EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: fivesixzero on July 03, 2021, 08:45:08 pm
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Hi Friends,
A classic Volt-Pac variac recently came into my posession. It was a freebie with the caviat that it probably wasn't going to work.
Naturally, I didn't plug it in - I took it apart! 8)
Most of the device is in pretty good shape given its apparent age. The "Type 50 - 1 Core" frame/chassis (http://www.farnell.com/datasheets/2119764.pdf) is in good shape, as are the connector block on the front, the carbon wiper, the indicator plate, and the knob. Movement of the dial is silky smooth! And the previous owner mounted it to a helpful wooden panel that makes it a bit less clunky to move around the bench, which is a nice touch.
Unfortunately (as the title spoiled a bit), not all is well with this little guy. While checking it out I decided to see if resistance was predictably linear across the range and, alas, it most definitely is not. Right around the spot where the wiper bumps past the high-resistance mark, I found a single copper winding wire that snapped at its bend.
This is the first time I've jumped into attempting a repair of a variac so I've got two questions.
Is this safe to simply solder together, maybe cover with some conformal coating, and call it a day? Or is there a 'trap for new players' here that'll lead me into a dangerous or otherwise undesirable situation?
Second, is it still safe to just use the variac within the range before the wire break? If so, it shouldn't be too hard to 3d-print a rig to stop the dial's rotation past a 'safe point'.
It'd be a shame to send this core to the copper junkers, so hopefully its still got some life in it. At the very least the chassis is still in good shape and could be of use to someone that has a bare coil Volt-Pac of the same type, I guess.
Here are some photos of the device and the damaged point in particular.
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*IF* you can gain access to the underside of the toroid, its perfectly safe to splice in a shortish length of the same gauge magnet wire *IF* you do so neatly, makeing and insulating the joints in a competent fashion, and also tack down the repair with a heat resistant clear varnish. The joints should be outside and inside the toroid, not between it and the base plate. Hopefully the toroid isn't adhesive bonded to the base plate . . .
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*IF* you can gain access to the underside of the toroid, its perfectly safe to splice in a shortish length of the same gauge magnet wire *IF* you do so neatly, makeing and insulating the joints in a competent fashion, and also tack down the repair with a heat resistant clear varnish. The joints should be outside and inside the toroid, not between it and the base plate. Hopefully the toroid isn't adhesive bonded to the base plate . . .
I'm actually not sure if the bottom plate or the paper-like insulator on there is adhered to the coil just yet. I was planning on giving it a full disassembly for cleaning but ended up stopping and moving on to the next project when I found the snapped wire. Next time I'm at the bench I'll give it proper teardown and see if thats the case.
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If it isn't removable, you could glob some clear epoxy over the broken end at the base corner for insulation, and short the end flapping loose at the side to whichever adjacent turn completes the circuit (but *NOT* to both). That will get it working again but give a 'flat spot', where the voltage doesn't increase over the angle one turn takes, and cause it to run slightly closer to saturation, so its a bit of a bodge.
If it is removable but the center is filled with a bonded in solid plug to mount the toroid, post a closeup photo of the damage from underneath, and we'll see if we can come up with any other ideas.
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It looks like there is more damage to the winding than just that, I see exposed copper next to the break, and some elsewhere to the right. It was probably overloaded and blew.
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Seems like it would take a hell of an overload to blow, especially without burning the vanish off the whole thing and making a stinky mess. I'm betting on mechanical damage, take it apart and see if you can get to both ends of the break, if you can do that you can solder it together.