Tenma 72-7245 power supply dead, open transformer, fuse? :-BROKE - Page 1
Forum section: electronics / repair

author: WolfmanTech
author_rank: Contributor
timestamp: November 15, 2016, 10:47:00 am
content: Hello all, I got this supply for free while volunteering at a recycling place and only recently got it out to see what was wrong with it.I found that the toroidal transformer was open on the shared primary wire (the one common across all voltage inputs) and after contacting someone that did rewinding years back, I was told it might be the thermal fuse.Unfortunately, I have removed all three light-duty secondary windings but still cannot get deep enough to even see if there is a fuse.So, I'm in need of someone else with this supply or similar that can tell me what the voltage output of all the various secondaries might be so I can attempt to power it up with several standard transformers and see whether it's worth rewinding or buying a custom replacement transformer. A schematic would be cool too!Thanks for any/all help!

author: CJay
author_rank: Super Contributor
timestamp: November 15, 2016, 03:12:21 pm
content: How many secondaries are/were there?Did you keep count of the turns on the windings you removed?

author: oldway
author_rank: Super Contributor
timestamp: November 15, 2016, 03:51:37 pm
content: Power transformer is the most expensive part of a linear power supply.For this reason, I think that your power supply must be considered as beeing beyong economical repair ...

author: Seekonk
author_rank: Super Contributor
timestamp: November 15, 2016, 07:27:08 pm
content: Too bad, if the transformer was 110-220 with that common winding it could have been just moved. You also could have just paralleled in another transformer.

author: WolfmanTech
author_rank: Contributor
timestamp: November 15, 2016, 10:55:56 pm
content: There were three three-wire secondaries removed thus far (two being single-winding, one being dual/center-tap), I did not count the turns, I got it for free and have a collection of transformers I could substitute in so I may be able to repair it if I can find out the right voltages. I know there are folks on here that have this unit or similar, but I don't know if those members are still active! The one I checked hadn't been in since 2013, have yet to check/message the rest.

author: CJay
author_rank: Super Contributor
timestamp: November 15, 2016, 11:31:39 pm
content: Quote from: WolfmanTech on November 15, 2016, 10:55:56 pmShame, because you may have been able to use one of those transformers to power a secondary winding and worked out the voltages before you tore it apart.

author: WolfmanTech
author_rank: Contributor
timestamp: January 02, 2017, 06:46:00 pm
content: Actually, I've only removed the low-power secondaries, there are still eight wires (six pairs of filaments or four with two center-taps, not sure) and they are the same thickness as the primary windings. Would those be useful?

author: Seekonk
author_rank: Super Contributor
timestamp: January 02, 2017, 07:53:44 pm
content: There probably isn't any room in the box to put another transformer. I've seen some lighting transformers in a nice external case and you could feed that AC into the high power secondary winding of supply transformer. Those other secondaries are low current and wouldn't place much of a load. Tou didn't say what the voltage range of the supply is. You could start with a 12V transformer and that would likely work with a lightly loaded 18V supply for most of the range.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 02, 2017, 09:24:11 pm
content: Can we see a picture or two? In particular, a clear picture of the main board. As a dual-tracking, 0-30V, 0-3A, series or parallel power supply there are going to be a few seperate supplies and relays for switching things around. There could be tap switching to reduce internal dissipation. That would require more leads or windings. It could get ugly.You're going to have to find a schematic or reverse-engineer it and create one to find out what the voltages are supposed to be.Then there's the basic question of what caused the transformer to fail in the first place. Thermal fuses rarely fail for no reason.

author: WolfmanTech
author_rank: Contributor
timestamp: January 03, 2017, 05:56:22 am
content: Seekonk- Well, since it had a toroid, there's actually a fair sized gap where it used to be, so we'll see! As Ed said, it's a 30V 3A max supply, so some of the supply voltages from the trans might be at least that.Edpalmer42- Pictures incoming! There are a group of relays, so you may be right, and there are quite a few windings/taps, yes. The caps are either 63V or 35V, so that could give us a ball-park to work from (since caps are often roughly twice the supply or at least 150% thereof. So I've heard, anyhow). As to what caused the failure, my guess would be one of the students (it used to be at a school) did something they shouldn't or perhaps ran it at its full ratings for a while... Being of Chinese make, that would likely lead to overheating! (so I've heard many times on this very forum, anyhow) Still, it was free and I'll try to learn something from it even if I can't fix it in the end! I think I'll try supplying voltages to the front panel with a PC power-supply (and/or a laptop supply) if I can get the right voltages, that is!

