Help, Micromill Smartstep/3 control blows fuse!

[milton d]

Hello all, my Micromill with Smartstep/3 control board died this morning in the middle of a big job; it blows the F1 on-board fuse repeatedly.

I examined the board and it looks like the device pictured below, a KBU806 rectifier may be fried. The only spot on the board that looks like it's been hot is where it's soldered on by its 4 pins. The solder around those pins has obviously gotten to the melting point and all other pins on the board look OK, slick & shiny.

The question is whether something else caused it to fail or did it just die of old age. I was worried about the big capacitor next to it because the cap is bulged but I posted that question on another forum and some posters said it was probably OK. It's been working fine for almost a year in light use. When checked with an ohmeter, it measures low resistance for a moment, then high. I think that's how you test 'em without a proper tester? Would a fault with that capacitor overload the rectifier?

I plan to replace both the cap & the bridge rectifier on Monday if I can find them locally and hope that fixes it. Anybody have experience/knowledge with this problem?

Lastly, if the poor thing is tango uniform, does anyone have a used one (controller board) they want to sell? I'm in the U.S.

Cheers,
Milton

[angel-tech]

bridge rectifiers tend to run a bit hot, check across the pins for shorts.

[milton d]

Can it be tested without removal? Could the ohmeter damage other components?

I tried to desolder & remove it but couldn't. I will have to crush it and remove the pins individually to replace it.

[angel-tech]

no probs with a meter, you should be able to see each seperate diode, any reading under 20 ohms should be a fault. Have a look at the face of the main power driver chips (L298), as they can fail and it usually shows as damage to the front face of the chip. The two in the pic look ok.

[angel-tech]

you'll need a good strong hot iron to take out the bridge rec, as it tends to dissipate the heat very well.

[milton d]

Thanks! All three driver chips look good as new so I'm hoping they're OK.

I'm having trouble getting all 4 pins hot at the same time to pull it out so I'll try test it in-situ so as to not destroy the board trying to get it out.

'Tis a moot point I reckon as I'm going to get a new one Monday & destroy the old one. It'd just make me feel better today & tomorrow if I knew the old one is bad. There's a big, money-making job hanging on getting it running again!

[angel-tech]

to get it out with a single iron, you have to heat the first two legs and tease it out a bit, then repeat with the other two, and on and on until it works it's way out. Applying new solder improves the heat conductivity and helps removal.

[angel-tech]

if you have to destroy the bridge rec to get it out, you'll probably damage the board a fair bit as well.

[milton d]

Hmm, I'll try some extra solder 1st. I was thinking that a big pair of Vise-Grips would crush the plastic case leaving the internals exposed so that they could be clipped out carefully leaving the wire pins exposed to be removed individually. Mind you, I've never actually done that before but it seems plausible.

[angel-tech]

That may work

[milton d]

I thought I'd post the results of my struggles in case anyone has a similar issue.

I got it fixed and working well by replacing the bad rectifier with an 8 amp part from Radio Shack...5 min from my house and $2.59 + tax! https://www.radioshack.com/product/index ... Id=2062583

It was configured to mount in the horizontal mode rather than vertical as on the original but I was able to bend the pins around to fit the original holes and solder it in easily. I removed the original part by slicing it off with a Dremel cutoff wheel horizontally as close to the board as possible, then slicing it vertically leaving 2 separate pieces with 2 pins each. It was then easy to desolder and remove the 2 pieces.

After putting it all back together, I ran parts for a while & checked the temperature of the new part and it was around 150F. The old one died from heat so I added a chunk off a old treadmill speed controller and it now runs 85F.

[angel-tech]

sorted

[Denford Admin]

Nice one - thanks for keeping us informed.

That area on the base board around the stepper chips does get really hot and needs a good airflow.

[milton d]

That area on the base board around the stepper chips does get really hot and needs a good airflow.

Yep, after running a couple of hours, I noticed they were running hotter than I liked so I added an additional small 12v computer fan that blows across the rectifier heatsink and onto the driver chips. That keeps everything below 90 deg F. I'm hoping that will help it last for long time.

[angel-tech]

you could mount the bridge rec on the main heatsink and hard wire it to the board