Rotel RA-930BX

[sakis]

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

[ashok]

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.

[Karl vd Berg]

Guys, you're simply the best!!

Originally Posted by jaycee

Dont confuse VA rating of the transformer (secondary voltage * current) with voltage...
The transformer you linked to is 225VA, which means 2 * 30V * 3.75A = 225VA. Higher VA is OK electrically, the problem is only that the size of the transformer may be too large to mount in the case.

30V AC secondaries would be a bit too high in my opinion, this would produce 42V DC supplies, and probably more like 45-48V with ~250V AC on the primary. You would risk causing too much heat in Q901/902 short term, and in the long term risk damage to the output transistors.

The original transformer has a centre tapped secondary. Referencing the diagram on that website, you would connect the Yellow and Blue wires together, and this would connect where the original Black wire connected. The Red and Grey wires would connect where the original Red wires connected.

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)??

Originally Posted by sakis

i stand corrected about the fuse thanks JC you are right .

if mains measure for some reason above 240 obviously the transformer is toasted ( count this worst in case that happens in long term )

But sure you need to check this out there is going to be more failure coming in case and for any reason your mains is 254 volts

happy holidays
sakis

PS
The amp will make it also in case of rail voltage is a bit higher than specified though 2X24 will e also easy to find ..That high rails will not support loads specified lower than 8 ohms , then heatsink will be too small to handle the extra power ( already heatsink is small ) ,secondary power supply via zener and resistor might suffer more ... so yes if its for casual listening in low power and easy load ....if pushed or low loaded the amp will eventually fail and take with , precious speakers since the 930 features also no DC protection

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...

[jaycee]

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 ?

[Karl vd Berg]

Originally Posted by jaycee

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

Yes! The original transformer itself (still inside the amp) measures 80mm (width) by 95mm (depth) and roughly 120mm (diagonals)

Shall I order it on-line right now??

Thank you!

[Karl vd Berg]

Another question Jaycee! The new toroid is rated 2.40A!

Any advice on that? The original fuses has to be changed?

And finally, the white wires on the primary side will go in 0V and AC3 in the main board, right?

[jaycee]

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.

[hitachi_nut]

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.

[Karl vd Berg]

Originally Posted by jaycee

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.

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.

[Karl vd Berg]

Originally Posted by hitachi_nut

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.

Thanks for all the info!

No! the amp has no voltage selector. It's AC240V @ 50Hz...