How to splice and solder this damaged winding? - Page 1
Forum section: electronics / repair

author: artvandalai
author_rank: Regular Contributor
timestamp: February 05, 2025, 12:52:26 am
content: What would be the best way to repair this shorted winding on this variable transformer? Can this be spliced and soldered? What materials should i use to properly repair this?

author: benj38
author_rank: Frequent Contributor
timestamp: February 05, 2025, 09:34:51 pm
content: I guess you mean "open" not "shorted" winding.Super easy. This is just enameled copper wire. Get a similar piece of wire to splice in; scrape the enamel of the ends; solder with a regular soldering iron, solder , and rosin flux; put some lacquer on the repair to insulate it again. Done.Note that noting bad will happen if the spliced piece slightly overlaps the existing wires (which will make the soldering job so much easier).

author: Ian.M
author_rank: Super Contributor
timestamp: February 05, 2025, 10:50:11 pm
content: It looks like it may be 'cooked' by overheating. The ends of the damaged turn appear to be melted, not cut or abraded, so the first thing to do is a Hi-Pot insulation test from both ends of the winding to chassis to see if it is worth trying to salvage it.When preparing the ends to splice, you must *NOT* disturb the flattened top face of the winding as any roughness will tear up the carbon brush on the wiper. Because the break is so close to the top edge, splicing it is not going to be easy.

author: artvandalai
author_rank: Regular Contributor
timestamp: February 06, 2025, 02:36:57 am
content: Thank you mates for the help. I will post the results of the repair when i get everything gathered.

author: coppercone2
author_rank: Super Contributor
timestamp: February 06, 2025, 02:51:41 am
content: check the brush, it could be a worn brush, the reason why there is a brush is to spread the current out across wires. if you have a worn brush it makes a point contact with one wire and I think can fry it.'The relatively high resistance of the brush also prevents it from acting as a short circuited turn when it contacts two adjacent turns. 'In that case you need to replace the brush too.

author: bdunham7
author_rank: Super Contributor
timestamp: February 06, 2025, 03:00:06 am
content: Quote from: coppercone2 on February 06, 2025, 02:51:41 amIt's actually pretty much the opposite. It's all the same winding, so there aren't multiple wires for the current to flow in, all of it goes through that single strand. If you connect two of the wires, you have a shorted turn and high current in that loop. That might fry a wire. A typical variac brush is designed specifically to only contact two of the flattened windings at any one time and then to have high enough resistance in the direction between the windings so as to limit the current in the shorted turn. Stopping at a point where it only contacts one is the ideal case.

author: coppercone2
author_rank: Super Contributor
timestamp: February 06, 2025, 03:38:22 am
content: i mean to say it make point contact with one gap and there is a shorted turn, someone said it makes a over stress if this happens. there is a brass dome under the carbon puck. If it gets completely worn down it looks 'correct' when you clean it, because it does make some amount of sense that a brass dome sliding around makes a good electrical contact because it looks very similar to a switch contactso its supposed to be a carbon cylinder on a leaf spring but it turns into a dirty brass dome (blackened) on a leaf spring which turns into a clean brass dome on a leaf spring (which is similar looking to the contact rivet (silver or tungsten) on a switch 'blade') after only mild cleaning and that leaves the variac in a state that it can burn windings supposedly and if you don't read about it you will never figure that its improper. I thought that was supposed to be clean brass because the residual carbon brush came off pretty much as easily as 'switch dirt' that you find in a highly used relay or switch. I expect alot of people might come on this problem when they get a clapped out first variac for $5 on ebay that has only the afterimage of a brush on itIt just happens to look electrically correct when its totally gone and cleaned, which is a nasty design consequence. most people learn to clean switches and relays way before they get into trying to restore a variac, so its a trapI think the roller is a much better design

author: artvandalai
author_rank: Regular Contributor
timestamp: February 06, 2025, 04:28:27 am
content: While i have you guys here, why doesnt this amp meter panel give any reading when the variac is turned on? This is a different transformer which tests good with the multimeter.

author: bdunham7
author_rank: Super Contributor
timestamp: February 06, 2025, 04:51:51 am
content: Quote from: artvandalai on February 06, 2025, 04:28:27 amIs there a load on it? If there's no ouput load the ammeter probably won't budge.

author: artvandalai
author_rank: Regular Contributor
timestamp: February 06, 2025, 05:13:10 am
content: I turn the variac all the way up to 150v and it doesn't give any reading.

author: bdunham7
author_rank: Super Contributor
timestamp: February 06, 2025, 05:14:01 am
content: Quote from: artvandalai on February 06, 2025, 05:13:10 amBut you have nothing connected to its output, right?

author: artvandalai
author_rank: Regular Contributor
timestamp: February 06, 2025, 05:16:18 am
content: Yes I plugged in a 20in box fan to the output and still no reading.

author: bdunham7
author_rank: Super Contributor
timestamp: February 06, 2025, 05:20:51 am
content: IDK, if the box fan runs I'd expect a little bit of a reading. However, on closer inspection I don't actually see an indicator needle in there? If it's there and I just can't make it out, perhaps try a larger load like a toaster. Don't turn it all the way up with the toaster connected though.

author: artvandalai
author_rank: Regular Contributor
timestamp: February 06, 2025, 05:34:46 am
content: Yeah there's a needle in the pictures. im able to move it with a magnet. Ill try out the toaster and check back.

author: artvandalai
author_rank: Regular Contributor
timestamp: February 06, 2025, 05:58:56 am
content: I tried a toaster and blender and the needle didn't budge

author: bdunham7
author_rank: Super Contributor
timestamp: February 06, 2025, 05:57:34 pm
content: Quote from: artvandalai on February 06, 2025, 05:58:56 amAssuming your toaster toasted and the blender blended, then the ammeter is broken or miswired. I don't have a diagram and don't know the particulars on that ammeter, but the wiring looks pretty non-standard. Try to hunt down a manual for that exact model number and get a diagram. IDK what the rectifier is doing in this case, it doesn't look very factory.

author: artvandalai
author_rank: Regular Contributor
timestamp: February 07, 2025, 11:16:05 am
content: I found the part number Modutec 920049Analog Panel Mount DC Amp Meter10-10 Milliamperes RangeHow do i test this amp meter panel to verify it works?

