Buggeration - Blew My Mc40 Drive, Any Ideas?

[mattthemuppet2]

hi all,

moment of great excitement today - wiring up a treadmill motor that I'm almost ready to put on my drill press. Wired up the plug>fuse>on/off switch>board with the trim pot and motor plugged into their respective spots.

Nothing, not a sausage.

Checked that there was AC going into the board but no DC out. Tried the motor on my lathe drive, worked fine. Tried the drive on my lathe motor, nothing. So at least the motor's fine.

Kept having problems picking up AC at the board, so decided to wire the board directly to the power cord. Trim pot at off. Plugged in the power cord - FLASH - unplug power cord.

Two diodes (1N4005) blew. One wired to each leg of the AC input, with their output legs on a common connection

They're the 2 diodes on the left, under the middle yellow wire. In happier times


close up of blown diodes


diodectomy


So, I'm guessing that these diodes are some kind of protection for the control circuitry on the board as the SCRs are wired directly to the 2 legs of the AC input and DC output. Other than that, I've no clue. I have a couple of suitable replacements, but don't want to put them on the board only for them to blow too. Any ideas why this happened? Why when directly wired into the mains but not via the plug and switch (have to DMM that set up to find out what's going on too)? The trim pot was at the off position, so there shouldn't have been any current flowing through the board. No obvious shorts or issues elsewhere.

thanks!

 

[British Steel]

Hi Matt,
I'm not familiar with that board, but the KB KBIC boards have the speed control pot terminals at about half supply, shorting them to earth (ground) will do similar damage to the supply to the control electronics...

Dave H. (the other one)

 

[John Hasler]

I'm guessing that those are rectifiers for the low-voltage supply for the control circuitry, but I'd need to see the board to say for sure. Everything is isolated from the case, right? Are the tabs of the SCRs isolated from the heatsink? Some SCRs are internally isolated, while others have the anode tied to the tab.

 

[mattthemuppet2]

Hi Matt,
I'm not familiar with that board, but the KB KBIC boards have the speed control pot terminals at about half supply, shorting them to earth (ground) will do similar damage to the supply to the control electronics...

Dave H. (the other one)

Thanks Dave. No idea to be honest - I used a pot that came from the treadmill with that controller (or similar MC model). I can check to see if it's behaving as intended

I'm guessing that those are rectifiers for the low-voltage supply for the control circuitry, but I'd need to see the board to say for sure. Everything is isolated from the case, right? Are the tabs of the SCRs isolated from the heatsink? Some SCRs are internally isolated, while others have the anode tied to the tab.

Thanks John, that'd be my guess too - all the SCRs are off on much thicker traces, including ones connected by those yellow wires. The tabs are not isolated from the heatsink, just thermal compound same as the original heatsink/ shelf it came attached too. I'd assume that it would be safe to use them as they were used originally, but I can check the traces to double check. I do remember that a couple of the SCRs have the tab/ middle leg soldered into the same trace as one of the outer legs, but that doesn't necessarily mean the tab is live or does it?

Possibly silly Q - does it matter which way round the AC wires are? I wouldn't have thought so, just wanted to check.

 

[John Hasler]

It doesn't matter which way around the AC wires are as long as the cirucitry is isolated as it should be. Was the heatsink in the original assembly grounded? In any case I suggest that you unscrew the devices from the heatsink and use your ohmmeter to check for continuity between the tab and each pin of each device. If I had the information printed on the cases of the devices I might be able to look them up for you.

At least one of those devices is a diode, by the way.

 

[mattthemuppet2]

neat, thanks John! The board was originally mounted direct to an aluminium heatsink which in turn was mounted to the steel frame of the treadmill. The ground was bolted to the steel frame.

I had a look at the SCRs/ things again and several of them have each of the 3 pins connected to a different trace, if that helps. I'll read off the device info in a bit (working, hammock, cat on lap, hard to type!) and also remove the heatsink to check continuity.

Also checked the trim pot and it's behaving as it should - full resistance (9.5k) across the red and black terminals, from 0 to 9.5k from white to red/ black depending on rotation of the pot. direction obviously depends on red to white or white to black. plug and switch also checked out fine too. Don't know what I was doing wrong before.

 

[mattthemuppet2]

woohoo! Fixed it!

First, no continuity between middle leg and tab.

Second, replaced the diodes with some 1N4002 diodes I scrounged from some random piece of electronic equipment.

Third, it works! Well, just for a little bit until it blew the tiny fuse in the fuse holder I think it was a 3A one, but I'd have to check. Didn't have anything bigger in that size (GMA), so soldered an AGC 5A fuse to the blown one and popped it back in. Yeah yeah, it's not going to stay that way Anyway, works - from 0 to full speed. Woohoo!

Quick Q for the electronics gurus. The 1N4002 is rated at about 1/6 of the max reverse voltage as the 1N4005. Is it safe to leave them in or should I find some 1N4005? I have two 1N4007 (almost twice the rating of the 1N4005) I can put in - is higher better in this case? I'm guessing yes, as they're not meant to be fuses (!), but guessing and electronics got me electrocuted once so I'm a bit wary of that.

 

[John Hasler]

Those 1N4002s are 100V diodes. Replace them. The 1N4007s will work fine. Check around under the board and make sure that there isn't a component lead or something making intermittent contact with the case. Something had to be shorted to draw enough current to smoke those diodes.

 

[mattthemuppet2]

thanks John, will do. I tested it outside the case, although there shouldn't have been any possible routes to a short you never know.

Checked it again this morning - no start. Double checked connections and fuses, all good. Then suddenly it starts in the middle of the trim pot range and blows the fuse. Bletch.

I'm starting to wonder if this board was from the treadmill I found that nearly spat my wife off the back into the bushes. Good for me at the time (I got to dismantle the treadmill), perhaps it's now coming back to bite me. I think I'll take a ride out to the scrapyard and see if they have any treadmills in

 

[mattthemuppet2]

well I replaced the fuse, then it worked, then it didn't. Rather than spend a bunch of time stumbling around scratching my head I bought a working replacement for $30. I can use the other one for spares. thanks for your help everyone!