up, I checked that the fuse was the proper rating.? ?I didn't check when I bought the used machine. ? I tore down the faulty switch.? The copper colored stud contact is damaged where the brass colored spring loaded tab makes contact.? It looks like there was arcing in the switch as the 'copper' has migrated. ? I cut away the faulty transformer's insulation tape, and it did not have a thermal fuse.? None of the visible magnet wires showed sign of damage.? I have not yet unwrapped it to see if there was a possible arc over on the primary before becoming an open.? ? One idea I had was to not switch any 120V AC through the pot/switch.? Rather switch low voltage DC as the control input to an optically isolated relay (low voltage, low current through the pot/switch).? This circuit could use either the unusual as-built pot/switch which is closed when CCW, or the much more common open when CCW.? ?The 12V fan circuit _might_ be able to supply enough extra current to power a relay.? I've used relays such as in the past.? Had LMS not had the pot/switch in stock in a reasonable time, I would have done this.? ?I mention this alternative in case someone else is having issues with their pot/switch and can't find a suitable replacement.? As much of this post is only in the spirit of sharing my findings for others to find, and if someone has more insight... I'd be happy for them to share with me. ? Regards, Gary ? ?

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