I also am suspecting that there must be another factor in the failure. Possibly a burst of higher voltage, high enough to do damage but short enough to not pop fuses. But the first thing to check, because it would be obvious and easy to detect, will be continuity between the power transformer and the supply rectifiers. If that continuity is OK, or even if not, check the rectifier diodes for being either open or shorted. A burst of over-voltage can cause that sort of failure.
Then check for the voltage across the 10,000 mfd capacitors, relative to the supply common, which might be chassis ground..It should be close to the posted value of 41 volts. If not, the rectifier bridge is suspect. But if the 41 volts is good, then check the regulator outputs, 15 volts positive and negative.
Either the supply is working or not, it is the easiest to check and the cheapest to repair.

 

Good plan. Thanks. Will report back. Sunday, I hope.

 

Keep an open mind. It could have been a latent defect in the capacitors themselves. Given that they were probably side-by-side they were probably also side-by-side in the reel of components used in the machine that placed them on the PCB, which might mean they sisters or even twins.

 

Mackie is noted for making fairly high quality products, and they have a good reputation. In addition, it is my impression that this monitor is not new, nor a recent acquisition. Maybe they did just fail for no obvious reason, but given their position in the circuit, it should work quite well without those devices, unless their demise broke the power feed path, which might have happened. That was the reason for my most recent suggestions.
This might be the same TS who had recently experienced other problems, I think. Those seemed to be related to mains supply issues.

 

One remote possibility is that the connection to the centre tap of the transformer was broken by a bad connection at J13-2 and there was much more load on the negative rail than the positive rail C52 and C53 could see almost the full 58 volts AC acress them. This would still only have a peak value of about 82 volts. This still should not be enough to damage them as they are rated at 100 volts.

Les.

 

It is a bit puling because C52 and C53 are across half of the 24 volt power transformer. As I examine the circuit I see noway a believable failure of any component could have caused them to fail. And unless they have damaged the transformer by excess current. So I suggest first clearing away the damaged capacitors and then checking to see that the transformer is still able to deliver the original output voltage. Possibly, if the amplifier voltage input switch was set for 120 volts mains and the power applied was 340 volts, that might cause the problem.
I also suggest checking the two fuses, F1 and F2, a shorted circuit probably took one or both fuses out.

A lot of this I agree with. But excessive current shouldn't be an issue for those caps. I AM on board with the notion of being over-voltage'd. At either 120 or 240VAC, if either got through the transformer i.e. a short between primary and secondary, that could be the root of the problem. Someone mentioned an intermittent short where when overheated the short may appear (or disappear). I'd test the transformer completely disconnected from ALL other components, wires, fuses, switches - everything. Check for continuity of any resistive value between the secondary (either lead) and (one at a time) the primary coils. There are two because the unit can be powered from 120VAC or 240VAC. Depending on how the switch is configured.

On its own, if the switch was set to 120 and the unit was plugged into 240, the secondary output still would not exceed the cap's rated 100V. At worst, that voltage would reach (accepting the schematic values) 58VAC. Since rectification is taking place AFTER D24, no compensation for the voltage on any of the caps prior to D24. AND since each cap is connected to center tap, should the tap fail, the voltage STILL would not come close to the 100V cap rating. Therefore I strongly suspect a bad transformer.

I assumed one of the internal fuses had opened but they were fine.

Going to say this - you might think I'm suggesting you don't know how to test fuses; well, there have been visitors here who truly had no idea how to test a fuse, so please don't take any offense. Fuses need to be tested OUT OF CIRCUIT. If you tested them IN circuit there's a possibility you got a false reading.

I can't see any reason that this failure would, on its own, stop the amp from working.

Agreed. Looks like a robust design. But even with the best engineering things can still go wrong. Just ask (if ever you could) the crews of Space Shuttle Challenger or Columbia.

Fully diagnose the problem before throwing parts at it. Make sure the transformer has no issues. As for finding an intermittent issue - - - that truly takes some damned good luck.

One other possibility comes to mind: Was there a local thunderstorm during the new-found failure? If so then there's also possibility that other issues may be lurking on the board(s). Lightning has been known to do some strange things. Striking a bicyclist four miles away, under clear skies. And four miles is the average length or reach of a strike. Lightning on its own is a very interesting subject. But I'm not taking this discussion away from the issue; fixing all the failures. An "Ash filled crater" suggests quite a violent hit. I suspect you're going to find more issues. Even if you're lucky enough to prove the transformer 100% reliable or highly suspect.

 

Killing 0.1u 100V is not easy and they are being used as simple high freq filter caps. IMHO, the only way they "toast" is overvoltage caused by a primary to secondary short in the transformer. As others suggested, measure resistance between primary wires and secondary. And be aware, transformer winding shorts can come and go due to heat. So the short may only happen after a period of time while under load.

