Check/repair 120/100v step-down transformer? - Page 1
Forum section: electronics / repair

author: CZroe
author_rank: Contributor
timestamp: February 17, 2022, 04:59:37 pm
content: I needed a high-wattage 100v step down transformer for a fixture intended to test rare/valuable Japanese 100v electronics. I managed to find a 2,000w VT-2000J from VTC for about the price of the 500w version being sold as “open box” on eBay.When I picked up the package it felt like a large and heavy rock was freely tumbling around inside. Inside the box was a form-fitting styrofoam enclosure that should not allow the contents to move around. While trying to slide that out of the box a washer and a small rock chip fell out of the package, making me think that there really was a big rock rolling around inside.To my surprise, I found the transformer housing secure but with something heavy rolling around inside. I still half-expected to find a rock in there since the rivets that secure the transformer were removed. That would also explain the chip of rock that fell out.There was a large dent on top emanating from the inside caused by whatever was banging around loose. The mains switch was dangling out the back, seemingly unable to fit when you try to snap it back in to the panel. Along with clear signs that this was from an return lot sale (“Amazon Warehouse” tape + salvage store labels) everything just SCREAMS “return fraud.” Perhaps someone tried to gut it or swap the innards with a lesser transformer for a fraudulent return to Amazon. Amazon subsequently sold the return lot to a salvage dealer and the salvage dealer had it in their store and on eBay.I opened it up on camera:https://youtu.be/Otfv3PNkfDUNo rock. Definitely a transformer inside, though I don’t know how to tell if it is correct. Is there any way to tell without plugging it in?I don’t want to take the L here but I do want to keep the transformer if it’s the correct one. I’d rather fix it than force the seller to take the complete loss with return shipping. It’s dangerous to plug it in to check without repairing it first but I need to know ASAP so I can decide whether to repair or return. Is there a safer way to use my meter to check?I still need to notify the seller but I’d like to know if it’s the right transformer first in case I can offer a solution. Thanks!

author: CZroe
author_rank: Contributor
timestamp: February 17, 2022, 05:04:43 pm
content: It was pretty banged up in there but the only stray wires are from the busted indicator lamp. The mains switch does not fit back in to the panel housing and the red wire is badly frayed from being wrenched around with the dangling switch. The soldering in the switch is atrocious so it’s pretty clear that someone swapped it.If it’s the right transformer then the repair plan would be to replace the broken lamp and lens with another 7mm 220vac neon lamp, replace the broken Alco fuse holder, straighten the bent mounting tab on the transformer, check/secure all connections, mount transformer with new rivets, then mount the mains switch to the panel even if I have to widen the opening. Finally, I’ll make sure nothing “hot” has continuity with the housing before I plug it in to test voltages. Sound good?Also, some of the plates on the transformer look like they got banged and misaligned. Guess I should loosen the screws to see if I can line those back up.

author: FIXITNOW2003
author_rank: Regular Contributor
timestamp: February 18, 2022, 08:55:00 am
content: if the transformer lines up with the rivet holes chances are it correct ,as a smaller transformer would have different mounting holes ,have you metered the windings to check its not burnt out ?

author: SeanB
author_rank: Super Contributor
timestamp: February 18, 2022, 11:38:02 am
content: Looks more like the box was dropped in shipment, and the transformer broke loose in the case. Probably fell out of a truck, and they sent it back as damaged after opening on the ground, and seeing the dents. Screws probably pulled from impact a few more drops later on and likely at install they were half stripped anyway. Case a can of matt black will fix it, knock the dents flat with a large and small hammer and a cloth to protect the metal, and it will work.Switch and fuse holder typical sweat shop soldering, just they used the cheapest ones, which are pretty poor and break, and the indicator was taken out by the transformer. Disconnect the indicator and insulate the wires to it temporarily, temporarily put in a fuse holder and 5A fuse, and plug in and turn on, them measure the output voltage, likely it will be 100VAC, if the right transformer, or 230VAC, if they put it in the wrong way. then you can consider to buy a decent switch and fuse holder, solder in the fuse holder, and use spade connectors for the switch, and heat shrink sleeving over the connections after installing, and use it. Use small nuts and bolts, along with washers, to hold the transformer down, not the tiny rivets they used.

