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Old 17th January 2007, 11:41 PM   #1
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Default Rotel RA980BX

At the moment Im unfortunately working without a circuit diagram (failed to find one yet but Im still looking - I can only see the RA970BX which I assume is different), but I do have the benefit of one channel working and one not so I have some sort of reference. Can anyone give me any pointers based on experience with this (or other) amps please ?

My Rotel RA980BX 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 !).

I am currently not 100% on this, but I suspect this fault might be intermittent because I was able to play loud volumes without distortion yesterday but the fault has returned and seemed permanent previously. It could have been the music content, although I doubt it - as I say this one Im unsure of.

Anyway, the fault lies in the power amp, because swapping the preamp to power amp channel connectors over the fault remains in the left channel. The same is true of swapping the speakers with cables over, ie the fault remains in the left channel. Given that it isnt surprising that the fault remains regardless of input source. So I conclude its the left channel power amp stage.

Assuming I can find my way around the board, my first port of call would be to check the power rails (excepting I dont know what they shoule be), 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 one with experience in this or other Amps can give me any pointers (or even a diagram as well !) ??
Terry.
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Old 18th January 2007, 12:03 AM   #2
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Look the pcboard over real closely you may just have a cold or cracked solder connection. Tapping on the pc board with a non-metallic probe sometimes helps to locate faulty connections.
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Old 18th January 2007, 12:55 AM   #3
Pars is offline Pars  United States
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Rotel normally gives out the service manual/schematics for free. Email them.
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Old 22nd January 2007, 04:54 AM   #4
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Thanks for your replies. And for the Rotel tip - of course you were right and I got a diagram directly from them. I would never had guessed they would give those out, but there you go !

Looks like it isnt a dry joint, the bias on the output transistors is all wrong, and it looks like at least one transistor in the bias circuitry has gone. When I can get hold of a replacement Ill see if it just went in isolation or took something else out with it ....

thanks again.

t
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