Maypole Turbo 120 Mig Welder MP 620 (N412337/89)

[Monty DIY]

From your measurement of the "open-circuit" primary of the small potted transformer, yes, you have to replace that component. With a new transformer there will be power on the PCB, the trigger signal for the Triac will be available as normal through the original torch switch, and hopefully the welder will be fully functional again!
( Regarding the "open circuit" : I may have been low with my estimate of 200 Ohms for the primary resistance. If it is actually 300 Ohms, that would not show on a meter set to the 0-200 range, so maybe you need to test again on the 0-2K Ohms range. Hopefully you already did that.)
To remove the transformer from the PCB, the normal method would be to use a solder sucker, or solder wick. With only 4 pins, it should be possible to just heat the joint at each pin in sequence, whilst levering it carefully upwards.
To find a generic replacement transformer, search for "12v PCB transformer", or "potted" or "encapsulated". E.g.:

Vigortronix VTX-121-3015-212 Encapsulated PCB Transformer 230V 1.5VA 0-12V | eBay

Single primary. Ambient temperature: ± 40°C - 70°C. Temperature Class B (130°C). Power: 1.5VA 50/60Hz. Dual secondary windings.

www.ebay.co.uk

Thanks, Eddie to really help me out. Much appreciated. I checked the small potted transformer again by setting the meter at different Ohms setting - 0-200, 0-2000 & 0-20K.
At 0-20K the primary reading shows 2.89 ohms but nothing at 0-200 or 0-2000. Does this change your advice to replace the transformer? Also, thanks for clarifying to MoreWellie, both of your advice have been brilliant.

 

[eddie49]

In working order, the transformer primary has to be under 1k Ohms. It may not show conduction and report a value on the 0-200 Ohms range, but certainly would read on the 0-2000 range. The result that you got on the 0-20k scale represents 2,890 Ohms, which is definitely too high. I am sure the transformer primary is open-circuit. The 2.9k value is some other component in parallel with the transformer primary, for example the zero-crossing-point detector in the white optocoupler chip.
This can be confirmed when you unsolder the transformer from the board, so that it can be checked on it's own.

BTW, the replacement transformer may not be a perfect fit, the pin spacing may differ from the original. You may have to solder short wires to the pins and bend them to fit the board layout. Additionally, there may be two 12volt secondaries. These could be wired in parallel, to share the output current.

 

[eddie49]

Hopefully the transformer you buy will be labelled or will come with documentation.
In the case of a transformer that has 2 x 12v secondaries, it may look like the one in this Radiospares listing:

44057 | Myrra 12V ac 2 Output Through Hole PCB Transformer, 1VA | RS

uk.rs-online.com

The label on the side refers to pins 1 and 5 for the primary, then 6 and 7, and 9 and 10, for the two secondaries.

In the underside view, there are just 2 pins on the left/top side. These are the 230v AC mains primary. The pin numbering is 1 and 5 due to the unused three "blank" positions between them. These two pins would be linked to the two widely-spaced solder points on the board.

For the two secondaries, at the right-hand-side, starting from the bottom right the pins are 6, 7, blank, 9,10.
To wire the two 12v secondaries in parallel, use short insulated jumper wires to connect 6 to 9, and 7 to 10. Then add a wire to each of the "6 + 9" and the "7 + 10" junctions. These two wires become the two 12v AC outputs to the closer-spaced solder locations on the printed circuit board.

Making multiple solder joints to the pins will be easier if you use thin solid-core wire, and make mechanical joints by wrapping the wire around the pin before soldering.

Depending on size and space, you may find it is easier to place the new transformer on it's side, or even upside down ( i.e. on it's back ), using longer wires to make the connections, and then glue or cable-tie the transformer to the board.

 

[Monty DIY]

Good Evening Eddie, many thanks for yet another posts from Friday & Saturday. I have been busy changing a clutch plate on my Citron as I need the car.
I find you are very passionate about your work and I much appreciate your detailed help. I have not yet had the chance to ordered the transformer and need to fully understand your instruction on how to replace one.
Do you carry out repair work? If you do, I do not mind sending it to you with return postage and pay for the service, as you will know what to do.
I will keep you posted.

 

[eddie49]

I have also got a Citroen that will need a new clutch soon - it is a Berlingo Multispace MPV. In addition, it needs new sills on both sides....
If you send the board to me, I would be happy to remove the old transformer and fit the new one ( FoC ). You could order the new transformer ( e.g. the eBay link in post #18 ) and enclose it, or I could order it and fit it here. I will PM you my address.

 

[Monty DIY]

I have also got a Citroen that will need a new clutch soon - it is a Berlingo Multispace MPV. In addition, it needs new sills on both sides....
If you send the board to me, I would be happy to remove the old transformer and fit the new one ( FoC ). You could order the new transformer ( e.g. the eBay link in post #18 ) and enclose it, or I could order it and fit it here. I will PM you my address.

Hi Eddie, just finished the final work on Citron. To support me I used to replace the clutch I used YouTube https://www.williamsmobileclutch.co.uk. The guy does it in a couple of hours. They quoted me £335 including the material to do it at home. Also, the gearbox is quite heavy, so I rigged it with a timber bar and rope to hold the gearbox.
I would be happy to send you the board and you repair it I pay for the material & service. I have very basic tools for electronic repair, so would not mind a professional like your self repairing it as you will know what else is wrong. I am not sure how we can exchange info without going public. I am not sure if there is a private messaging service in this forum?
Let me know.
Thanks
Monty

 

[a111r]

Yep. To PM, hover over the user name and 'start a conversation'.

 

[eddie49]

Monty, I have already sent my address to you by PM. You should have an alert under "Conversations" ( the "Envelope" icon, top right ).

 

[eddie49]

In working order, the transformer primary has to be under 1k Ohms. It may not show conduction and report a value on the 0-200 Ohms range, but certainly would read on the 0-2000 range. The result that you got on the 0-20k scale represents 2,890 Ohms, which is definitely too high. I am sure the transformer primary is open-circuit. The 2.9k value is some other component in parallel with the transformer primary, for example the zero-crossing-point detector in the white optocoupler chip.
This can be confirmed when you unsolder the transformer from the board, so that it can be checked on it's own.

The above statement is not correct. In fact the tiny - 1.5VA - potted transformer was OK, and it does have the surprisingly high primary resistance of 2,900 Ohms.
The fault was due to the 2N3055 power transistor, which controls the speed of the wirefeed motor. It was base-collector short circuit, and this fault also stopped the optocoupler from triggering the Triac to power-up the main transformer.

 

[Monty DIY]

The above statement is not correct. In fact the tiny - 1.5VA - potted transformer was OK, and it does have the surprisingly high primary resistance of 2,900 Ohms.
The fault was due to the 2N3055 power transistor, which controls the speed of the wirefeed motor. It was base-collector short circuit, and this fault also stopped the optocoupler from triggering the Triac to power-up the main transformer.

Hi Eddie, many thanks for sorting out the PCB. I very much appreciate your professional advice and support.
I have fitted the PCB and the Mig Welder does work now and feeds the wire.
Once again many thanks and keep the great work up.
B Regards
Monty