author: CZroe
author_rank: Contributor
timestamp: February 20, 2022, 12:05:55 am
content: Quote from: FIXITNOW2003 on February 18, 2022, 08:55:00 amThanks, but that’s exactly what I was hoping to find information on how to do. I have no idea what measurement I’m looking for. It seems to be an auto transformer with only one winding and a center tap. Two of the three taps appear To be connected at the input voltage selector switch. The other measures 1.2ohms with the first two.That sounds low but it’s EXTREMELY heavy gauge, so maybe that’s normal for such a transformer that only needs to step down 117v to 100v? Flipping the input voltage switch doesn’t seem to change anything.As for aligning with the rivets, my suspicion is that someone who burned one out may have bought a replacement, swapped the insides, and returned the bad one.I was confused by it having no secondary winding but I googled and came across this highly-abrasive explanation:https://www.doityourself.com/forum/electrical-ac-dc/467216-3-wire-step-down-transformer-230-115-where-do.html“Obviously.” LOL! Not for the guy asking… “obviously.”Quote from: SeanB on February 18, 2022, 11:38:02 amPerhaps but I’d still expect to find some remnant of the other three rivets rattling around, which I did not. I’m pretty sure someone has been in there, especially since the panel-mount switch will not fit back into the panel. It seems the exact correct size if it didn’t have the panel mounting clips… but it does. My attempts to push it back in (before I realized there was no way it would fit) only damaged the plastic that is supposed to squeeze through.Thanks for the help. I’ll be executing your plan as soon as I get back to the workshop tonight.Edit: We’ll, I just noticed that they had hot going to the wrong terminal of the fuse holder. Since the holder was broken and the fuse was pushing itself out it’s yet another way I could’ve got a nasty shock from this thing (or worse). The end cap was falling out with the outer contact still “hot” should the fuse touch it in the way out while switched on, as shown here…https://youtu.be/bvrO7g7BnIA

author: bob91343
author_rank: Super Contributor
timestamp: February 20, 2022, 03:44:02 am
content: It all depends on what knowledge and equipment you have. First, measure the winding resistances with an Ohmmeter. If all connections appear to be reasonable, you could measure the inductance with an impedance bridge. Failing that, you could apply power to the highest resistance winding connection, via a variac while measuring the current draw.In other words, the more you guess and the more you take for granted, the more likely you are to not get the desired result.

author: CZroe
author_rank: Contributor
timestamp: February 21, 2022, 02:26:38 am
content: Quote from: bob91343 on February 20, 2022, 03:44:02 amOf those, all I have is the ohmmeter with no idea what measurements would be “reasonable.” There is only one winding and I only got about 1.3 ohms. That said, I bypassed the mains switch and replaced the fuse holder and it gives me ~115v on the 120v outlet with 98.8v on the 100v outlets. I think we’re good.Already testing some Japanese PlayStation with PS1Digital and XStation before I send it back to a guy from the UK. Next up is a Japanese-exclusive NEC SuperGrafx with NEC ROM² Amp. Thanks, guys!

author: bob91343
author_rank: Super Contributor
timestamp: February 21, 2022, 02:35:15 am
content: Good work CZroe! Some luck was involved too, I am sure, but the bottom line is what counts.

author: CZroe
author_rank: Contributor
timestamp: February 21, 2022, 02:59:58 am
content: Quote from: bob91343 on February 21, 2022, 02:35:15 amThanks. …and, yeah, f I had blindly plugged it in as received I would’ve got a nasty shock or blown the circuit breaker for sure! Would’ve been hard to miss the exposed contacts on the switch though.