Can a Toroid Transformer "pop"? - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: September 07, 2025, 02:30:18 pm
content: I have a Proceed BPA 3 amplifier that I'm repairing for a friend. The unit has always remained powered on and one day recently a loud "pop" was heard, and another about a minute later. No smoke, no smell, just the pop.Yesterday I opened the cover, and, from tracing wires, it looks like: the main board (which doesn't have many components) has three fuses (one for each channel - R/L and center), each fuse is connected to one of three Toroid Transformers, the output of each transformer goes to one of three boards (each board is dedicated for each channel and appear identical), and each board has two wires feeding back to the main board. I suspect these wires tell the main board whether a fault is occurring on one (or more) boards. When I measured the voltage output of each transformer, I found the voltage immediately went to zero after pushing the power button indicating my assumptions on the feedback wires are correct.Each board has two LEDs that seem to illuminate when the board is working; I assume one LED for the positive rail and one for the negative.From what I observed, the only components that can "pop" as loud as described would be the Toroid Transformer or a capacitor (each board has two large caps).Initially I suspected the left channel because one of the caps has a slight bulge on top (hard to tell because it has plastic coating) and yellow stuff on the bottom. I can't tell if it's dirt that got in from the vent holes on the bottom or something leaked from the cap. The bulge isn't bad enough to make such a pop, but who knows.After digging deeper, I realized the right channel is the culprit. The LEDs aren't illuminating and the slow blow fuse has high resistance(it was low enough that the meter beeped when I measured but I saw it was slightly burnt so I measured the resistance and found it was high).Update: that was silly of me to say. With the fuse out of the holder, the terminals measure about 3ohms. For some reason I was thinking maybe the slow blow didn't fully open. Needless to say, I removed the fuse and it's fully open.Seeing how the entire system only has Toroid Transformers and six caps (two in each channel), these would be the only things that would pop loud enough (unless the right channel sent a surge to the speaker and the pop was actually the speaker).The Toroid Transformers have the following on the label:Toroid International LtdP/N TI-61342Madrigal P/N 290093SL 9738Unfortunately I haven't found any info on this and suspect it's a custom transformer.My question: can Toroid Transformers short or whatever and cause a pop? I don't see any burn marks on the transformer, no smell, nothing.Personally I've never experienced a failed transformer, especially one that could have popped, so thought to ask.

author: radiolistener
author_rank: Super Contributor
timestamp: September 07, 2025, 05:09:16 pm
content: You might want to check the power transistors as well. High-power transistors can actually explode quite violently - sometimes ejecting large chunks of plastic from the package. These pieces can be thrown with significant force and could even cause injury if someone is nearby when it happens.Yes, a transformer can fail too. For example, a winding can short, insulation can break down, or if the mains voltage is too high, the core can saturate, causing overheating and potentially damaging components inside the transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2025, 05:18:08 pm
content: IMHO if a transformer 'popped' it would stink. Lots of burnt enamel insulation to build up enough gas pressure. OTOH a power semiconductor 'pops' much more easily, and only the burnt epoxy immediately next to the die will be at all stinky, so if it either blew a fuse or broke the die bond wires and went open circuit when it popped, removing the heat source there might be next to no smell.

author: bostonman
author_rank: Super Contributor
timestamp: September 07, 2025, 08:07:25 pm
content: I said something incorrect above about the fuse being high resistance and fixed the wording. The fuse is blown, not high resistance (although technically yes, it's very high resistance), and, the terminals without the fuse measure about 3ohms. Initially I measured the fuse in the circuit and thought, hmmm 3ohms, maybe it just opened slightly (it's a slow blow), but guess I was still waking up.Interesting information about a transistor giving off a pop because I've never seen such an occurrence. Due to a pop being heard, I was focusing on the components I considered that could give off such a sound; but felt a transformer or such a large capacitor would be much louder.Figured I'd start with inquiring about the transformer, but usually the go to is caps and anything connected to a heatsink (i.e. the transistors). Normally I would just remove the board and start with transistors, but I'm a little concerned the caps may have a charge and can't access underneath. Also, this amp is built quite well with lots of screws, so I wanted to start off cautious with my approach.I'll double check the transformer to see if I notice any burning. My other thought was connect right speaker transformer to the circuit of another channel and see if the fuse blows, then it may indicate something. Unfortunately all I have are 1A fast blow and not 2.5A slow blow.Also, I measured the resistance of the transformer with a milli ohm meter and it seems to be similar to the center channel transformer (the assumed good channel). I got the following:Right channel (the one blowing the fuse): 1.797ohms primary and 1.035ohms secondary.Center channel (assumed good one that doesn't blow fuses): 1.796 ohms primary and 0.989 secondary.Going by the resistance measurements, seems the transformer is good.

