practice have proven that in long term this is creating trouble especially when trafos are marginal from any aspect either primary or secondary
add to the above that trafo will be also loaded from secondary circuits while operating at 254 volts so its a no all the way through
It will survive for a short term but in long terms to my understanding it will fail
kind regards
sakis
add to the above that trafo will be also loaded from secondary circuits while operating at 254 volts so its a no all the way through
It will survive for a short term but in long terms to my understanding it will fail
kind regards
sakis
Right about the long term effects . The designers would not have factored in the increased dissipation due to different conditions on the circuit side. Probably it is a good idea to have a mains supply monitor which tells you if it has gone beyond limits and possibly sounds an alarm. For people who have long spells of high voltage the only solution is to use a voltage stabiliser.
Guys, you're simply the best!!
2x25V 80VA ringkerntransformator - Ringkerntrafo.nl (80VA)
or even
http://www.ringkerntrafo.nl/120va/453-ringkern-transformator-230v-2x6v-15va-.html (120VA)
For the mains VAC sometimes it can reach incredible 260V AC like yesterday!!
So then here we have a problem? These strange long peaks of voltage can last for more the an hour and perhaps this was the main cause of this failing transformer? I only hope the new toroid could manage +/-10% of voltage surge.
Jaycee, according to the new toroid diagram, if we put the Yellow and Blue wires together then we'd double the secondary voltage, right? I was thinking to put together the Yellow and Grey for the original Black wire, and the Red and Blue for the AC1 and AC2 secondary inputs (Rotel Manual page 4)??
Now (15:50PM) it's back to the normal 226V AC. I noticed that the 260V peak occurs mainly during the nights than during the day time...
Right, so are these ones better?
2x25V 80VA ringkerntransformator - Ringkerntrafo.nl (80VA)
or even
http://www.ringkerntrafo.nl/120va/453-ringkern-transformator-230v-2x6v-15va-.html (120VA)
For the mains VAC sometimes it can reach incredible 260V AC like yesterday!!
So then here we have a problem? These strange long peaks of voltage can last for more the an hour and perhaps this was the main cause of this failing transformer? I only hope the new toroid could manage +/-10% of voltage surge.
Jaycee, according to the new toroid diagram, if we put the Yellow and Blue wires together then we'd double the secondary voltage, right? I was thinking to put together the Yellow and Grey for the original Black wire, and the Red and Blue for the AC1 and AC2 secondary inputs (Rotel Manual page 4)??
Yes, as I say before, yesterday the mains was at 254V AC, reaching sometimes 260V AC!
Now (15:50PM) it's back to the normal 226V AC. I noticed that the 260V peak occurs mainly during the nights than during the day time...
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Karl, can you measure where the transformer would go, so that I can advise by size what VA rating to go for ? The 2x25V 120VA transformer should fit (diameter 92mm) but it is best to check!
The way I described connecting the secondaries is correct. It would only double the secondary voltage if you were only connecting the Red and Grey wires to the amp, but you aren't. By connecting Yellow and Blue together, you create a centre tap and end up with 25-0-25 V AC, similar to the original transformer. This is then rectified and smoothed to produce the +35v and -35v DC supply rails that the amp runs on.
If you see the schematic diagram that shows the original transformer connection, it should make sense
edit: The high mains is probably what has killed the original transformer. Did it used to buzz at all ?
The way I described connecting the secondaries is correct. It would only double the secondary voltage if you were only connecting the Red and Grey wires to the amp, but you aren't. By connecting Yellow and Blue together, you create a centre tap and end up with 25-0-25 V AC, similar to the original transformer. This is then rectified and smoothed to produce the +35v and -35v DC supply rails that the amp runs on.
If you see the schematic diagram that shows the original transformer connection, it should make sense
edit: The high mains is probably what has killed the original transformer. Did it used to buzz at all ?
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Yes! The original transformer itself (still inside the amp) measures 80mm (width) by 95mm (depth) and roughly 120mm (diagonals)Karl, can you measure where the transformer would go, so that I can advise by size what VA rating to go for ? The 2x25V 120VA transformer should fit (diameter 92mm) but it is best to check!
The way I described connecting the secondaries is correct. It would only double the secondary voltage if you were only connecting the Red and Grey wires to the amp, but you aren't. By connecting Yellow and Blue together, you create a centre tap and end up with 25-0-25 V AC, similar to the original transformer. This is then rectified and smoothed to produce the +35v and -35v DC supply rails that the amp runs on.
