Looking at your photo I recommend you check the 3 areas marked, these connections need to be clean, new washers and bolts would be a good idea, also check the ring crimps are good.
Lastly check the 3 bolted connections on the rectifer they look ok, but always a good idea to give them a clean.
Picture of complete machine here https://www.mig-welding.co.uk/forum/threads/sip-autoplus-120-repair-options-weak-spark.19487/
I have attached 2 diagrams, but I have no more info, PCB diagram sadly lacks component values.
The wiring is correct, if wrong the voltage would not increase in the 4 ish volt steps, also on High-Max you blow the fuse a soon as you pulled the trigger.
The reading might be a wrong due the waveform, you are measuring a non loaded choke smoothed supply, I supect the output is more rectified...
Yes the mod would work on the Autoplus 180 and 150 but not the 130, the 130 would need original relay removing from the PCB ands a wiring change to the mains side.
One of 2 problems 90% of time relay contacts welded and you will need a new relay or there is a short in the trigger circuit.
Possible to do a temp fix on the relays with a clear cover by cleaning the black ones cannot be fixed.
The more you do the quicker you get, the second one did I used a taper punch, missed the inner part of the bearing and got the punch stuck in the bearing cage for a while.
Fine emory and finish off with 800 and 1500 grit wet dry.
Ok on pump name the pre 2000 ABAC pumps were 2 number, B39 was replaced by the newer B3800 the other pumps gained 00 at the end. B39 was a great pump but now long gone.
The B2800 is a much smaller pump than the B3800 see below, not my image the left hand pump is a B3800B due to the aftercooler...
The wolf is fitted with lets call it a copy of the B3800, the orignal B3800 was introduced by ABAC in the late 1990s and is currently produced by FNA
The B3800B and B2800B are pumps but fitted with aftercoolers so thay can run them faster.
No idea on the B38 name bit, they have always have been...
Avo 8 Mk4, good diagram and info here http://www.richardsradios.co.uk/avo8.html
Blown section looks like it's R10 0.45 ohm shunt for the 1A current range
Sorry if my post caused confusion, should be a case of new bridge rectifier, check the capacitors, new chip and 2 new diodes, offline PWM power supplies are very simple the 8 leg chip does 99% of the work. Check the chip for its number and grab the datasheet.
You could try it with the rectifer...
The problem area looks like it could be a psu section for the main control system, I suspect it uses something like a Viper-22A 8 leg offline PWM chip, PCB connections seems to match that.
Recovered 1000s of bit, only melted a few, therma shunts not seen used for a long time, might still be needed for some devices.
Thing with smd is most are soldered on using hot air or in an oven, just dont over do the heat.
Hold the PCB vertical in the vice, heat the component leg side with the hot air gun until the solder melts, will still heating the leg side the components can but pulled off the other side and drop in to the metal roasting tin, for smds is just heat the PCB and bursh the into the tin with a...
If the pcb not to be repaired I use hot air gun (electric paint stripper) a vice and a metal roasting tin.
If I need to take more care I use de-soldering station
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Also a selction of soldering irons upto 200w, solder suckers and desoldering braid