What's going on with this transformer? - Page 1
Forum section: electronics / beginners

author: AgentDelta
author_rank: Contributor
timestamp: December 22, 2024, 04:09:49 am
content: I'm troubleshooting an A/V receiver that uses a transformer with three sets of outputs, marked AC12, BL, and BH. When I look at each of the outputs on an oscilloscope, the AC12 and BH outputs look roughly like what I would expect to see, but the BL output waveforms seem out of sync with each other (see the attached screenshots). Any ideas as to what's going on with the BL output?I haven't been able to track down a spec sheet for the transformer, so I don't have any official word on what each output is supposed to look like. Any help understanding what's going on with the BL output (I'm mainly trying to determine if there's something wrong with the transformer, and if so, what's wrong) or pointers to resources I should be looking at would be greatly appreciated. Thanks!

author: Analog Kid
author_rank: Super Contributor
timestamp: December 22, 2024, 04:16:31 am
content: 2nd pic shows the two voltages exactly 180° out of phase, in other words polarity reversed.Dunno about the 1st pic; that looks more problematic (not 180° out of phase).

author: AgentDelta
author_rank: Contributor
timestamp: December 24, 2024, 03:01:54 pm
content: Yeah, that first pic is what has me concerned that something's wrong with the transformer; I just don't know what might cause output like that.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 24, 2024, 03:10:37 pm
content: I would guess that the winding is floating. There is still some signal because of the interwinding capacitance but you get a bug phase shift. Use an ohm meter when the transformer is deenergized to check for the winding configuration.

author: AgentDelta
author_rank: Contributor
timestamp: March 23, 2025, 11:13:06 pm
content: Apologies for the delayed response; things got a bit busy the past few months.I've checked the windings with an ohmmeter, but I'm not sure what to make of the results.There is no continuity between either of the wires on the primary winding and any of the wires on the secondary/tertiary windings.I measured 1.1 ohms between the two wires that feed the primary winding.The transformer has two output connectors, one of which has six wires (Red1, Red2, Orange1, Orange2, Black1, Black2) and the other of which has three wires (Yellow1, White, Yellow2).The labels on the circuit board where the connectors connect indicate that Red1 and Red2 are both AC-BH, Orange1 and Orange2 are both AC-BL, Black1, Black2, and White are all E, and Yellow1 and Yellow2 are AC12. I included oscilloscope traces for AC-BL and AC-BH with my first post, and I've attached a trace for AC12 to this post.The wires connect to the transformer in the following order, from left to right: Red1, Orange1, Black1, Black2, Orange2, Red2; Yellow2, White, Yellow1I measured the following values with the transformer completely disconnected from all power sources (except my ohmmeter) and circuit boards:Red1 to Black1&2; Red1 to Red2; Orange1 to Black1&2; Orange1 to Orange2; Red1 to Orange2; Red2 to Orange1:2.41 MOhmsRed2 to Black1&2:0.5 OhmsOrange2 to Black1&2:0.2 OhmsBlack1 to Black2:0 OhmsRed1 to Orange1; Red2 to Orange 2:0.3 OhmsYellow1 to White and Yellow2 to White:1.3 OhmsYellow1 to Yellow2:2.6 OhmsI haven't been able to track down a specification sheet for this transformer, so I don't have a list of known good values to compare my measurements to. Do any of the measurements seem problematic or provide an indication of where to look next?

author: showman
author_rank: Contributor
timestamp: March 24, 2025, 12:52:05 am
content: Looks like this, right?Code:|| +--- red1|| | 0.3 ohms|| |--- orange1|| x break (should be 0.2 ohms)primary1 ---+ || +--- black1 & black2| || | 0.2 ohms| || +--- orange21.1 ohms | || | 0.3 ohms| || +--- red2| ||primary2 ---+ || +--- yellow1|| | 1.3 ohms|| +--- white|| | 1.3 ohms|| +--- yellow2

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: March 24, 2025, 07:48:23 pm
content: I think Showman has the right idea, it looks like one of the sections of that secondary is open. Perhaps a termination point broke.Just out of curiosity what style is this transformer? is it a toroidal or something else?