author: bostonman
author_rank: Super Contributor
timestamp: September 07, 2025, 11:52:52 pm
content: I removed the right channel board (the one blowing fuses) and to my surprise, not a signal component looks blown. In fact, the writing on most components (and resistors and resistors with color bands) are quite readable.It's actually rare I've dug into something so old and not seen high wattage resistors have some discoloration, so I'm impressed that all the components still look new.Some device I've never seen before is mounted to one of the transistors (or MOSFETs). It's a Thermik H06. I assumed it was some sort of thermal sensor because it was screwed onto the front of a large transistor, and, a quick look online confirmed it. This stuck out because the two wires soldered onto the PCB were low resistance, but the other channels were the same.The only anomaly I saw was a Darlington TIP120 that ironically looks like it was replaced (although when my friend bought this many years ago, it was sold as "new"). It's measuring much lower than the expected approx. 1.4v drop, however, I know from experience when blindly measuring without schematics that components go through other low resistance components. It's not until I go through the trouble of removing the part that I discover this; so I'm not getting too anxious that I may have discovered a problem.Anyway, I don't want to deviate from the initial topic. It was a help getting better feedback on what to expect from a Toroid Transformer.Also, I did a closer inspection of the transformer and all the white tape is still white and didn't see a single area where it could have overheated.

author: Ian.M
author_rank: Super Contributor
timestamp: September 08, 2025, 12:10:00 am
content: Maybe you heard the fuse blow?

author: bostonman
author_rank: Super Contributor
timestamp: September 08, 2025, 01:00:03 am
content: It was my friend who heard the pop.I thought it could be the fuse too, but the story is two pops occurred about a minute apart.I'm thinking the sound came from the speaker. Possibly something in the amp blew, sent a pulse to the speaker, maybe the first pop was the speaker and the second was the fuse.

author: Ian.M
author_rank: Super Contributor
timestamp: September 08, 2025, 01:09:28 am
content: Probably both pops were the speaker - first a failure that put DC through it making a sudden displacement of the coil and cone, then when the fuse blew the DC stopped resulting in an equal and opposite displacement returning to their neutral position, drowning out the possible noise from the fuse.

author: bostonman
author_rank: Super Contributor
timestamp: September 08, 2025, 01:22:19 am
content: That's a much better technical explanation that I had.My friend was in the other room and said he jumped from the "pop". Since my theory about the speaker causing the "pop", seems plausible, then it make sense I'm not seeing anything burnt.

author: bostonman
author_rank: Super Contributor
timestamp: September 09, 2025, 01:19:37 am
content: I've checked every transistor including checking the heat sink on the bridge rectifier (it's screwed into the heat sink) to see if it's shorted to chassis ground.Nothing is discolored, never mind burnt. All transistors seem find, etc...I found a schematic online (see attached) created by someone who traced the circuits, so I'm using it as a reference, however, I can't power this board even if I wanted to risk shock because the AC shuts off immediately due to (I assume) the feedback line telling the supply to turn off due to a problem; also note the schematic was originally downloaded blurry for some reason. I just found a better copy.At this stage I can either connect the right channel (the channel that's blowing a fuse) Toriod Transformer to the center channel (all three channels are identical) and see if the right channel fuse blows again. If it does, it indicates the transformer is bad.I can connect the center channel Toroid Transformer to the right channel board, and, if the center channel fuse blows, that tells me the problem is on the board.I'm not a fan of swapping and rather take some sort of methodical approach. Would it make sense to try powering the Toriod Transformer directly from AC (a VARIAC), slowly increase the voltage while measuring the secondary?The secondary on all three Toriod Transformers have three wires; I assume it's center tapped. The very large capacitors are rated for (I'll double check) 70V. I'm guessing one wire to the center would be about 30V since the capacitors are probably rated for twice the working voltage.Since the fuse that blew is a 2.5A SB (the primary side of the transformer - one fuse for each transformer), I'm thinking of connecting a 60W light bulb to the secondary with a VARIAC on the input. That would load the transformer and give me a better idea whether it could have an issue. Since I'm guessing about 30V on the secondary, I can connect the light bulb across the two phases leaving the center tap disconnected. At approx. 60V AC, the light bulb should light dimly.Does this seem like an idea way to test the transformer (besides that I already measured the resistance with a milliohm meter).?

