Repairing a Smootharc Multiprocess 175 Welding Machine

[Low Fat Ergos]

I've been given a BOC Smootharc 175 Multiprocess welder that released some magic smoke...

Apparently, the welder was used a few times then sat in storage for a few years, and when it was turned on again after a long time some smoke came out of the case when it was just turned on. Now the machine looks pretty dead; the fan doesn't turn on and none of the LEDs on the front panel turn on.

I checked the switch, harmonic filter, and input bridge rectifier (2 in parallel) and they all seem to be working OK. I thought it could be a dried-out capacitor, but it looks like the input caps are in good condition. After turning it on briefly it looks like the caps charged up as I can short them out with a screwdriver. I looked at the high voltage side of the isolation transformers going to the low voltage side and there seems to be voltage there, but there are LEDs thee also that don't light up (I'll have to try and reverse how these are connected). Initially, I didn't see any obvious charing, burned-up traces, or heat damage, but then I found C11 which was a tantalum cap with a hole burnt in its top forming an almost dead short (removed in the photo). I took it out and tried powering the device again but didn't seem to get any voltage on the LV side over the capacitor.

The welder looks to be some pretty generic Chinese rubbish so I don't think the brand will actually be that helpful in finding a schematic, but someone might recognise it under a different name. I'm wondering if anyone has the schematics (or service manual)? My plan was just to start looking at LV (DC) voltage for the control/logic side of the circuit and see if I can find anything else that's blown, though the conformal coating is making probing around a bit tricky. I'd expect if the capacitor was shorted out there will probably be a linear voltage regulator or something along those lines that needs replacing too. Though, what would cause a capacitor to fail in this way? There's a relay on the HV side which I think is part of a soft starter circuit, which should prevent a large current draw/voltage spike at startup.

[t1d]

I see this is your first post. Welcome!

Firstly, welders involve high voltage that can kill you. If you are not qualified to work on such equipment, stop and seek professional help. I assume no liability for any risk that you take.

For example, discharging caps... particularly high voltage caps, like what might be in a welder, with a screwdriver is bad and dangerous practice. It can weaken and damage the cap. It can arch and throw molten metal into your eye, etc. Yes, discharging the caps, before working on any electronic device is essential. But, you need a proper discharging tool to do that. They can be made easily with a bit of wire and high value/wattage resistors. But, you need to acquire that tool, before continuing.

With the liability stuff out of the way, let's see if we can get you better positioned to get the help you need. Firstly, you have taken a general and casual approach to writing your post. Therefore, the best that you can expect is general and casual answers. And, this forum tends to deal in specifics. Meaning, your post will more likely be passed over.

So, let's get some details and give purpose to your path... Firstly, we need to know about you... Your electronics knowledge and skills levels. What tools and resources you have, or can get access to. Details about your multimeter, soldering iron, oscilloscope, etc.

You have obviously done some tinkering around and visual inspection. That's a good place to start. But, as that didn't reveal any easy fix, you might want to formalize your approach...
- Start by replacing the known bad Tantalum cap. A simple cheap component can shut down the whole system. It needs to be repaired, before troubleshooting further, because it may be effecting things downstream.
- Divide and conquer... Break the device into parts and isolate the failure. My guess is that the high voltage welding side is okay and that the issue is with the control side.
- Once the failure is isolated to a zone, start with the basics... Verify the various power supplies. Start where the power enters from the wall and follow it out to the low voltage regulators.
- Take lots of written notes, as to in what order you did things, how you did the test and the results. Of particular importance is the exact symptoms you are seeing. No fan, no LEDs, but, if the welder is a wire-type, will the wire advance? Report that back to us.
- With that information, we will continue to divide and conquer, breaking the circuit down and eliminating possibilities. Eventually, we will hit the jackpot.

I hope that helps. My true intention is to get you the help you need. I do not mean to seem as if I am talking down to you. We just need some information. I look forward to working on this project with you.