Question about Rotel RMB-1095 Power Amplifier

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Rotel RA 9808X Amplifier

Im not sure how technical the kind assistance on this site is, but let me try. I should first mention that I do have electronics knowledge, but that is different to troubleshooting and repairing audio amplifiers particularly without a circuit diagram which is frustrating. My Rotel Amp has started distorting on just one channel with moderate listening volumes. At lower volumes there is no distortion. As the volumes increase so does the distortion, almost as if the Amp is clipping or the tweeter is faulty (but dont let this subjective info bias you !). The fault lies in the power amp, as with swapping the preamp to power amp cables the fault remains in the left channel. The same is true of swapping the speakers over, ie the fault remains in the left channel/speaker. Happily I can use the working Amp channel as a reference as a poor substitute for circuit diagram ! Assuming I can find my way around the board, my first port of call would be to check the power rails, although Im unclear how one channel can be fine and one not in this scenario. My next thought would be its a capacitor breaking down somewhere, which is going to be a little more tricky without a circuit diagram. Im wondering if any experienced wizards read this board and can help me out with some tips and experience, or failing that perhaps where to look for the same ? Terry.

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  • Terry Whit Jan 19, 2007

    Thankyou for the pointers, and thanks for your response ... I will take a look first up at all the points you mention and report back in due course for interests sake. Im currently searching for a schematic to assist in general but also in case all your ideas check out OK ....

    T

  • Terry Whit Feb 26, 2007

    After a long delay getting a replacement transistor, it turns out the problem was further back in the final stages with a transistor involved in setting up the bias voltage for the output stages. This had gone short circuit and the result was a +9V bias on three of the output transistors that were supposed to be biased at -0.4V. Of course this meant that half the output waveform was "truncated" (!) naturally causing the distortion heard in one channel.

    I have to say that following advice on the Web I contacted Rotel for a schematic, who were very obliging. Without this I would have been unable to find the fault, and in my book is a big plus for decising to buy Rotel if you're up to fixing stuff yourself.

    My remaining puzzle is why this component failed. All the ref. voltages now check out, so I cant see anything else that would have caused it to fail, however there's still a nagging doubt its going to fail again. Maybe its just paranoia.

    Anyway, I just wanted to update this thread in case its of use to anyone else. Many thanks for the initial posting, which was a pretty good starter without further diagnostic information. In fact checking the emitter resistors initially showed no bias current to the final stages, and I was able to work back from there after a few red herrings !

    many thanks again.

  • Terry Whit Feb 26, 2007

    After a long delay getting a replacement transistor, it turns out the problem was further back in the final stages with a transistor involved in setting up the bias voltage for the output stages. This had gone short circuit and the result was a +9V bias on three of the output transistors that were supposed to be biased at -0.4V. Of course this meant that half the output waveform was "truncated" (!) naturally causing the distortion heard in one channel.

    I have to say that following advice on the Web I contacted Rotel for a schematic, who were very obliging. Without this I would have been unable to find the fault, and in my book is a big plus for decising to buy Rotel if you're up to fixing stuff yourself.

    My remaining puzzle is why this component failed. All the ref. voltages now check out, so I cant see anything else that would have caused it to fail, however there's still a nagging doubt its going to fail again. Maybe its just paranoia.

    Anyway, I just wanted to update this thread in case its of use to anyone else. Many thanks for the initial posting, which was a pretty good starter without further diagnostic information. In fact checking the emitter resistors initially showed no bias current to the final stages, and I was able to work back from there after a few red herrings !

    many thanks again.

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I'd suspect that one of the two output transistors on that channel has gone bad. If you are unsure how to test or troubleshoot this, then you might try replace both the NPN and PNP outputs and catch the bad one that way. If your amp uses an output module then you would of course have to replace the module which would replace all the output transistors on both channels. Good luck. Another thing to look out for, is the emitter bias resistors. They are the large white ceramic ones near the outputs. Sometimes one of those will open up too. They will be of a very small resistance value like .5 ohms (point five ohms) or something in that general ballpark, and have a wattage value of typicaly 5 watts. So they are pretty big resistors. Be sure to check them all even if you do find a bad output transistor. They will often go out with the transistor. Good luck.

Posted on Jan 18, 2007

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