author: AgentDelta
author_rank: Contributor
timestamp: March 27, 2025, 01:22:51 am
content: Thanks, Showman! I think you hit the nail on the head; that's exactly what I'm seeing. Now to see if I can find exactly where the break occurred.The transformer doesn't look like a toroidal transformer to me. I've attached a couple pictures (one of which includes the label; so far I haven't been able to track down a datasheet or replacement, but if anyone is able to, I'm all ears) for reference. The brown and grey wires attach to the primary winding.

author: tooki
author_rank: Super Contributor
timestamp: March 27, 2025, 08:37:47 am
content: Please share the make and model of the receiver. Maybe there’s a service manual out there.

author: AgentDelta
author_rank: Contributor
timestamp: March 29, 2025, 03:28:41 am
content: It's a Yamaha TSR-7810 (AKA RX-V781, from what I’ve been able to determine). I was able to track down a service manual athttps://elektrotanya.com/yamaha_rx-v781_tsr-7810_rx-a860_sm.pdf/download.html, but it doesn't seem to have much information about the transformer.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 29, 2025, 05:24:18 am
content: Quote from: AgentDelta on March 29, 2025, 03:28:41 amWhat info were you looking for? The transformer is shown on page 97 with a simplified schematic. The full schematic is on page 132 but doesn't show much more. The simplified schematic shows that the 5-lead bundle should all be connected in series. If there's an open between black1-black2 and orange1, that winding is blown. The question is why? Since the unit is rated for a Dynamic Output Power of over 200W per channel it'd take a lot to blow that transformer. You might be looking at shorted output transistors. Is the main fuse (8 Amps!) blown?The transformer is custom-made for Yamaha, so your only chance for a replacement is Yamaha - but that's doubtful since it looks like that manual might date from 2016.You might luck out and be able to repair the transformer. You need to expose the winding terminals and see if there's a broken wire right at the terminal. Cut open the blue cover. Be careful not to dig too deep. Follow the black and orange leads that should be connected and look very carefully at the terminals they connect to. Look at the wires that come from inside the wire layers and connect to the terminals. You might need to use a magnifying glass or similar. If you find a broken wire there, you need to carefully scrape the enamel insulation from the end of the wire and resolder it to the terminal.I've been able to do this repair on a couple of pieces of equipment. On one unit, both of the primary windings were open. I was able to resolder both of them. That was a very low power unit with hair-thin wires. I'm expecting that you're going to find melted wires!

author: edpalmer42
author_rank: Super Contributor
timestamp: April 01, 2025, 10:11:13 pm
content: It occurred to me that rather than try and replace this transformer - probably impossible - the OP might be able to replace just the blown winding with a seperate, much smaller transformer. Connect the secondary of the new transformer in parallel with the blown winding so that everything looks normal to the rest of the circuit.I don't remember hearing about anyone doing this. Is this a reasonable thing to do?

author: AgentDelta
author_rank: Contributor
timestamp: April 06, 2025, 03:24:48 am
content: Quote from: edpalmer42 on March 29, 2025, 05:24:18 amI was hoping the service manual would have a listing of expected resistance values for each winding and voltage levels for each output to compare my results against, but that so far hasn't proven to be necessary.The receiver was in this condition when I purchased it at an estate sale, so I don't have any insight into the circumstances at the time the damage occurred. The main fuse is fine, and the unit will (when assembled) power up and immediately power back down because one of its protection circuits reads values that are out of bounds. If I override the protection check, it will stay powered on and let me cycle through the menus.I suspect this is likely beyond repair (at least at my skill level). I've cut open the blue cover, but I don't see any obvious signs of melted wires, loose connections, or other damage (pictures attached). Unless there are any other recommendations, I might dig deeper to satisfy my own curiosity, but I expect that would be a one-way journey. I'm certainly open to trying your idea of wiring in a separate transformer to replace the blown winding, but I would need more information about what to look for in the separate transformer and the best way to go about wiring it in.

