2017-11-06 12:37:03 +00:00
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date = "2017-11-06"
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title = "Repair: Rotel RB-970BX"
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tags = ["rotel", "repair", "audiorepair"]
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categories = [ "Audio Repair" ]
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description = "Another lucky find: Rotel RB-970BX Power Amplifier with one defective channel. Let's fix this!"
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slug = "repair-rotel-rb-970bx"
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draft = true
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![The Rotel RB-970BX](/img/rotel-rb970bx-front.jpg)
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I did another great find on _Marktplaats_: a Rotel RB-970BX. This is a power amplifier, meaning it has no
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volume or tone controls whatsoever. You'd normally pair a RC-970BX pre-amp with one or two of these
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power amplifiers.
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The amp can run in stereo mode at 60 Wpc or in _bridged mono_. In bridged mono you can use the entire
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amp to amplify _one_ channel and get 180 Wpc. Of course, for stereo, you'd need two of these
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amplifiers in your setup.
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![Bridged Mono on the RB-970BX](/img/rotel-rb970bx-bridged-mono.png)
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## Diagnostic
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I bought this amp with the remark that one channel did not produce any sound. This could be due
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to a number of reasons, but most likely one or more components in the circuit will have failed.
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Before testing the amp to determine _which_ channel is not working, I opened up the Rotel and
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checked for any obvious failures: burnt resistors, exploded or leaking caps, and blown
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fuses. Besides some oxidation on the mechanical parts and jumpers I could not find anything
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out of the ordinary.
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![The insides of the RB-970BX](/img/rotel-rb970bx-pcb.jpg)
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After that I did some quick measurements (still with the unit powered off):
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* Check for shorts on the transistors on the heatsinks
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* Resistance of the bias resistors (the larte white ones)
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Both of these showed that none of the power transistors had shorts, and that all
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bias resistors were well within spec (0.22Ω).
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Next I powered up the amp for the first time - with nothing connected to it. I measured
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for DC on the outputs but both were in the mV range, so nothing out of the ordinary
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there.
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2017-11-06 19:22:53 +00:00
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With that I continued to hook up my dummy load and test which channel - _if any_ - was not
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functioning. The right channel was working just fine, but the left channel appeared to be
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dead as a doornail.
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2017-11-06 12:37:03 +00:00
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Because the amp is pretty bare bones, my next step was to test the transistors on the
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PCB for the left channel. The first one I tested (Q615, a humble 2SC2910 NPN transisor)
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showed a short between _emitter_ and _base_.
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A quick probe around showed no other damaged parts. So, I ordered a few new 2SC2910's from
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a local supplier and waited for them to arrive.
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## The Repair
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The repair is as simple as replacing the damaged transistor.
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Since I'm there, I've also cleaned the oxidation from the jumpers and mechanical parst,
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cleaned the PCB and replace the four capacitors for good measure.
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## Conclusion
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A single failed transistor took out the entire left channel. Replacing a €1 part revived
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this power amplifier to its former glory.
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## Links
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[Rotel RB-970BX Datasheet at Hifiengine](https://www.hifiengine.com/manual_library/rotel/rb-970.shtml)
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