diff --git a/content/electronics/repair-rotel-rb-970bx.md b/content/electronics/repair-rotel-rb-970bx.md new file mode 100644 index 0000000..7797a48 --- /dev/null +++ b/content/electronics/repair-rotel-rb-970bx.md @@ -0,0 +1,71 @@ ++++ +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 ++++ + +![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) \ No newline at end of file diff --git a/static/img/rotel-rb970bx-bridged-mono.png b/static/img/rotel-rb970bx-bridged-mono.png new file mode 100644 index 0000000..20444df Binary files /dev/null and b/static/img/rotel-rb970bx-bridged-mono.png differ diff --git a/static/img/rotel-rb970bx-front.jpg b/static/img/rotel-rb970bx-front.jpg new file mode 100644 index 0000000..f375ccb Binary files /dev/null and b/static/img/rotel-rb970bx-front.jpg differ diff --git a/static/img/rotel-rb970bx-pcb.jpg b/static/img/rotel-rb970bx-pcb.jpg new file mode 100644 index 0000000..6ed5b71 Binary files /dev/null and b/static/img/rotel-rb970bx-pcb.jpg differ diff --git a/static/img/~~rotel-rb970bx-front.jpg b/static/img/~~rotel-rb970bx-front.jpg new file mode 100644 index 0000000..2772ee2 Binary files /dev/null and b/static/img/~~rotel-rb970bx-front.jpg differ