author: najrao
author_rank: Regular Contributor
timestamp: April 06, 2025, 08:25:01 am
content: Agent delta,I feel sad that you find your skill level inadequate to repair this, that too having gotten thus far. Your picture shows (to me) that there just cannot be any miracle behind the open connection between the red and orange, or the orange and black (all at one end of a center-tapped continuous winding merely tapped for lower dc voltage by the two orange wires). The winding wire is pretty thick, and will not have fused or ruptured.You CAN easily locate the discontinuity by removing the top layer of (transparent) tape, inspect the soldered joints and reflow them if necessary. It may just be that the enamel on the winding wire was not cleared enough during jointing.If still no cigar, slit the underlying blue paper? on both sides and lift up the jointing area, and see if one of the outgoing leads has a problem. Finally, you can redo the entire lot of leads out with new stranded 120+C wire. Good luck.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 06, 2025, 08:56:51 am
content: Yeah, you're not going to get broken wires on that!However, I'm not impressed with what I can see of the soldering. I'd peel off the tape that's covering the soldered connections and inspect them more closely. Wouldn't it be crazy if it was a bad soldering job on one of those connections? I'd resolder them all, just in case.If you have to replace the blown winding, remember that the transformer is symmetrical. The voltage needed for orange1 to black is the same as orange2 to black. Just measure it and see what you get. The full schematic shows that the voltage on that bus is 61V peak which would be ~43Vrms. You'd have to look further for a note to see if that's with or without the boost from the speaker impedance switch but the position of the relay appears to show the lower voltage. Those pieces of info give you more than enough data to home in on an appropriate voltage. It doesn't have to be perfect - the voltage at this point is unregulated. Just make sure that the peak voltage is well below the main capacitor's rather odd rating of 71V.The current rating is harder to estimate, but the bridge rectifier is rated for 6 amps so that puts a limit on the transformer rating. Since this is for an audio amp, it will almost never reach its maximum current drain and, even if it does, it will just be for a fraction of a second on some huge drum beat. Something in the 4 or 5 amp range should be fine.Two points regarding testing:1. Use a dim bulb tester. Search the forum for info on this.2. For test purposes, forget about the transformer. Use an external power supply to provide DC power on that bus. Connect it across the main capacitor. Power both the amp and the power supply via a switched power bar so that you can turn them on together. 61V is a strange value, but if you've got a lab-style supply that can provide 50V @ 1A, that might work. I see that there's a 56V regulator so you might have to use a voltage high enough to keep that happy. The 1Amp current limit will protect the system in case something else in the circuit is fried.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 06, 2025, 09:32:00 am
content: Would it be an option to take a different approach - rather than trying to repair the transformer, could you perhaps replace the whole PSU with a new one? Or, perhaps intercept the existing PSU after the transformer and rectifiers, thus keeping all the filtering, regulation and protection circuitry.Worth considering? It was the thought of writing off the whole thing that made me think there must be alternative approaches.

author: David_AVD
author_rank: Super Contributor
timestamp: April 06, 2025, 09:44:35 am
content: What DC voltages do you get on the collectors (tabs) of the output transistors?Just measure any two as long as they are next to each other and report back.What is the actual fault symptom(s) that lead you to believe the amplifier is faulty?

author: edpalmer42
author_rank: Super Contributor
timestamp: April 06, 2025, 07:18:21 pm
content: Quote from: SteveThackery on April 06, 2025, 09:32:00 amYes, the power supply could be replaced. In this instance it's a bit complicated because if you look at the simplified schematic on page 97, you'll see that there's a speaker impedance relay that changes the supply voltage. The multiple voltages (+18, -18, +61, -61) also make it a nuisance. Since this amp includes a phono preamp, minimizing power supply noise is mandatory. So do you use a switching supply with its potentially noisy outputs but small size or do you use one, or two, completely new linear supplies? Is there room to mount two new linear supplies? Maybe. Another thought that occurs to me is that the high voltage, unregulated output on the original power supply will inherently sag when it's driven hard and automatically recover when the load goes away. Regulated power supplies aren't typically speced for that situation.There aren't any really nice solutions to a situation like this.AgentDelta, don't be intimidated by this amp. The money's been spent. The amp is currently scrap. If you take it to a repair shop, they'll likely tell you that it's BER (Beyond Economical Repair) - and they might charge you a diagnostic fee. Think of this as a learning exercise. If you manage to fix it, you'll have something to be proud of. If you aren't able to fix it, you'll have learned a lot along the way.DAMN THE CONSEQUENCES!! FULL SPEED AHEAD!!

