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trying to convert a child toy from 4 D batteries to 6v AC supply. However, I can't get the toy to work when I attach the wires to the battery locations. I can see what's the positive (flat) and negative (spring) sides.

I'm pretty I'm getting power since I'm getting 6.2 when I attach the multimeter.

It looks like the battery leads are all attach to a single piece of metal so I'm not sure how positive/negative are even determined.

Thoughts on how I can connect this? Any videos out there that show me how to do it?

You need to convert your 6V ac to dc using a bridge rectifier https://en.wikipedia.org/wiki/Diode_bridge

Another method is to use a "wall-wart" available from most hardware stores for a fiver. Be sure to get the dc voltage and polarity right.

After converting it to DC and adding a few decoupling capacitors, when you do attach them, they must be attached at opposite ends of the battery holder because the batteries are connected in series. This means that you attach the positive to positive (flat) at the left-most battery position and the negative (spring) on the right-most battery position. If that doesn't work, try reversing positive to the right-most and negative to left-most battery positions.

Bear in mind that adding decoupling capacitors changes the voltage. You may end up with the peak voltage which is root two times the rms value.

Best to talk f2f with an expert, local college lecturer or ham radio guy.