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Old 17th March 2009, 12:29 AM   #1
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Default Rotel RB850 transformer blown?

Hi, i have a Rotel RB-850 power amp that had failed on one channel. I opened the amp up and tested all output transistors for short's but all were fine, then my ear was drawn to the fact that only one of the transformers was making the normal noise. I then did a voltage test on the wires coming directly from the left hand transformer to the pcb - no voltage present. The right hand transformer tested good so my thinking is that it is the left hand transformer that has blown?? Could it be anything else? Where can i buy one from?
Thank you.
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Old 17th March 2009, 01:38 AM   #2
h_a is offline h_a  Europe
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I cannot imagine that your transformer blew without other damage - except if it has a thermal fuse. Check first for fuses in the primary - you don't state wether any voltage arrives at the transformer at all

Have fun, Hannes
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Old 17th March 2009, 08:57 AM   #3
jaycee is offline jaycee  United Kingdom
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Disconnect the primary of the suspect transformer and measure it with an ohmmeter. If it's open, it's dead.

It's unlikely you will get an exact replacement EI transformer, but you could mount a pair (to keep it matched) of toroids in there.

A schematic suggests that this amp has 42V rails. In which case get yourself a pair of 30-0-30 V toroids. My guess is 100VA each will be plenty here, but it'll depend on the available mounting space.

You might want to replace the 4 power supply capacitors in the centre. They're probably long over due for replacement at this point.

Check also the bridge rectifier in the channel with the dead transformer. It's possible it failed, shorted teh secondaries, and that ruptured a thermal fuse. IMO, use some 400V 35A spade connection ones (GBPC type) and bolt them to the bottom of the case.
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Old 17th March 2009, 01:02 PM   #4
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yes there is voltage going to the transformer. There are no fuses around this area of the amp only up stream from it (2 per channel on main pcb).

I have contacted Rotel and they say they do not keep that transformer anymore, so i will either go with 'jaycee's' approach or just hang on for another rb850 for spares/repair so that i can make a good one. They don't tend to go for a lot on ebay even when working.
Either that or i will sell it as is and then buy something else.
Thanks for your help.
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Old 17th March 2009, 04:38 PM   #5
jaycee is offline jaycee  United Kingdom
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The likely cause if there is no primary fuses for the transformers, that there is a thermal fuse embedded in the primary windings of the transformer (these are required to pass certain CE safety tests for example)

Pretty much any RB-850 you get is going to have main supply capacitors well past their prime, and require renewing.

What size are the original transformers, out of interest?
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Old 16th April 2009, 02:59 PM   #6
gni is offline gni  United States
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This thread may help:

Rotel RB850 Still Current? Schematic Included

I have the US model . . . 120VAC. . .could have sold you a pair. . working on replacing
them with toroids.
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