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Scott Welder - portable 135A petrol engine stick welder

Scott welder

My friend and neighbour Bob recently gave me an old Scott 135 Amp petrol motor welder, which he had been given by his old employer Flextool. I’ve had some trouble getting this welder going, and I’ve found very little useful information online to help me.
With the welder, I was given the Maintenance Manual. I’ve scanned the manual, and publish it below so that it’s available for anyone else it may be useful for – see images below my text.

Why a motor-driven welder?

As we live on remote solar power (not connected to mains power), we have limits to how much power we can draw from our electric system. This means we can’t run a welder from our 240V power supply (we also can’t run a standard electric air compressor). For many years I’ve had a petrol motor-driven arc welder for metalworking. This is way less convenient than mains welders: noisy, can be hard to start, etc., but it keeps my workshop capable.


The first problem, after getting the petrol engine going, was that the welder didn’t make a spark – no weld power at all. It made sense to me that this would be due to loss of residual magnetism in the alternator – I’ve had this before in my other petrol motor welder after overhauling. Re-exitation of the alternator was achieved very quickly and easily by connecting the negative lead of the Scott welder to the positive terminal of a lead-acid car battery, and touching an electrode in the (positive) welding lead to the negative terminal of the battery. There was an instant arc and from then on the welder kept giving an arc.
If left for a day or so, the alternator seemed to lose magnetism and would take a minute or less to self-excite. In the wiring diagram I found there was an excitation wire from the welder to the motor magneto kill switch, which seems to trickle a little current to overcome the need for residual magnetism. I reconnected that and excitation doesn’t seem to be a problem any more.

Lack of welding power

Even though I had an arc, it was very weak at first. I could just run a bead, maybe 60 or 70 amps by my estimate. But certainly not good enough to use on jobs.
Using the Maintenance Manual I followed the trouble shooting instructions. They indicated that the problem would be in the control circuit, as the open circuit voltage of the welder was only 45V. After checking everything else, I finally found the problem was in the capacitor in the control circuit. I replaced this (with an oversized capacitor from a computer power supply) and the welder then worked fine.
Now the welder has more than enough welding arc current – too much for the engine to maintain at higher settings. The 7hp Mitsubishi is unable to maintain rpm on higher currents, even when governed at 3600rpm. The welder as it currently runs is good for 2.6mm electrodes, but can’t power a 3.2mm electrode for more than a few seconds.

DIY welder?

I wish I could say this unit has shown me how to make another one from a big alternator. Unfortunately the windings in this one are pretty highly customised. The key thing seems to be the special windings added to generate the field current, separate from the main power windings. Some online methods use another alternator to generate the field current, which seems to work but makes for a complex unit.
Note the field current windings, in thinner wire, inserted beside the main windings, foreground
This is the welding circuit inside the box (sorry to cut off the edge). the unused circuit with fuses is the old 120V accessory power circuit. The big white heatsink blocks of diodes connect straight to the electrode terminals. 

Along with the welder, I was given the Maintenance Manual. I’ve scanned the manual, and publish it below so that it’s available for anyone else it may be useful for.

To download the Scott 135 Amp Welder Maintenance Manual as a pdf document use this link:

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