author: temperance
author_rank: Super Contributor
timestamp: April 06, 2025, 08:15:35 pm
content: Some transformers contain a thermal fuse. You might be able to find it's connections buried between the layers of tape.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 08, 2025, 06:01:18 am
content: Quote from: temperance on April 06, 2025, 08:15:35 pmTrue, but I'm not seeing any evidence of overheating. And typically, the only way to get to the fuse would be to destroy the transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: April 08, 2025, 06:31:14 am
content: Also, the thermal fuse, if present, is almost invariably in series with the primary, which was tested and has continuity.

author: AgentDelta
author_rank: Contributor
timestamp: April 10, 2025, 02:56:35 am
content: Quote from: edpalmer42 on April 06, 2025, 08:56:51 amI haven't been able to spend much time working with the solder joints this week, but I think you're on the right track. While I still don't see an obvious defect to hone in on, after moving the Orange1 solder joint around in the course of inspecting it, I decided to test it again. The resistance between Orange1 and Black1&2 is now measuring 0.6 - 0.7 Ohms, which still doesn't match the 0.2 Ohms between Orange2 and the Black leads, but is a world of difference from the 2.4 MOhms I was measuring before.I long ago moved this receiver into the learning exercise category, and regardless of how things turn out I've already gotten more than my money's worth in knowledge gained. To clarify my previous statement regarding the unit being beyond repair, I was thinking that the lack of obvious defects meant disassembling and potentially rewinding the transformer. I'm perfectly willing to try that if that's what it comes to; I'm just not certain how successful I'd be.I'll continue working on the solder joints as I get more time (and I appreciate all the information regarding potential repair methods and alternative power sources for the receiver). My bench power supply maxes out at 30 volts, so I'd have to look at other options if I were going to try to power the receiver without the transformer. However, given the results I'm seeing so far, I'm optimistic that more work on the solder joints will get the transformer where it needs to be. If anyone has any tips for the most effective way to redo/reflow the solder joints, I'm all ears (and definitely open to picking up an entry-level hot air rework station if that would be helpful for these connections).Many thanks for everyone's insight, advice, questions, and general willingness to help! Please keep the ideas & advice coming.

author: David_AVD
author_rank: Super Contributor
timestamp: April 10, 2025, 03:14:16 am
content: So, before you pulled the transformer out, what were the actual fault symptoms?

author: AgentDelta
author_rank: Contributor
timestamp: April 10, 2025, 03:50:03 am
content: Whenever I pressed the power button, the unit would turn on then immediately power off. If I overrode the protection circuit, it would stay powered on and let me get into the self-diagnostic menu. All the self-diagnostic values were normal except the value for PS1, which steadily cycled between 0 and 255 instead of staying within the range of normal values listed in the service manual (pg. 72).I checked the output jacks while it was powered on in self-diagnostic mode and didn't measure any voltage on them, but I didn't measure the output transistors. I also traced the PS1 circuit in the diagrams in the service manual and found portions of the circuit on one of the digital boards (pg. 122 of the service manual) and the main board (pg. 132 of the service manual). When I unplugged the cable that PS1 uses to travel between the two boards, the PS1 value listed in the self-diagnostic menu stopped cycling and stayed at a steady 255.

