Sunko 709 Modifications: due to bad planning

barry_rawn

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Jan 12, 2021
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Folks, have to confess that I ordered a 110V version of the 709A, even though I'm living in a country with a 220V 50Hz supply.

I opened the unit to have a look at the transformer, and I saw that the circuit board that probably implements the single phase triac


is labelled 110/220V.

Question is: does anyone know if I have a hope of modifying this PCB (assuming I could get it out) to be happy with 220V? I would be thrilled and willing to put in the work, because the alternative is dropping 200-300USD for a high-rating step down transformer, or stubbornly spending 100 USD to build one, which probably won't be as good.

I have also submitted a question to Sunko itself but I don't know how responsive they are.

sheepishly,
Barry
 

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LithiumSolar

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I don't have a direct answer to your question, but I've seen several people point out this on the transformer now. What I don't understand is, if it's as simple as soldering to a different terminal to use a different winding... why would they not simply put a 120/240 selector switch on the device rather than making two separate versions? :/
 

barry_rawn

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I don't have a direct answer to your question, but I've seen several people point out this on the transformer now. What I don't understand is, if it's as simple as soldering to a different terminal to use a different winding... why would they not simply put a 120/240 selector switch on the device rather than making two separate versions? :/
Seriously that would be a big help. It's possible they weighed this, and decided the cost of the switch wasn't worth the number users who would care. For them, this is just a slight variation in assembly process I guess. ..

..but facing the mod, I do tend to think, better the factory person who does this all day, than me, making this adjustment! It's really not idea, but it's a huge relief. Will let the forum know how it goes.
 

barry_rawn

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I have the 240V version. I will open it tomorrow and take a picture of how it's connected to compare to yours (assuming nobody beats me to it by then).
Wow, fantastic! Thanks, that would give me more confidence to make the modification, and it would be helpful to others in the future.
 

RolandW

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Looks like the transformer has 2 windings already. Does that transformer step up or step down to the secondary coil? You could measure the resistance of the coil which is used now, and find the other one, which should have around double the ohms if stepping down, as the 220v needs more windings to do that, or it should have half the ohms if the secondary side is stepping up...
 

barry_rawn

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Looks like the transformer has 2 windings already. Does that transformer step up or step down to the secondary coil? You could measure the resistance of the coil which is used now, and find the other one, which should have around double the ohms if stepping down, as the 220v needs more windings to do that, or it should have half the ohms if the secondary side is stepping up...
It seems like we can reason from the schematic Jon of "jonshobbies.com" provided https://www.jonshobbies.com/modify-sunkko-737g-from-120v-to-240v-operation.html (also see attachment) There are two input windings and one output winding. The circuit board decides whether the input windings are connected in series or parallel. It's kind of elegant: we know the power coming across the magnetic interface is the same either way, but their design means you either have twice the voltage step down, or you have a 110V appropriate step-down with twice the current rating. Pretty cool.
 

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barry_rawn

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Damn although I have the 709, and not the 737. The SUNKKO people wrote back saying this:

"Thanks for contacting us.
I'm afraid no. The mainboard and transformer for 110V and 220V are different."

So since they're responsive I sent them Jon's site and my photos and asked them if they're sure. :LOL: Let's see

I guess the rational thing would be to maybe disconnect some of the wires and perform some tests to check whether or not the transformer has those multiple windings or not.
 

Cypher618

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Jan 14, 2021
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I have the sunko 737G which I have modified to run on 240V from the pictures from johns hobbies.

I also added my own spot welding leads. I made them out of 1AWG MTW. I stripped back about 1/2" of the cable, stuck the sunko tips in the wire, and soldered them in. I turned the transformer around then added a terminal strip and a circuit breaker in the machine. In my case this was needed because my 240V plug is on a 50A breaker. Changed the plug on the unit out for an L6-50P. Works quite well now.

No magic smoke yet. I would only recommend doing this if you are knowledgeable on and comfortable with mains voltages. Do not work live and modify at your own risk!
 

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