author: themadhippy
author_rank: Super Contributor
timestamp: September 09, 2025, 01:32:04 am
content: Id just totally disconnect the suspect tranny,chuck some mains up it and see wot it does

author: bostonman
author_rank: Super Contributor
timestamp: September 09, 2025, 01:10:03 pm
content: I thought of that, but, since the unit is cutting off power because something is wrong, I may turn a small problem into a catastrophic one before the fuse blows.Also, as stated, I was trying to approach it methodically and get comfortable running a few simple tests with the transformer to expand my knowledge of them.

author: themadhippy
author_rank: Super Contributor
timestamp: September 09, 2025, 02:07:01 pm
content: Quotethats why i saidQuotewith nothing connecting it to the rest of the circuit it cant do further damage and in one hit you've proved if the tranny is good or not

author: bostonman
author_rank: Super Contributor
timestamp: September 09, 2025, 02:15:56 pm
content: What if the transformer has a short and the output voltage is higher? Then I'll blow the board for that channel.Right now I suspect a sense circuit is checking voltage and/or current, without that shutdown circuit in place, I can blow up components.

author: themadhippy
author_rank: Super Contributor
timestamp: September 09, 2025, 02:25:41 pm
content: QuoteHow? read slowly until it sinks inQuote

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 09, 2025, 04:11:45 pm
content: Transformers don’t blow and increase their output voltage.themadhippy provided the test. Disconnect the secondary from the board and power the primary without light bulbs, variacs, etc. There is no risk to anything, other than the fuse.If it blows the fuse, the transformer is bad.If not, measure the output voltages. End to end and ends to center tap.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 09, 2025, 05:41:24 pm
content: If you conclude the pop was from the speaker, you can't confine your investigation to the power boards. The drivers or the speaker may have demanded full output, which the board handled until the fuse blew. This is completely consistent with the two pop report. The first pop was the initial failure. The protection line shut things down and something recovered enough to let it try again. The fault was still there and this time the fuse (possibly still warm from the prior event) was blown on this occasion.According to your posts all the power boards, including the channel with the blown fuse pass a visual inspection. And are reported to be functionally identical or nearly so. If you want to pursue the hypothesis that the power board is the failure, you have the best of all worlds for diagnosis uninformed by schematics or understanding of the circuitry. Two reportedly identical and probably working power boards. You can compare point to point resistances between the "known good boards" and the suspect board, focusing on the left channel as a reference because it is least likely to have differences from the right channel. If there are no large differences I would broaden your investigation. If there are they should lead you quickly to the problem.

author: bostonman
author_rank: Super Contributor
timestamp: September 09, 2025, 07:35:29 pm
content: QuoteI must have misinterpreted the suggestion. I thought the suggestion was to keep the transformer connected to the power board and power the transformer directly to AC with an inline fuse.CatalinaWOW,Just to clarify,youryou're providing good feedback as always, but I understood your message to imply the unit was playing music. I may have typed my initial message implying the wrong thing because (as I sometimes do) think people know what I'm thinking.The amplifier has been powered 24/7, however, it was not playing music at the time; nor has it played anything in quite a while. My friend always left it powered for some reason (it's only the push of a button to turn it off - guess he's just lazy).Also, just to make sure I/we are using the correct terminology. The "main board" is the board that AC feeds into and has three fuses each going to the transformer, this board has very little electronics on it. The unit has three "power boards", one for each channel (R/L and center), and each "power board" is fed by one of the three transformers.By stating a winding may be shorted to another, I was thinking on the lines that say the the first turn had a section of insulation melted and it melted the (as an example), the fiftieth turn, then it would change the ratio of the transformer.I'm just trying to understand transformers a bit better. Obviously I'm not trying to criticize any feedback/suggestions.