author: David_AVD
author_rank: Super Contributor
timestamp: April 10, 2025, 07:45:15 am
content: A lot of Yamaha models have issues with the SMD resistors that tap off the various voltage rails and feed the protection sense lines, but you don't normally see it cycling between extreme values like that.Another issue is dead regulators. You may have one that's cycling on / off, although they usually just drop dead. Check the manual for which rails the PS1 line monitors for that model.Resolder the transformer wiring and if all the windings appear to have continuity reassemble and check the PS1 live reading again. Don't obsess over the exact readings as they will all be quite low and the meter leads will cause a measurement error.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 11, 2025, 02:45:25 am
content: Quote from: AgentDelta on April 10, 2025, 02:56:35 amFolks, it looks like we have a winner! Remember that the secondary winding is symmetrical. The resistance from black to orange1 should be the same as from black to orange2. There might be a bit of variation, mostly due to your meter, but they should be close. The same applies to the other windings; red-orange and yellow-white.QuoteI've never rewound a transformer. It sounds tedious AF. You'd likely have to strip off all the secondary windings and rewind all of them. The only way I'd attempt it is if the unit was an absolute classic or had a bucket full of sentimental value.QuoteThis is not a job for a hot air rework station - unless you're thinking of one that has both hot air and a temperature-controlled soldering iron. That would be fine. In any case, don't use a tiny tip. Big wires require a big tip.The wire strands look like they're heavily oxidized so it might be tricky to get solder to stick. But if the old solder protected the wires, it might be an easy job. I'd grab my bottle of liquid solder flux and apply it to the joint before applying heat. Since you probably don't have that, just add fresh solder and see what happens. If it looks ugly, use a solder-sucker or vacuum desoldering gun to clean it all off and start again. Remember to do *all* the joints, not just the one you found. If one is bad, there could be another one that's hanging by a thread.Do the job as quick as you can. If you sit there with the soldering iron on the wire you could heat up the wire enough that it starts to melt the plastic form that the wire is wound on. It's hard to get the scale from the picture, but it looks like the plastic is only about 20mm from the solder joint. A large tip will help you get in and out quickly. A small tip will force you to sit there for an extended period of time and start melting the plastic. Sounds backwards, but it's true.Once the connections look good, you can either use the over-under layer of tape that was originally used or slip pieces of heat shrink tubing over the connections. Tape the cover back in place and you're done. I'd only use Kapton tape. Ordinary electrical tape is rather thick and won't tolerate the heat very well.I don't know what your background is so some/all of this might be well known to you. If that's true, I'm playing to the audience. That's my story and I'm sticking to it.Ed

author: AgentDelta
author_rank: Contributor
timestamp: May 03, 2025, 07:03:54 pm
content: Quote from: edpalmer42 on April 11, 2025, 02:45:25 amApologies for the delayed response, but I finally got a chance to circle back to this project. Many thanks for the advice! As to my skill/background, I did post this in the Beginners section after all, and I'd much rather receive a thorough response.Unfortunately, I think I'm likely at a dead-end. Despite liberal application of liquid flux, solder would not stick to the transformer tap wires. I was able to get it to the point where I could measure a couple Ohms of resistance between Orange1 and the Black leads, so I taped it back up, hooked everything up, and powered up the receiver. There was a spark from the transformer, but I was still able to put the receiver in override mode and check PS1, which showed a steady reading of 211. That's outside the normal range listed in the service manual, so I disconnected everything to do some more work on the transformer. (As a side note, before I started troubleshooting the transformer, I tested the resistors feeding the protection sense lines and didn't find anything unexpected).In the course of twisting the wires back together after cleaning them up (I was having trouble getting consistent readings when probing the wires, so I had untwisted them so I could use some fine-grit sandpaper to remove whatever was on the surface of the wires), one of the wires coming out of the transformer broke off flush with the plastic such that the tip of the wire is just barely visible/accessible. If there's a good way to attach a new wire under the circumstances, I'm all ears, but I suspect this might be the end of the road for the transformer.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 03, 2025, 07:43:38 pm
content: Ouch! That hurts.At this point, your only practical - well, somewhat practical - solution would be to replace the bad winding with a seperate transformer. That speaker impedance thing really complicates this situation. I'd be tempted to go with two transformers. Use a small one to replace the low voltage windings (24 Vrms CT @ 1 or 2 amps). Use a large one for the high power winding (~86Vrms CT @ 4 or 5 amps). Check that voltage and confirm that it's the lower of the two possible voltages. Then disable the speaker impedance relay or rewire the contacts so that the same lower voltage is used for both settings. That might limit the maximum output power when using a higher impedance speaker, but you won't notice that.While somewhat ugly, this will give you a working, usable receiver instead of a piece of scrap.

