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Old 22nd December 2012, 09:32 PM   #11
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Quote:
Originally Posted by jaycee View Post
I have seen instances where the bridge rectifier has failed in these amps. They are only 4A rated units and that is a bit tight. Replace with at least an 8A one if it has been damaged.
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Originally Posted by sakis View Post
upgrade remarks correct

rectifier can be confusing from trafo windings but also voltage that is stored inside the caps

Kind regards
Sakis
Bad news, my friends! The rectifier is still good! But I measured the Transformer's secondary output and there's no voltage! Is that possible it is the transformer?? The main power switch is working normally...
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Old 22nd December 2012, 10:17 PM   #12
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Schematics from RA-930AX suggest a secondary output of 28VAC with the fuses rated for 5A...
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Old 23rd December 2012, 02:14 AM   #13
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I'm assuming when you say "the main power switch is working normally", that you are seeing the correct voltage across the transformer's primary.

The transformer will have a thermal fuse in the primary. Check and see if the primary is open circuit. If it is... the thermal fuse has blown. It will be practically impossible to replace this as it will be buried deep inside the primary winding.

The transformer has 28-0-28VAC secondaries. A standard 2x25VAC toroid would work to replace it, with only a tiny loss of power output - a watt or two, nothing to worry over. I'd guesstimate the VA rating is about 120VA, judging by the size of the transformer.
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Old 23rd December 2012, 08:17 AM   #14
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guesstimate...= English language evolution ?

did you check the fuse ?
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Old 23rd December 2012, 06:24 PM   #15
jaycee is offline jaycee  United Kingdom
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guesstimate = guess + estimate

There is no primary side fuse on this amp.
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Old 24th December 2012, 02:27 AM   #16
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[Repeated post. See below]

Last edited by Karl vd Berg; 24th December 2012 at 02:47 AM.
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Old 24th December 2012, 02:46 AM   #17
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Quote:
Originally Posted by jaycee View Post
I'm assuming when you say "the main power switch is working normally", that you are seeing the correct voltage across the transformer's primary.
Yes, correct! But I found that the WHITE wire (from primary bottom windings) is NOT closed or connected to any other (BLUE, BROWN or RED). It is OPEN to them. The other RED, BLUE and BROWN terminals (primary) still are closed circuits between them. According to the manual service on line you provided some would go for the 120VA. However, there's an open winding at the primary... The secondaries are ok...
Quote:
Originally Posted by jaycee View Post
The transformer has 28-0-28VAC secondaries. A standard 2x25VAC toroid would work to replace it, with only a tiny loss of power output - a watt or two, nothing to worry over. I'd guesstimate the VA rating is about 120VA, judging by the size of the transformer.
I found a toroid of 30VAC secondaries and rated 3.75A (???) In Holland we have 230VA mains, although I just measured 254VA right now!!

Now a question is if a 30VAC secondaries would force to a change with some resistor or is it still in the "good limit"?

Another question is linked to this page (type the code to access). The original Rotel transformer (model TT-240GF-2) has 2 REDS and one BLACK wires for its secondaries. How would be for the toroidal replacement having it 4 different colors for its secondaries?

Thank you so much, guys!!

Last edited by Karl vd Berg; 24th December 2012 at 03:15 AM.
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Old 24th December 2012, 06:55 AM   #18
jaycee is offline jaycee  United Kingdom
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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.
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Old 24th December 2012, 09:50 AM   #19
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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
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Last edited by east electronics; 24th December 2012 at 09:55 AM.
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Old 24th December 2012, 10:03 AM   #20
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Sakis, wouldn't a 230 V transformer be safe even if it runs hot at 254 V ? I'd expect the transformer designer to allow for a +/- 10% variation on mains supplies. Is this a real problem in practice ?
Here we never have that problem as the voltage is always on the lower side like 210 to 220 V. Sometimes hovers around 260 V for short periods of time !
But many people have ( controlled) motorised Auto transformers that could apply high voltages for short periods of time while it corrects itself.
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