author: bostonman
author_rank: Super Contributor
timestamp: September 09, 2025, 11:33:16 pm
content: I performed the test suggested; although still a bit reluctant to go directly to the wall so I went with a VARIAC.The VARIAC was plugged directly to the wall, a 1A fast blow installed for safety (although turning on the VARIAC even without the amplifier plugged in sometimes caused the fuse to blow), primary of the Toriod Transformer connected directly to the output of the VARIAC, and the meter across the secondary of the transformer.Measurements as follows:25V = 18.28V output, 50V = 37.2V output, 75V = 55.25V output, and 100V = 74V output.Connecting one side of the output to the center tap (I took less measurements):25V = 9.36V output and 100V = 37V output.I also connected to the other phase output to the center tap and the voltages at 25 and 100 were the same.My question regarding transformers for educational purposes: would a bad transformer (assuming it doesn't have burn marks and looks physically good) fail in such a test above without being loaded (I assumed a true test was loading it which is why I suggested a light bulb on the output)?Also, do transformers always fail catastrophically meaning it either outputs the correct voltage or it doesn't?

author: lowimpedance
author_rank: Super Contributor
timestamp: September 09, 2025, 11:55:22 pm
content: You have two other identical transformers on working modules, yes !,so Follow the same measurement procedure on those and compare.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 10, 2025, 12:35:43 am
content: Quote from: bostonman on September 09, 2025, 07:35:29 pmMy suggestion does not require or imply that music was playing. A component failure or short (tin wiskers perhaps) anywhere in the sound chain can cause an impulse through the system. If the failure is early in the chain there will be a lot of gain on what is probably already an abnormally large voltage change.It is good to learn about the transformers, and this is as good a vehicle for that learning as any. I am only suggesting that you don't get too focused on one potential failure area. So far you have two pieces of solid evidence, the report of the pops and the failed fuse. And a possibly suspicious capacitor. You can either think through the possible causes consistent with the data, and then conceive tests to support or reject each of those possibilities, or you can go through the entire unit and methodically verify operation of each stage and/or component.

author: bostonman
author_rank: Super Contributor
timestamp: September 10, 2025, 01:06:20 am
content: Thanks for clarifying.You're correct, it's a great opportunity to learn more about transformers. I've never had to question a transformer before, in fact, don't believe I ever come across a bad transformer. Unfortunately I'm trying to avoid too much tinkering with a $3k amplifier that's not mine. It's one thing if I blow up my amplifier, but I'd hate to tell my friend I blew up his.Usually my approach to repairing stuff is methodical and I like to rule out the essential circuits regardless if it's a waste of time. Sometimes I learn more as a result and/or other times it allows me to retain the layout so later on I can give thought to the problem.At least now I can (or at least believe I can) rule out the transformer since it seems to work. As suggested, I can test another to confirm the ratio.Thankfully I have a verbal explanation on what occurred during the failure, and, after looking at all the components, somewhat alludes to the "pop" coming from the speaker.After I test another transformer, I'll focus on the power board since it's the only thing left.Is it safe to say the transformer doesn't need to be loaded with a light bulb or something else to confirm it's good and measuring voltage in/out is adequate?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 10, 2025, 02:21:02 am
content: Quote from: bostonman on September 10, 2025, 01:06:20 amTransformers will usually fail open, or shorted windings.In rare cases you might get primary to secondary shorts, so you should take a little care.A shorted turn usually means significant current, which is normally visible on an outside layer.Measuring the idle AC mA current of the 3 transformers would be useful here.

author: bostonman
author_rank: Super Contributor
timestamp: September 10, 2025, 03:05:05 am
content: I'll take the measurements tomorrow.How come idle current is adequate and not under a load?If a transformer fails shorted primary to secondary, such a failure should be catastrophic and obvious. Also, open would be easy to check since either side (or maybe both) would measure open.What about in the case such as a situation I mentioned where a winding shorts to a winding on another turn (believe I'm using the correct terminology)? I know such a situation would be hard since most likely the transformer is wound tightly, but say the primary had 100 turns and turn 1 shorted to turn 20.If I'm thinking of this correctly, the ratio (primary to secondary) would change. Shorting turn 1 to turn 20 may not be increase the current enough to be noticeable, but would change the ratio so the secondary would see more/less voltage (depending if it's step up or down).

author: Gyro
author_rank: Super Contributor
timestamp: September 10, 2025, 12:00:17 pm
content: Anyshorted turn will cause raapid overheating, either local (more likely on mains primary), or the transformer as a whole, in addition to locally - thicker low voltage secondary windings. In a torroidal transformer, the primary winding in almost invariably wound underneath the secondary where it is inaccessible.A shorted turn will generally result in the affected part of the winding, or primary, burning out, unless a suitably rated primary fuse blows first. It won't result in a change in turns ratio without other overheating symptoms or failure.