"Shorts can come and go due to heat." If you've ever noticed a street lamp that lights up for a few minutes then extinguishes for a few minutes, then repeats that cycle, that's an example of a broken wire in the ballast transformer. When cool, metals contract and the break makes temporary contact. But once current starts flowing the transformer heats up, expands and the break opens up again. In order to test the transformer you're going to have to test it when cold, then connect it to power, then apply some kind of load to the secondary to cause the transformer to heat up. I have no idea what kind of current you'd need to draw to cause the transformer to heat without blowing the fuses. Nevertheless, proceed with caution.

[edit]
When testing while powered you're going to have to monitor the secondary voltage output. IF there IS a short between a primary and the secondary, and you're connected to 240VAC, be advised, you can see voltages on the secondary somewhere close to that potential. I'm expressing the concern for danger when testing line voltages because they are dangerous. Be careful not to let wires touch each other OR something grounded. The sparks can be quite exciting and may even result in a visit from your local fire department.

 

and you're connected to 240VAC

line voltages - - - are dangerous.

Even 120VAC is dangerous. But good advice for sure.

 

I have no idea what kind of current you'd need to draw to cause the transformer to heat without blowing the fuses

One method I can think of is to connect an ordinary house lamp to the secondary. There should be 58VAC across the lamp and it may even glow. The lamp should be a normal incandescent bulb, not CFL or LED. Should the transformer short primary and secondary, the lamp will glow at full brightness. Otherwise under normal conditions it should glow dimly but glow it will. You may have to let it run for a few hours under that condition. And start with something small like a 60W bulb. If after a few hours there is no sign of troubles, switch to a 100W bulb if you can find one. I would imagine, and like Pan said, I don't have any real good ways of determining what kind of amperage you may have to draw in order to get the transformer to overheat.

Given that there's a center tap you may have to test that as well. Just test it as many ways as you can think of. If after all efforts to repeat a failure fail, then maybe, just maybe, the transformer is not the problem. It may just be a simple of a lightning strike finding its way into your device. But fully diagnosing ALL potential issues is not going to be easy.

I had an amp that failed for no apparent reason. Turns out it was MY D24 (bridge rectifier). It simply was too small for the load I was putting it through. One of the diodes shorted and blew the main fuse. After fully diagnosing the problem I replaced the BR with a more robust one and the amp has worked wonderfully for many years. You could solve your problem quite easily. But again, fully diagnose the problem.

 

What I was intending to convey, regarding an over voltage, was a transient spike on the power line. A spike of much higher voltage, possibly as long as a millisecond, far faster than a common fuse could respond to, which would cause a breakdown of the capacitor, and possibly the breakdown of the power rectifier diodes as well. Not a shorted connection between primary and secondary, but a pulse that contained enough power to pass through, transformed, to the secondary. I have had damage from what must have been such a spike that it jumped the contacts of the mains switch and failed the rectifier diodes in a tube-type device many years ago. The smoke did not appear until it was switched on some days later, resulting in the 100 ohm resistors in series with the diodes starting to smoke and stink. The resistors were in place to keep the output voltage where it would have been if a tube rectifier had been used, But they served to protect at a lower cost than actual fuses.

 

Anything is possible. Unlikely a voltage spike of that magnitude could come through and do that much damage; at least that's what I think.

 

All thespike would have to do is cause a breakdown in the dielectric insulation. Once a breakdown occurs current starts flowing and that leads to a temperature rise, which leads to more current flowing and more temperature rise. A bit like a sharp pin and a balloon, except a lot slower.
I am thinking that this is the same TS that had issues with the street lights flickering when some audio equipment was not operating correctly., so there could be some serious disturbances in the mains power feed.

 

Am I the only one who noticed that the two blown caps are on a single side of the center tapped secondary? A transient spike should have hit the whole thing, not just half.

 

MTT727 How's it going? Any progress?

 

We are over-analyzing this. Speculating on way too little information. Let's wait and see if merely replacing the caps solves the problem.

 

Look at the circuit and observe that those caps are not required for correct operation. The big question is about the power supply and the diodes. Any spike that could fail those capacitors could also damage a diode. And we still have no information about the condition of the conductors on the PCBin the area where the capacitors were. A simple power off check with a multimeter can tell us if the diodes are open or shorted or OK, and also if there is continuity between the bridge and the transformer. Which I suggested checking long ago.

 

Personally I'm convinced it's a blown Johnson Fragulator.

 

Personally I'm convinced it's a blown Johnson Fragulator.

My comment was serious, not joking.