Replacing Transformer - Page 1
Forum section: electronics / repair

author: cplusplus
author_rank: Newbie
timestamp: February 15, 2020, 02:54:00 am
content: I have a transformer that has a blown fuse inside that I can't get to because the laminated core is welded shut. I've attached it's schematic.Is it correct to say that there is a single primary winding and three secondary taps that are center-tapped? I'm unsure of what is going on for the fourth tap on the top of the primary side. The voltage leaving there is around 15v and they come straight out of the top of the transformer while everything else is through-hole.Thank you for your time.

author: bob91343
author_rank: Super Contributor
timestamp: February 15, 2020, 05:23:17 am
content: It's just drawn that way for convenience. Yes there are four secondary windings, each center tapped, and one primary. If you can't get to the fuse, you are out of luck.Having said that, you have nothing to lose by getting aggressive with the case and cutting it open somehow in hopes of getting to the fuse. Be advised, however, that you are mucking with stuff that has safety considerations involved and you could be asking for trouble.

author: cplusplus
author_rank: Newbie
timestamp: February 15, 2020, 07:45:35 am
content: I meant to include that looking for the fuse was the first thing I tried. It must be pretty deep in there unfortunately.I don't think I can find an equivalent replacement for this. It looks like my options are:Use two transformers with two secondary windings external to the unitFind someone to rebuild this transformerI'm open to any suggestions or ideas I'm not aware of.

author: Chris56000
author_rank: Frequent Contributor
timestamp: February 15, 2020, 01:17:00 pm
content: Hi!Could you give us a model no and / or a link to the Service Manual please?The Chinese now offer a good range of "R Core" mains transformers suitable for 115/230V supplies in a very wide variety of power rantings and voltages, with two or more secondaries, and I'm sure I can draw up a repair suggestion from the Service Manual for you!Chris Williams

author: cplusplus
author_rank: Newbie
timestamp: February 15, 2020, 08:40:31 pm
content: The SM is not uploaded on the internet unfortunately, but I have uploaded a more complete image I chopped together.

author: james_s
author_rank: Super Contributor
timestamp: February 15, 2020, 08:50:14 pm
content: I repaired a transformer once that had the fuse blown by a lightning strike. It was on the top surface of the windings under just a few layers of tape, I did not have to do any major disassembly to get to it so you might try that first. My grandmother used the dishwasher for another decade after that and it was still in service when the house was sold after she passed away.

author: bob91343
author_rank: Super Contributor
timestamp: February 16, 2020, 11:03:40 pm
content: There is no magic. Just keep digging until you find it.Another approach is to find the winding end that goes to the fuse and bring it out separately. If there are many primary turns you can afford to go to a place on the winding instead of trying to find the end.Of course the whole unit may not be worth all the effort.

author: andy3055
author_rank: Super Contributor
timestamp: February 17, 2020, 01:51:18 am
content: Can you post a picture?Also, most often, the primary winding is the inner and the secondary is on the outside. According to the schematic, the fuse is on the primary and this means you may have to undo the secondaries before you get to it. That is if you can get the covers out!But sometimes, the fuse is inserted close to the core but can be accessed from out side top or bottom.

author: cplusplus
author_rank: Newbie
timestamp: February 17, 2020, 02:57:39 am
content: Quote from: bob91343 on February 16, 2020, 11:03:40 pmExciting news. There are several through-hole posts on the primary side of the transformer. While examining it, I noticed that there was an additional wire around two posts next to each other. I ran a jumper between them on the bottom of the PCB and got power again on the secondary side! I ran it for nearly an hour with no issues other than the transformer itself becoming a little warm. How can I make this safer since I can't get to the fuse inside?

author: coromonadalix
author_rank: Super Contributor
timestamp: February 17, 2020, 03:27:28 am
content: Normally if you bypass the thermal fuse .. the simplest thing is : to have an primary fuse in case of ........ i would add one in the primary circuit if the schematic is the right one.Done that in the past, never had any problems.

author: Electro Detective
author_rank: Super Contributor
timestamp: February 17, 2020, 09:38:20 am
content: I'd be very curious/wary if jumping it bypassed a fuse, or tapped the transformer differentlyi.e. running warm may result in running cold eventually

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 17, 2020, 09:40:23 am
content: How about sticking in one of those thermal fuses like is used in fan heaters and toasters etc in place of the busted one?Just stick it in place with some silastic in the corner between the inner windings and the core.