author: bostonman
author_rank: Super Contributor
timestamp: September 10, 2025, 09:54:36 pm
content: I measured the input current with the secondary open, and, while at it, measured the primary and secondary resistance on the left channel (previously I only did the center and right).Adding to my previous measurements:Right channel (the one blowing the fuse): 1.797ohms primary and 1.035ohms secondary.Center channel (assumed good one that doesn't blow fuses): 1.796 ohms primary and 0.989 ohms secondary.Left channel (assumed good one that doesn't blow fuses): 1.764ohms primary and 1.013 ohms secondary.Measuring input current with 100V AC (through a VARIAC), open secondary, and a meter in series with the input side:Right channel: 181mACenter channel: 172mALeft channel: 185mASeems all three transformers are fine based on them all measuring approximately close.Unfortunately I made two errors even after going over all the connections several times due to being overly cautious about safety. First was not disconnecting the secondary side of the transformer from the bridge rectifier to the center channel Power Board and then bringing up the VARIAC. Although I doubt it's an issue, I avoid such situations where I under-voltage a circuit.The second was setting myFluke 79112 or a 179 (I have both, but don't have access at the moment to confirm which one) on mA, connecting it in series to the input side of the transformer, and accidentally left the leads connected to V and GND. After I increased the voltage to about 100V AC, the meter was reading 43mA and was decreasing very slowly.Doesn't seem like the meter suffered any damage, and, after looking online, it seems Fluke designs for such mistakes.Just goes to show, no matter how many times you go over something for safety reasons, you can still overlook something.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 11, 2025, 12:46:36 pm
content: Measuring the primary idle (magnetizing) current is quite informative. It indicates the core has not been damaged and that there are no shorted primary or secondary windings, either turn to turn or winding to winding. It may not catch a rare case where a shorted turn or two may not be detectable but then it’s unlikely to clear a primary fuse in an instant as well.Clearing fuses is a usually a sign of a catastrophic active component failure. A shorted bypass cap or something similar that allows large amounts of uncontrolled current to flow, not a resistor that has drifted out of tolerance.I think it’s a safe bet you have a bad amp board, so do you have a lab supply that can power the +/-15V op amp rails directly? Then you use the variac at slowly apply the +/- HV and monitor the current. Start by removing Q2 & Q16 and walk the voltage up. You’ll know in an instant if you have a blown output transistor. If you have a thermal camera, you can find a bad device that way as well. Lots of options, just need to know what you have to work with.

author: bostonman
author_rank: Super Contributor
timestamp: September 11, 2025, 01:23:32 pm
content: Good to know the transformers are good and I learned a bit about how transformers fail (although I thought the cost of learning was blowing my meter when I saw the meter probe was in the volts hole and the meter was measuring the 120V current).One or two concepts I didn't quite grasp. When the secondary is open, the magnetizing field still occurs (obviously it does since I measured mA current)? I wasn't sure if an open secondary would cause the current to be limited by the resistance of the primary winding only.Also, on the opposite, if the secondary is shorted, would the primary be higher current and not noticeable, or the magnetizing field would cause the primary to be almost equal to the shorted secondary current?As for repairing the amp, I had a question regarding the schematic and started a thread regarding the circuits. I didn't want to deviate from the topic of transformers since this was more about expanding my knowledge of them and feel maybe this other thread is more suited for repairing the circuit.Here is the thread I started:https://www.eevblog.com/forum/projects/question-about-amp-schematic/To answer your questions, I have several pieces of test equipment, so my options aren't limited. I'm trying to avoid experimenting too much if I can because it's not my amp, it's an expensive amp, and the problem should be quite obvious. If the schematic is correct (and I feel it's not), I could just disconnect the Zener that is fed by the input of VR1. If the fuse blows, it would indicate an issue within the first stage. Unfortunately I can't trust the schematic and randomly disconnecting could result in over-voltage somewhere else.Update: just for clarification from above, it wasn't a Fluke 79 (I crossed it off in my previous message). I just realized it was 112 or a 179 (I have both, but don't have access at the moment to confirm which one), but a 79 and 112 or 179 look physically different making me realize it wasn't my Fluke 79; noticed when I was looking at the manual which showed the picture.