If you see the schematic diagram that shows the original transformer connection, it should make sense
Shall I order it on-line right now??
Thank you!
The transformer sounds like it should fit. You should be able to cut a small bit out of the PCB if you need to, as there is quite a lot of empty space on the side nearest the transformer. The RA-931 amp is the same circuit and PCB, but they used a toroid transformer.
The fuses should be fine the way they are. I suspect they have rated them at 5A to counter for the initial charging of the supply capacitors. Really, the fuses should have been AFTER the capacitors.
The white wires do connect where the original transformer's primary wires would connect. The service manual is a bit confusing here. It shows the mains cord connected to 0V and AC3, and the transformer connected to 0v and AC4. As long as it is wired so that the power switch breaks the circuit, it will work The extra 120-220-240 terminals seem to just be holders for the wires, and not actually connected, so you may ignore those.
The fuses should be fine the way they are. I suspect they have rated them at 5A to counter for the initial charging of the supply capacitors. Really, the fuses should have been AFTER the capacitors.
The white wires do connect where the original transformer's primary wires would connect. The service manual is a bit confusing here. It shows the mains cord connected to 0V and AC3, and the transformer connected to 0v and AC4. As long as it is wired so that the power switch breaks the circuit, it will work
The extra 120-220-240 terminals seem to just be holders for the wires, and not actually connected, so you may ignore those.If it was me, I would just replace the thermal fuse on the original transformer. The thermal fuse is usually located on the outer section of the transformer. Who in their right mind would locate the thermal fuse on the hottest section (inner) of the transformer? Besides, it complicates the transformer winding process and leads to early transformer failure if the thermal fuse is located other than the outer part. Remove the copper shield and look for the bulge on the primary side, the thermal fuse is usually inside a fiberglass tubing. The transformer bobbin might have separate sections for the primary and secondary windings which makes servicing easier. The transformer is already broken so why not open the thing? You got nothing to loose at this point.
You don't have a short on the secondary side so chances are the transformer windings are still good. This is not a high spec amp and lacks decent speaker protection (no relay). I wouldn't throw that much money upgrading an amp like this. There is no user-replaceable fuse on the primary, add one while you are at it.
This is a simple problem, the thermal fuse might have failed because it was not spec'd properly, premature failure or the line voltage was too high for a long period causing the transformer to cook. The output bias might have been set too high causing the idle current to be high. Continuous high current draw translates to more transformer losses which equates to more heat generated especially if the line voltage lingers on the high side too long. A lot of amps I've seen, even factory set ones have their bias set too high by BLINDLY following the service manual suggested value. The right way to set the bias is with a THD/THD+N analyzer while monitoring the primary/secondary current. You would think all manufacturers do it this way but they don't.
One other thing, does your unit have the voltage selector? Set it to 240V. Your line frequency is at 50Hz. Transformers designed for 50/60Hz operation run hotter at 50Hz BTW.
You don't have a short on the secondary side so chances are the transformer windings are still good. This is not a high spec amp and lacks decent speaker protection (no relay). I wouldn't throw that much money upgrading an amp like this. There is no user-replaceable fuse on the primary, add one while you are at it.
This is a simple problem, the thermal fuse might have failed because it was not spec'd properly, premature failure or the line voltage was too high for a long period causing the transformer to cook. The output bias might have been set too high causing the idle current to be high. Continuous high current draw translates to more transformer losses which equates to more heat generated especially if the line voltage lingers on the high side too long. A lot of amps I've seen, even factory set ones have their bias set too high by BLINDLY following the service manual suggested value. The right way to set the bias is with a THD/THD+N analyzer while monitoring the primary/secondary current. You would think all manufacturers do it this way but they don't.
One other thing, does your unit have the voltage selector? Set it to 240V. Your line frequency is at 50Hz. Transformers designed for 50/60Hz operation run hotter at 50Hz BTW.
Yes, the fuses F901 and F902 (next to the rectifier) are rated 5A, and the other F601 and F602 (next to the power caps) are rated 4A.
Thanks for all the info!
No! the amp has no voltage selector. It's AC240V @ 50Hz...
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I'll try to open (???) the transformer to see where is the thermal fuse...
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