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2025, 03:06:53 pm
content: Quote from: artvandalai on February 07, 2025, 11:16:05 amIt's a DC meter and your variac and mains are AC.The meter is only rated to 10mA. If used it to measure the current flowing through a toaster, then it's almost certain you've destroyed it. If you're lucky and didn't complete the circuit, then it'll be fine.Test it by connecting to a 9V battery, via a 1k resistor. It should read just under 9mA, assuming the battery is fresh.

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2025, 04:04:33 pm
content: Quote from: Zero999 on February 08, 2025, 03:06:53 pmAFAIK there's supposed to be a shunt and rectifier in there but those pictures don't look promising. Also, I'm unsure he has the right part number because it says 10-0-10mA meaning a center zero and this one isn't. I don't see a manual for his variac anywhere.

author: Gyro
author_rank: Super Contributor
timestamp: February 08, 2025, 05:09:28 pm
content: Looking at the meter internals, I think that is a moving Iron meter (backed up by the OP being able to move the needle with a magnet). The current carrying conductor passes straight through the meter, meaning no shunt required. I don't know about the 10mA reference, that doesn't sound/look right. I'm puzzled by the diodes though (there looks like another one under the meter), a moving Iron meter ought to be able to read AC or DC.The soldering to the meter terminals is terrible. I wouldn't be surprised if there was a dry joint there.Edit: I could be wrong though, the wires going to the 'device' under the meter look thicker than the diode's and discoloured. Maybe that is a wire current shunt, with the meter reading half-wave current. If there is a dry joint then there's a possibility that the unit may be outputting half-wave DC.

author: benj38
author_rank: Frequent Contributor
timestamp: February 08, 2025, 05:44:51 pm
content: Something does not look right. Can it be that the wiring is wrong? It should be super simple:https://variac.com/staco/EngineeringDrawings/3PN1010BA.pdfUnfortunately, the diagram in the URL shows the ammeter as a single two-wire part, and does not specify any internal structure.Perhaps the ammeter is a moving-magnet type. This would explain the rectifier, and the fact that the needle reacts to a permanent magnet.@artvandalai can you please figure out where the wires/leads hidden in the images behind the ammeter go to?

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2025, 07:54:12 pm
content: Quote from: benj38 on February 08, 2025, 05:44:51 pmActually I agree withGyro, it probably should have an AC ammeter without any shunt or rectifier. It would just be too complicated otherwise as either the rectifiers have to be very large or the shunt would have to have excessive drop. Whatever is soldered in there is probably not right and perhaps the meter movement isn't either.

author: Poroit
author_rank: Frequent Contributor
timestamp: February 09, 2025, 09:08:02 am
content: G'day artvandalai,Can you remove the meter ,open the case and post a pic of the internals?We can then tell if is Moving Coil or Moving Iron.920049 may just be the Plastic Case part number.

author: artvandalai
author_rank: Regular Contributor
timestamp: February 23, 2025, 03:50:24 pm
content: Will this splice repair work? I used 18awg enameled copper wire and clear insulating varnish after soldering.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 23, 2025, 04:01:52 pm
content: Looks good ... have you tested it?

author: artvandalai
author_rank: Regular Contributor
timestamp: February 27, 2025, 02:26:14 am
content: how many coats of varnish do to put on? Ive added a few coats already and the wire doesnt look coated.

author: benj38
author_rank: Frequent Contributor
timestamp: February 27, 2025, 07:32:11 am
content: Quote from: artvandalai on February 27, 2025, 02:26:14 amIf you put on two or three layers all is good. It is simply that your varnish is transparent, while the old varnish has a bit of a dark tint to it.

author: MathWizard
author_rank: Super Contributor
timestamp: February 28, 2025, 02:22:14 pm
content: Quote from: Zero999 on February 08, 2025, 03:06:53 pmSo what normally burns out in a current controlled DC meter ? Is it the fine windings that can only take a few mA ?? I'm guessing the coil is fairly high DC resistance. So is there a lot of voltage across it and the insulation is too thin and break's down ??

author: tooki
author_rank: Super Contributor
timestamp: February 28, 2025, 06:51:46 pm
content: Someone please correct me if my memory/understanding is mistaken. I would assume the wire overheats and fuses open. It’sextremelythin wire so as to get a large number of turns with very little weight.

author: coppercone2
author_rank: Super Contributor
timestamp: March 01, 2025, 03:47:39 am
content: look at it with strong magnifying glass, you can't tell with your eye. You can see the sheen of varnish under bright light and magnification to make sure its coated right. Its possible if its dirty its not coating (oil, grease), but more then likely your coating is fine and you just can't see it

author: Poroit
author_rank: Frequent Contributor
timestamp: March 01, 2025, 06:07:49 am
content: According to my workbooks....DC mA meter coil resistance is normally between 1-3 ohms and are wound from 30 B&S wire.(Moving Coil type)If you over range it you would normally severely bend the pointer plus burn out either the hairspring or coil.As @gyro posted .....yours is a probably a Moving Iron."im able to move it with a magnet"They are tougher to destroy and you would have to burn out the coil.