[Emon]

A while ago I shorted one of the TO-220 diodes to ground, and the STEPS died. I fiddled with it a bit, but I was about to leave for school (for 10 weeks) so I didn't bother.

I'm home on break now, and since I needed to make a Mouser order anyway, I ordered four replacement diodes and a new voltage regulator incase that was bad. All the other passive components looked just fine so I thought it might have been one of the semiconductor parts. Replaced all the diodes (even the small ones near the LM317), despite the fact that they tested okay on my DMM.

Plugged it in and...nothing. Fiddled with it for a while, plugged it in again, and I get AC arcing under the line filter choke! I actually had this back in September but I figured it was another faulty component causing it. Being someone afraid of leaving a powersupply on while it was arcing, I took out the choke. It was in perfect condition, no burn or scortch marks anywhere, on the choke or on the board. I did get that typical electronic burning smell, I figure it's the insulation on the board burning away.

So then I bypassed the choke, and turned it on. Yay, no more arcing and nothing has exploded. Time to take some measurements. I get no DC voltage and about 0.3 straight from the transformer. Huh? A dead transformer? It's encased in plastic so it's kind of hard to tell. It certainly didn't "blow up" or anything extrodinary, otherwise I would have noticed.

I've replaced all the semiconductor parts and checked all the caps and resistors, and everything looks perfect. What could be causing the arcing, and why is my transformer dead? Or is it? I did some searching but this isn't a very common topic, apparantely. I don't mind ordering a new transformer, but I'd like to be sure it's the problem before I spend $17 on it, only to have DigiKey reem me with a $5 surcharge because my order is less than $25.

Anyone know what's going on? I got my DT 990 Pros in the other day and I need a working Millet to try them.

 

[Teerawit]

BTW, if you do decide to get a new transformer from digikey, just go ahead and get another $8 of stuff instead of paying the surcharge. It's like getting $8 of stuff for $3 (if you were to pay the surcharge = $22).

 

[ericj]

At this hour of the night i can't bring to mind exactly what would cause the arcing, but transformers do fail sometimes.

They often have a weak point in the winding analagous to a fuse that will create an open circuit if you short circuit the output, and maybe that's what you did when you shorted a rectifier to ground.

This is a safety feature that prevents a major meltdown and possible fire in short circuit conditions. afaik it's probably mandated for UL certification.

If i were in your situation, I'd desolder the trafo and use an ohm meter on the secondary (and maybe also the primary) to see if it's open.

Since the windings are just big coils it should read a bunch of ohms but not, you know, megohms. low kohms tops.

That would explain the 0.3vac reading, at least, in my experience.

 

[Emon]

Hmm, thanks. I'll try that out tomorrow.