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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.

Pulleys and belts

My welder arrived with an SPZ 750 belt: that's SPZ size, which is a narrow vee belt, which is 750mm long. I bought a few of these on ebay, but they wore out in only a few hours of work. The welder was clearly losing power from slipping belts, and costing too much in wrecked belts, so I felt the need to do some work on the pulley and belt system.I have subsequently fitted an SPA 750 belt: “A" section belts ares a couple mm wider and are stronger than Z section. This required me to change the pulley on the alternator to another wider one from an old alternator. I also did some adjustments to the main case so that the pulleys align better.

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:


  1. do u know where to get the little drive belt, cant seem to find one.

  2. Hi Danny, funny you should ask! I've been having a bit of trouble with drive belts, they've been wearing out very fast. My welder arrived here with an SPZ 750 belt: that's SPZ size, which is a narrow vee belt, which is 750mm long. I bought a few of these on ebay, but they have each worn out in only a few hours of work. I recently did some work on the welder so I could fit an SPA 750 belt, which is a couple mm wider, and stronger. This required me to change the pulley on the alternator (to another wider one from an old alternator). I also did some adjustments to the main case to better align the alternator and motor pulleys. I also made a hook which I fitted to the alternator so I can properly tension the belt (which is otherwise very difficult).
    I recommend you start with an SPZ 750 (I bought mine on ebay) and see how you go from there. It is possible you need a different length or section, but you'll find out fast. I'll endeavour to post some relevant pictures etc in my main post.

    1. i mite go with a dual row #25 chain and spockits, i read some where that these welders are under powered and the thin belts slip all the time, i have an 8hp wr motor, the amp rating wood requer about 10hp. Iam not having any problems my belt has some real small frays thats it just wonted an extra belt incase it broke. but ya thanks for the info.if the belt breaks and i go with the chain drive i will post pics. Dan

  3. well that belt broke and damaged the alt pulley, after looking at the pulleys and belt there is know way the belt and pulleys are rated for the 8hp motor. in fact they look alot like one,s i seen in the old 1 ton ibm copy machines.
    iam going to replace the with a #25H nickel chain and sprockets, they do make the dual row #25 and they make the chain in a stainless version, but its expensive. so going with the nickel version. will post pics, for anybody that may wont to upgrade the drive train.

    1. Thanks for the update Danny. I think you're right about belt and pulleys being under-rated. I'm having trouble still with the belts, the SPA belt just flipped over under load and seems to be permanently damaged.

  4. i think these welders where ment for the acousonal welder, but i like this welder and i dont think the duty cycle is more then 40% but thats enough to weld someting and then go do someting els for a wile then come to the welding.
    i vented bottom and my add a fan, i have lath and mill so i will be making something that will work.
    i have 3 larg welders but i see to use the littel welder its simple it welds smoth.will post pics.

  5. well got the alt spocket made and mounted, next the bottom spocket,
    iam going to have to make it from scratch, cant find 1 allready made for a resonable price.

  6. heres pics of the alt spocket.