high vac at speaker terminals rotel rb1070

chefsinc · 3rd January 2012, 10:26 AM

Dear Hi Fi Gurus,

I am having some issues with my Rotel RB 1070 power amp. I have measured the vac on the speaker terminals at idle nothing connected and it is giving me 92 volts on the right side and 38 volts on the left side.
there is no dc voltage at all on the terminals. I had to replace a 10ohm resistor in series with a 0.1uf cap going to ground at the terminals as it got toasted. the new resistors get hot but so far have not burnt out.

As a newbie to how electrical circuits work I would like for some comments or suggestions on how to fix it.I can send it off to the repair man but I think that would be passing up an opportunity to learn something new.
I would welcome any suggestions on this.

thank you very much in advance.

regards

jaycee · 3rd January 2012, 10:30 AM

It sounds like the amplifier is oscillating. The resistor/capacitor on the output terminals is called a Zobel network, and the resistor burning out is a sign of oscillation.

DF96 · 3rd January 2012, 10:57 AM

If it affects both channels then it could be a grounding fault. Look for loose connections.

Oscillation could also be caused by the Miller capacitors failing, but it is unlikely that they would fail in both channels at the same time.

3rd January 2012, 10:26 AM	#1
chefsinc diyAudio Member Join Date: Jun 2006 Location: malaysia	high vac at speaker terminals rotel rb1070 Dear Hi Fi Gurus, I am having some issues with my Rotel RB 1070 power amp. I have measured the vac on the speaker terminals at idle nothing connected and it is giving me 92 volts on the right side and 38 volts on the left side. there is no dc voltage at all on the terminals. I had to replace a 10ohm resistor in series with a 0.1uf cap going to ground at the terminals as it got toasted. the new resistors get hot but so far have not burnt out. As a newbie to how electrical circuits work I would like for some comments or suggestions on how to fix it.I can send it off to the repair man but I think that would be passing up an opportunity to learn something new. I would welcome any suggestions on this. thank you very much in advance. regards

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3rd January 2012, 10:30 AM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	It sounds like the amplifier is oscillating. The resistor/capacitor on the output terminals is called a Zobel network, and the resistor burning out is a sign of oscillation.

3rd January 2012, 10:57 AM	#3
DF96 diyAudio Member Join Date: May 2007	If it affects both channels then it could be a grounding fault. Look for loose connections. Oscillation could also be caused by the Miller capacitors failing, but it is unlikely that they would fail in both channels at the same time.

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