Draft: Rotel RB-970BX

content/electronics/repair-rotel-rb-970bx.md

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