I'm sorry that the new BT151 thyristor did not solve the wirefeed problem. It was just a guess. These SCR-based DC motor speed control circuits are simple but not easy to troubleshoot. I would start by using a meter on Diode Test to check the semiconductors, and on Ohms to test the resistors. A defective resistor is usually visibly damaged. If it has gone short or open-circuit, your meter will indicate that. For the diodes, expect "0.400" to "0.700" in the forward direction, and "OL" or "1." with the meter leads swapped over. The Diode Test range is indicated by the symbol for a diode. With most faults being open or short, usually you can get a reasonable go/no-go indication with in-circuit tests. If you get spurious results, this may be due to other components in parallel. If in doubt, unsolder and lift one leg of the component under test. For the MPSA42 transistor, that would mean all 3 legs. That transistor is available in single units:
I would start with the diodes. There are five of them. Often it's the Zener diode ( DZ1 ) that fails.
A very quick initial sanity test would be to short the thyristor and confirm that the wirefeed motor runs constantly at full speed. To do this, solder a short wire link across the cathode and anode of the BT151. With the flat metal tab at the back, that is pins 1 and 2 counting from the left. Then reinstall the board, plug in, power up, and squeeze the torch trigger. The motor should run flat out, regardless of the setting of the wirespeed pot. Then remove the link and troubleshoot the components on the board, as above.
Regarding the relays, as mentioned in my post #22, I am sure that they are fine. You proved this when you confirmed that the gas valve and the welding power ( arc ) operate when you press the trigger. The only fault is in the wirefeed, and the relays are not part of that circuit.
In step #28 you were measuring the voltage supplied to the relay coils. They operate on 24v DC, so I am surprised you only saw 1.5v.
In steps #30 to #34, you were measuring open-circuit voltage ( AC mains ! ) and closed-circuit continuity across the normally-open relay contacts ( the thick PCB tracks ). The relays ( two in parallel ) are just a switch, so with power on and the torch trigger not pressed there was no continuity, and there would be 230v AC across the open relay contacts. To answer your question #31, with the trigger squeezed, the relay coils pull in, the contacts close, and the 230v that was across them when they were open gets fed to the primary of the main welding transformer, so the voltage across the contacts drops to zero.
Regarding the circuit diagram referred to in my post #15, it does not apply exactly to your machine. The 2-pin connector CN1 on the PCB is not shown. There is no source of "standby" power on your board, so I assume that a small 24v AC transformer is mounted somewhere off-board, and connected via CN1. This would explain the lack of mains input pin JP1 on the PCB.
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A very quick initial sanity test would be to short the thyristor and confirm that the wirefeed motor runs constantly at full speed. To do this, solder a short wire link across the cathode and anode of the BT151. With the flat metal tab at the back, that is pins 1 and 2 counting from the left. Then reinstall the board, plug in, power up, and squeeze the torch trigger. The motor should run flat out, regardless of the setting of the wirespeed pot. Then remove the link and troubleshoot the components on the board, as above.
Regarding the relays, as mentioned in my post #22, I am sure that they are fine. You proved this when you confirmed that the gas valve and the welding power ( arc ) operate when you press the trigger. The only fault is in the wirefeed, and the relays are not part of that circuit.
In step #28 you were measuring the voltage supplied to the relay coils. They operate on 24v DC, so I am surprised you only saw 1.5v.
In steps #30 to #34, you were measuring open-circuit voltage ( AC mains ! ) and closed-circuit continuity across the normally-open relay contacts ( the thick PCB tracks ). The relays ( two in parallel ) are just a switch, so with power on and the torch trigger not pressed there was no continuity, and there would be 230v AC across the open relay contacts. To answer your question #31, with the trigger squeezed, the relay coils pull in, the contacts close, and the 230v that was across them when they were open gets fed to the primary of the main welding transformer, so the voltage across the contacts drops to zero.
Regarding the circuit diagram referred to in my post #15, it does not apply exactly to your machine. The 2-pin connector CN1 on the PCB is not shown. There is no source of "standby" power on your board, so I assume that a small 24v AC transformer is mounted somewhere off-board, and connected via CN1. This would explain the lack of mains input pin JP1 on the PCB.