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2017, 07:53:31 am
content: Windings are simple, but each side has a set of them. There is a 3 section winding, rated for 3A, giving 8VAC, 16VAC and 28VAC, for the main output. This is switched by the 3 relays on the power board, and feeds the regulator. Then there is a 9VAC at 1A supply, used to drive the 2 LED displays on the side. There is another 18VAC-0-18VAC winding, rated at 0.5A to provide the relay drive and to power the regulation circuitry.Duplicate for the other side, so you have 6 separate windings on the toroid. The reason yours probably failed is because it was run at full load for a long while, and the toroidal transformer it was designed for was underspecced and undersize core wise, winding wise and wire type wise. Thus the innermost winding, the mains primary winding, overheated and the insulation on a turn broke down and shorted. Then thus shorted turn blew apart, killing the transformer. There never is a thermal fuse on these cheap toroids, as it adds cost, needs extra assembly steps to tape it to the core with enough insulation and the wire itself makes a pretty effective fuse in any case. It is possible to rewind the toroid, but you will find it takes a lot of patience to do the winding, and even more to get the tapes on each layer as well without an automated winder.My rework on toroids always was limited to adding extra secondaries, as this on an unpotted one is very easy.

author: Seekonk
author_rank: Super Contributor
timestamp: January 03, 2017, 08:04:19 am
content: Just put a 24V transformer in parallel with one of the 28V windings and be done with it. It will never be the same , but usable.

author: WolfmanTech
author_rank: Contributor
timestamp: January 03, 2017, 08:27:14 am
content: SeanB- Well explained, thank you! Oddly enough, I've yet to find any signs of extreme heat or shorting, which is what made me think there may in fact be a thermal fuse.Shall see when I completely unwind it, for curiosity's sake if nothing else! There are plans online for making a winder, may eventually do that.Seekonk- May use multiple transformers, will update once I decide!Thanks for the help everyone!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2017, 12:47:40 pm
content: Winding such a transformer is tricky. Also the insulation is important, as this is the only thing between the output and mains.From the relays it looks like there are several supply taps used. So the transformer is specially made for such a supply. Taps could be something like 0 - 6 - 12 - 24 V or similar. Common transformers are more like 2 identical voltages.Using two or more transformers instead, could be tricky, as two transformers of half the rating are usually larger than the one big one. With these supplies there is usually not much spare power rating - more like to much heat already. So one might have to reduce the output current.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 03, 2017, 08:09:42 pm
content: Quote from: SeanB on January 03, 2017, 07:53:31 amSean, I only see two relays per side. What am I missing? In any case, two relays are enough to get 4 different AC voltages which would help reduce the internal dissipation. The heat sink doesn't look overly generous so that would be a good thing. I see a fan cutout on the rear panel but I can't see a fan. If there wasn't one, that would tend to cause the transformer and drive transistors to overheat under heavy loads. That could have been a factor in the transformer failure. I'd add a fan.

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2017, 08:49:52 pm
content: Sorry my bad, I was saying there are 3 taps, and the 2 relays switch them in in a sequence so the bridge is fed ( most likely ) with 8VAC with them both off, then one is energised to enable switching of the 16/28V supply in, and the second relay enables the higher of the two. This makes the logic easy, as you only need 2 comparators for output voltage comparison, and these can drive the relays simply using a transistor, and there is no way to have the relays energised to short the transformer outputs.The secondaries on all the windings have to be isolated from each other, as they will all be floating at some voltage in operation, and a direct connection inside will make a shorted turn. The 18-0-18VAC winding is likely connected to the output of the power supply section, and the 9VAC winding is also floating, as it drives a pair of ICL7107 LED display drivers, and these use a +/-5V supply with the input common referenced to this winding. This also is connected to the output positive.

author: kundarsa
author_rank: Newbie
timestamp: February 15, 2020, 04:19:22 am
content: Just got a broken one of these. It blows the 2 fuses (once a small load is applied) on the power supply board in the middle of the full bridge rectifier. Do you still have your unit?This unit really lacks doc's, my BK psu did way better for troubleshooting.