I have a Onkyo amp that the filters vent. I replaced them once and the replacements vented too. So this time, I measure the voltage. It's +/- 45 Volts. The caps working voltage is 35v!! Okay, so they're venting for a reason
The original caps were 35VDC rated.
The power transformer has a 2-wire primary.
The back of the set say's "US 120VAC 60Hz".
I've checked the rectifiers, they're okay.
The only thing that I can think of is that the transformer is for Japan market (100V), or that the transformer primary has shorted in a way to give a different turns ratio. ...A turns ratio that produces greater secondary voltage. I haven't seen this before. Is there something else that could cause the large secondary voltage? Has anybody seen this type of failure?
The original caps were 35VDC rated.
The power transformer has a 2-wire primary.
The back of the set say's "US 120VAC 60Hz".
I've checked the rectifiers, they're okay.
The only thing that I can think of is that the transformer is for Japan market (100V), or that the transformer primary has shorted in a way to give a different turns ratio. ...A turns ratio that produces greater secondary voltage. I haven't seen this before. Is there something else that could cause the large secondary voltage? Has anybody seen this type of failure?
Yes. I've got the AC feeding the unit going through a VariAc with a voltmeter on the output. I've also got a current meter in series with the load.
It looks like the unit was setup only for the US market. There's no indication that the primary can be wired in any other way (2-wire, no taps or jumpers). It doesn't look like the power transformer has ever been replaced.
I don't think it's a Japanese market transformer put into a US unit. I found while measuring the DC voltage on one of the filters, that only 58 VAC in turns into a DC output of 35VDC, ...the working voltage of the cap. So something is really messed up in this picture. I think I'm gonna use an isolation transformer on this one. Something is really weird.
Any shorted turn would cause the trafo to draw huge amounts of current - either the trafo gets REALLY hot or the fuse blows. Since that doesn't seem to be what's happening, must be something else...
Could the thing somehow turned into a voltage doubler circuit...
edit: otoh it could be that the short is from one turn to some other point "far' away. That would make sense...
Could the thing somehow turned into a voltage doubler circuit...
edit: otoh it could be that the short is from one turn to some other point "far' away. That would make sense...
Disconnect the XFMR AC output from the amp. Isolate the transformer. Measure it (ohms and volts). Do you have a voltmeter on your variac? Is the variac telling the truth? Was the amp brought to you broken this way? I have never heard of a damaged power supply delivering more voltage when it is broken, it kinda goes the other way, ya know. 
You sure do have something weird there.
Shawn.
You sure do have something weird there. Shawn.
Has the unit accidentally been rewired incorrectly by someone?
A simple power supply will have a XFMR like yours? say 20-0-20 VAC coming out but if wired wrong, you could drive 40-0 VAC into the rectifier and then reach the caps with a big DC voltage? I'm speculating of course. Perhaps someone else messed around with it before it reached you?
Shawn.
A simple power supply will have a XFMR like yours? say 20-0-20 VAC coming out but if wired wrong, you could drive 40-0 VAC into the rectifier and then reach the caps with a big DC voltage? I'm speculating of course. Perhaps someone else messed around with it before it reached you?
Shawn.
Well, he says 'The original caps were 35VDC rated'. But were they really the originals?
The reason I asked about other windings is that usually regulated supplies are run off of these, rather than tapping off of the main filter caps and regulating it down. I've no idea how Onkyo might have done it, but if the other windings look OK, and the regulators are not trying to fry themselves, then pinkmouse ought to be close to the truth, and I'd put some 50V caps in there and call it a day.
The reason I asked about other windings is that usually regulated supplies are run off of these, rather than tapping off of the main filter caps and regulating it down. I've no idea how Onkyo might have done it, but if the other windings look OK, and the regulators are not trying to fry themselves, then pinkmouse ought to be close to the truth, and I'd put some 50V caps in there and call it a day.
Thanks for the replies. This unit appears to have never been serviced. I'm wondering if the caps managed to keep it together in over-voltage conditions for a long time. That's frightening. It looks like the STK power IC's have managed to hang in there too. The transformer has (what looks to be) a single primary winding, then loads of secondaries. I haven't checked thoroughly yet, but I think that all of the secondaries are too high. The only yardstick that I have currently is the working voltage on the caps themselves. It seems the center tap fro ground is okay. I have symmetrical voltages off of the rectifiers. The transformer has never been opened. There's still un-cracked varnish sealing the bands etc.
I will order the roadmap for this one. I want to check if the transformer part number is correct. I assume it is, because even with a Japanese transformer installed, the secondary voltages are too high. Then Ill figure out what the normal turns ratio should be. If it turns out to be a transformer with short in the primary (tapping another voltage) and somehow not causing severe current draw, I'll be blown away. It's kind of what I'm leaning towards at the moment.
With regards to shorted turns causing current draw. I told someone once that they shouldn't mount a torroid in a metal enclosure with bolt going through. I told them that if there was an electrical conduction path, the mouting would look like a shorted turn. They tried it and proved me wrong!? There was no increase in VariAc current, no sparks, no nothing. Was I wrong in not recommending the metal box? Is there no problem. Is it possible the primary of my transformer wouldn't pull excess current if the original connections remained but there is a short existing somewhere along the windings?
This is one of the more curious failures I've seen. I'll start first with the service manual.
I saw the question whether these 35WVDC caps were factory. I believe so. All of the filters had some sort of white paint/adhesive holding them to the boards. The paint was not cracked yet. You're right, I had to ask myself the same question.
I will order the roadmap for this one. I want to check if the transformer part number is correct. I assume it is, because even with a Japanese transformer installed, the secondary voltages are too high. Then Ill figure out what the normal turns ratio should be. If it turns out to be a transformer with short in the primary (tapping another voltage) and somehow not causing severe current draw, I'll be blown away. It's kind of what I'm leaning towards at the moment.
With regards to shorted turns causing current draw. I told someone once that they shouldn't mount a torroid in a metal enclosure with bolt going through. I told them that if there was an electrical conduction path, the mouting would look like a shorted turn. They tried it and proved me wrong!? There was no increase in VariAc current, no sparks, no nothing. Was I wrong in not recommending the metal box? Is there no problem. Is it possible the primary of my transformer wouldn't pull excess current if the original connections remained but there is a short existing somewhere along the windings?
This is one of the more curious failures I've seen. I'll start first with the service manual.
I saw the question whether these 35WVDC caps were factory. I believe so. All of the filters had some sort of white paint/adhesive holding them to the boards. The paint was not cracked yet. You're right, I had to ask myself the same question.
Okay, so as to not leave anybody interested hanging. We have some professional relations with Onkyo. I did tell them my findings. They were going to send me the schematic, but now they're going to swap out the whole unit. No questions asked. So the bad news: they probably won't tell me what the failure really is. I've asked that they tell me the failure, but you know how that's going to pan out.
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