author: AgentDelta
author_rank: Contributor
timestamp: May 12, 2025, 03:10:59 am
content: Quote from: edpalmer42 on May 03, 2025, 07:43:38 pmIt was unfortunate, but that's how it goes sometimes. After taking a closer look at the transformer, it looks like I might be able to remove the housing and get access to the broken wire. If that pans out, I might be able to solder in some new wire to form the needed connections.If that approach doesn't pan out, adding new transformers to the mix sounds like a fun way to keep the project going, but I don't want to spend much more on it than I already have. Any recommendations for a good place to procure reasonably priced transformers that would be good enough (quality-wise) for this sort of project?

author: edpalmer42
author_rank: Super Contributor
timestamp: May 12, 2025, 05:01:19 am
content: I'm not sure that 'reasonably priced' exists these days. The high price of copper has had a big impact on the price of transformers but, it's the cost of doing business.Once you decide on the voltage and current ratings you want, the usual suspects would be places like Mouser, Digikey, Newark, etc. But you should also check the used market. I can't give you much help there. It's been a while since I did any serious looking and the current tariff situations have thrown old prices and info out the window.Before you buy anything, if you're going to be adding a second transformer, make sure there's room to mount it.

author: Ian.M
author_rank: Super Contributor
timestamp: May 12, 2025, 05:42:15 am
content: I wonder if the original transformer was wound with CCA (copper clad aluminum). That would explain the wire breaking when untwisted, as aluminum is far less malleable than copper amd tends to work-harden and fracture if repeatedly bent, and also the extreme difficulty getting solder to wet it when remaking the joints, as once the thin copper 'flash' coating has dissolved in the solder, anything that exposes the underlying aluminum will let its surface rapidly oxidise and effectively become unsolderable. If the wire is thick enough examining its broken end under a magnifier may be informative- if its copper color right across the end you are in with a chance as its probably not CCA, but if its got a silvery center its most probably CCA and that's game over unless you can scrape the end and properly crimp something solderable onto it e.g a duPont pin. I've done that repairing motors, but I doubt you'll be able to get enough length to crimp unless you are very lucky and its the top of the winding.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 12, 2025, 06:45:17 am
content: Quote from: Ian.M on May 12, 2025, 05:42:15 amI wondered about that, but if you look at the transformer teardown pictures the OP posted earlier, it's clear that the original connections were soldered. Since they were definitely the source of the problem, resoldering them was the obvious first choice. There's a cryptic note on the title page of the service manual that says " '16.07 ". If you interprete that as a date code of July 2016, was CCA being used in 2016? Did they initially think that the copper layer made CCA solderable? Is CCA susceptible to failure after 9 years?

author: Ian.M
author_rank: Super Contributor
timestamp: May 12, 2025, 07:34:12 am
content: You can solder fresh CCA *once*, maybe twice if its got thick enough copper cladding. If the copper has oxidised it may be unsolderable. Humidity + any improperly cleaned flux residue can set up a galvanic corrosion cell between the aluminum and the copper cladding and the aluminum center of the CCA just rots away.CCA was certainly around long before that. I first encountered it in a transformer from a '90's TV set, and it wasn't uncommon for TVs from that era to use CCA degaussing coils. The motor wound in CCA I repaired was in a pressure washer I purchased circa 2010. It was a universal motor and one of the stator coil wires had corroded through where it was crimped to a brass tag that connected to one of the brushes.

author: AgentDelta
author_rank: Contributor
timestamp: May 13, 2025, 02:21:38 am
content: I think Ian is right about the wiring being CCA. Looking at the end of the wire, it has a silvery center. If I'm able to get the housing off, I'll see if there's enough wire to allow me to crimp something onto it. Otherwise, I'll start looking around at replacement transformers to see if I can find something reasonably priced that will fit in the space available. Regardless of how things turn out, I've learned a lot, and I sincerely appreciate everyone's time and help!