Old Rotel Power Amp power input change - diyAudio
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Old 30th September 2004, 11:04 PM   #1
Elrin is offline Elrin  Canada
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Location: Prince George, BC, Canada
Question Old Rotel Power Amp power input change

Hi there!

I have a relatively old Rotel RB-971 Stereo Power Amplifier that I bought second-hand a while ago. It's been working beautifully powering my centre-speaker for years. I recently moved to Canada, and my Home Theatre just arrived by sea. I now need to convert all the units from a 240V model to a 120V model. Using an external transformer is just hellishly expensive when buying 1500Watt, and noisy too - besides, all the units have taps for different input voltages already...

All my other equipment (a bit newer) either have silkscreen printing, stickers or switches that will help me with the re-wiring of the power transformer primaries.
The RB-971 though, has none. I don't want to go and blow up my amp, but the $50 plus long distance shipping and long wait is a bit too much considering that I can do it myself if I am equipped with the right information.

From an initial investigation, I've gathered the following information so far without de-soldering anything:

There are 6 wires connected to the primaries; White, Green, Orange, Yellow and Brown.
Currently, for 240V operation, the White and Green are connected to the Supply Neutral, The Orange and Yellow are simply shorted together, and the Brown is connected to Live via the switch.

By looking at the resistances, I can gather that there might be two coils in series (or, with 6 wires maybe two parallel sets) between Green and Brown. These two coils are probably connected in series by shorting Orange and Yellow.

Can anybody perhaps help with the correct information on the layout of the coils in the transformer and how to best re-wire it's connections?

Thanks for any help in advance!
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Old 1st October 2004, 07:46 AM   #2
Netlist is offline Netlist  Belgium
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Here's the manual, just in case. http://www.xlfidelity.se/brux/
Usually for 110V the windings are in parallel and for 230V in series.
I assume you new this.
If you have a variac you could try disconnect the short between Orange and Yellow, connect Brown and Yellow to Live, and Orange, White and Green to neutral.
Then slowly power up and watch the Ampere meter.

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Old 1st October 2004, 12:53 PM   #3
Tensop is offline Tensop  Lebanon
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unscrew the rotel plate on the back of the amplifier if there is one, theres a voltage change junction behind it
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Old 1st October 2004, 02:27 PM   #4
Elrin is offline Elrin  Canada
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Thanks Hugo and Tensop for the reply!

I actually unpacked my soldering iron today, and will have to get a new tip for it too.
I don't have a variac (would love to have one of those), so I will just disconnect the secondaries for in case, and have to hope my guess about the winding configuration is correct.

Tensop, there is unfortunately no Rotel plate on the back. The utility power goes directly to the front switch and posts on a routing PCB.

Currently, I'm assuming there are 4 windings on the transformer. They are between:
White and Orange
Green and Yellow
Orange and Blue
Yellow and Blue

The thing that makes me jittery is that there appears to be a short between White and Green - and it does not seem to be on the PC board...

Hopefully I can better determine the coil configuration after I get my soldering iron working and can disconnect the wires... It's actually quite a challenge to convert so many tools - sometimes I have to convert several other things first just to have the tools to convert something else...
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Old 1st October 2004, 03:17 PM   #5
Netlist is offline Netlist  Belgium
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Originally posted by Elrin
I don't have a variac (would love to have one of those), so I will just disconnect the secondaries for in case, and have to hope my guess about the winding configuration is correct.
Make sure you put a fuse in series with the mains.
If you disconnect the secondary 1AT or less will do.
There should be no or a very small current draw
You could also use the good old light bulb in series.

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Old 15th October 2004, 04:12 AM   #6
Elrin is offline Elrin  Canada
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Location: Prince George, BC, Canada
Well, I finally did the conversion (after replacing my soldering iron, etc.), and thought I'd give some feedback:

First off - thank you for your advice everyone! Your suggestions helped me a lot.

For future reference (perhaps someone with a similar problem as me), here's what the system turned out to be:

The transformer primaries consist of two windings, initially (220V model) connected in series. I re-connected them in parrallel, and it's working beautifully now.
The windings are: White to Orange, and Yellow to Brown. The Green wire is connected to the White inside the transformer via a thermal switch.

Rotel have a little PCB with the on/off switch on with posts for connecting the primary wires. There was a "storage" post not connected to anything for the Green wire in the serial configuration. Another two posts were just connected to each other, providing a way to connect the two windings in series.
By using the linked posts to connect green and yellow, and wiring the live AC to orange and brown did the trick.

The secondaries (measured from center-tap (black) to the sides of the secondary) both output 36VAC when working correctly.

Thanks once again!

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Old 14th September 2005, 08:44 AM   #7
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Default Elrin are you out there?

Elrin, I'm in the same boat with an old Rotel wired for 240 voltage. I tried to send you an email, but you do not accept forum emails. Could you please take a look at a post I just put up showing pics of my Rotel RA-820 transformer? I'm hoping to do the same thing you did and convert this to 120v. Thanks!

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Old 18th September 2005, 07:09 AM   #8
Elrin is offline Elrin  Canada
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I'll took a look, and it looks familiar!

First off - do you have a 240v supply available (even on loan or very low wattage?) If you do, it will make discoverring the right settings much easier.

You will see that your transformer has several posts on the input side. Two on the left, and three on the right. Usually, those three on the right are different taps that will be for the different input voltages. You'll have to measure them carefully to make sure, but usually, they are for 100V, 120V and 240V. I can see the 2'nd and 3'rd from the right have two wires, so they are definitely taps. The exact voltages however, depends on the amount of turns around the core. The two posts on the left is probably before and after the protecting thermal switch most transformers have - almost like a fuse. You probably don't have to change anything there.

The easiest way to check, would be to remove the blue and white wires, and measure the resistance between the white wire post and each of the three posts on the right. You should see an increase in resistance in Ohms for each. The resistance should be relatively small, so you need at least a semi-decent multimeter!

If you scale the values up so that the right-hand one is at 240V (that's the known one), then you should see the voltages for the others. Because it is hard to measure the ration very accurately via resistance, don't worry if it's out by a bit. If it's in the right vicinity, it should be the right one.

Then, before you change the input, connect it to a 240v input. If you don;t have it, (like me) skip this step. It's not essential, just gives you extra peace of mind. Measure the voltages on the outputs of the transformer. Remember these. Then (very importantly) disconnect the outputs, and change the input to the correct voltage for where you live(120V). Then, measure the output voltage. It should be the same. If it is, power off, and re-connect the outputs, and there you go!

I hope this helps!
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