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Old 12th July 2005, 11:58 PM   #1
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Default Transformer for RA-931 Amp

I have determined that the primary on the mains transformer on my Rotel RA-931 amp has gone "open circuit". I don't have a schematic so don't know the secondary voltage or VA of the transformer to buy a replacement. Does anyone know or indeed have the circuit diagram? I guess it should be easy enough to get another if I can find the electrical details.
Many thanks in advance

Graeme
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Old 13th July 2005, 03:55 AM   #2
Eva is offline Eva  Spain
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Most EI-core transformers include a built-in thermofuse, it's connected in series with the primary winding and it has an ill tendency to fail open.

If your amplifier doesn't use a toroid transformer then this should be the first point to check. Note that encapsulated toroids may include a thermofuse too.
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Old 13th July 2005, 06:26 AM   #3
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Hi Eva

Thanks for your reply. After a bit of research I also discovered there was a thermofuse. I took the covering (mostly plastic) off the toroidal and did a bit of testing and yes, the fuse is blown! Checked with Maplin's who have suitable fuses, but I'm not sure which temperature to use? Perhaps you can help? There are two wiring possibilities on my transformer, one direct and one via the fuse. Is it safe to use the direct connection. I shall of course be checking for reasons why it blew in the first place.
Cheers
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Old 13th July 2005, 02:56 PM   #4
Eva is offline Eva  Spain
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Thermofuses usually show very poor reliability, no wonder they are the first thing to check when the primary of a transformer goes open. I would use a thermal relay instead, they are much more reliable. However, you may discard thermal protection as long as primary and secondary windings are properly fused.

The enamel coating of magnet wire is usually rated at 150șC or 180șC, but some of the plastic parts of the transformer may start to melt or lose their shape at lower temperatures. Also, note that it's quite hard to sense the exact temperature of the windings so it's a good idea to allow for some margin, I think that 75șC cutoff would be fine.
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Old 13th July 2005, 06:49 PM   #5
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Thanks for your help.
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