diff --git a/content/electronics/repair-rotel-rb-970bx.md b/content/electronics/repair-rotel-rb-970bx.md index 67c23e8..b77577a 100644 --- a/content/electronics/repair-rotel-rb-970bx.md +++ b/content/electronics/repair-rotel-rb-970bx.md @@ -1,11 +1,10 @@ +++ -date = "2017-11-06" +date = "2017-11-13" 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) @@ -14,58 +13,70 @@ I did another great find on _Marktplaats_: a Rotel RB-970BX. This is a power amp 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. +The amp can operate in _stereo_ at 60W per channel or in _bridged mono_ for up to 180W of power. In bridged +mono you can use the entire amp to amplify either the left or right channel to a single speaker. 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. +I bought this amp with the remark that one channel did no longer produce any sound. This Rotel +amp was a complete unknown to me, but my guess was it'd be easy enough to fix. ![The insides of the RB-970BX](/img/rotel-rb970bx-pcb.jpg) -After that I did some quick measurements (still with the unit powered off): +Before powering up an unknown device I always take of the cover and do a visual inspection. +This amp looks really clean and lightweight. There's not a single IC in there! I checked for +any obvious failures, like buldging capacitors, fried resistors or burn marks on the PCB. +Nothing looks out of ordinary, so that means there's a possible issue with any one of the +transistors on the board. - * Check for shorts on the transistors on the heatsinks - * Resistance of the bias resistors (the larte white ones) +The power transistors, those mounted on the heatsinks, are prime candidates for failure due +to the thermal stress they get. With my multimeter I did a quick continuity test to see if +any of these were shorted out - all appear to be fine. -Both of these showed that none of the power transistors had shorts, and that all -bias resistors were well within spec (0.22Ω). +Another quick test I performed is verifying the bias resistors (the large white ones). These +are high power, low resistance. All measured in spec at around 0.22Ω - so no open +or short circuits there either. 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. +for DC on the outputs but both were in the mV range. Right was close to 0V whereas the +left channel had a few mV of DC. Might be nothing, might be something. -With that I continued to hook up my dummy load and test which channel - _if any_ - was not -functioning. The right channel was working just fine, but the left channel appeared to be -dead as a doornail. +Next I hooked up a dummy load (8Ω 200W in power resistors) and applied a 1kHz sine +wave to the input. With my oscilloscope I quickly determined that the right channel was +in working order. Left was out completely. -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_. +Since I already checked the major culprits of a dead channel, I continued to check the +other transistors on the left channel circuit. The first one I tested (Q615, a 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. +A quick probe around showed no other damaged parts. Resistors showed their correct values +or I would get values that indicate a capacitor getting charged. All other transistors +checked out ok as well. + +After taking out Q615 I put it in my component tester and instead of an NPN transistor, it +showed up as two diodes. + +![Defective NPN transistor 615](/img/rotel-rb970bx-npn-defect.jpg) + +This is what a working 2SC2910 transistors looks like on my tester: + +![Replacemenet NPN transistor](/img/rotel-rb970bx-npn-okay.jpg) ## The Repair -The repair is as simple as replacing the damaged transistor. +The shorted transistors was quickly replaced (a local supplier has the exact model in stock). +This model has a removable bottom panel that gives easy access to the bottom of the PCB. -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. +![Bottom view during repair](/img/rotel-rb970bx-bottom-repair-annotated.jpg) -## Conclusion +While I had the unit opened up on my bench I also to the opportunity to clean the insides using +some rubbing alcohol and compressed air. -A single failed transistor took out the entire left channel. Replacing a €1 part revived -this power amplifier to its former glory. +After verifying correct operation with a dummy load and my scope I hooked up pair of speakers and +got to enjoy the beautiful sound of this Rotel amp. ## Links diff --git a/static/img/rotel-rb970bx-bottom-repair-annotated.jpg b/static/img/rotel-rb970bx-bottom-repair-annotated.jpg new file mode 100644 index 0000000..9adc722 Binary files /dev/null and b/static/img/rotel-rb970bx-bottom-repair-annotated.jpg differ diff --git a/static/img/rotel-rb970bx-bottom-repair.jpg b/static/img/rotel-rb970bx-bottom-repair.jpg new file mode 100644 index 0000000..5876666 Binary files /dev/null and b/static/img/rotel-rb970bx-bottom-repair.jpg differ diff --git a/static/img/rotel-rb970bx-npn-defect.jpg b/static/img/rotel-rb970bx-npn-defect.jpg new file mode 100644 index 0000000..e612d24 Binary files /dev/null and b/static/img/rotel-rb970bx-npn-defect.jpg differ diff --git a/static/img/rotel-rb970bx-npn-okay.jpg b/static/img/rotel-rb970bx-npn-okay.jpg new file mode 100644 index 0000000..1f65c2b Binary files /dev/null and b/static/img/rotel-rb970bx-npn-okay.jpg differ