83 lines
3.8 KiB
Markdown
83 lines
3.8 KiB
Markdown
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date = "2017-11-13"
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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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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 operate in _stereo_ at 60W per channel or in _bridged mono_ for up to 180W of power. In bridged
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mono you can use the entire amp to amplify either the left or right channel to a single speaker. Of course,
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for stereo, you'd need two of these amplifiers in your setup.
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## Diagnostic
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I bought this amp with the remark that one channel did no longer produce any sound. This Rotel
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amp was a complete unknown to me, but my guess was it'd be easy enough to fix.
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Before powering up an unknown device I always take of the cover and do a visual inspection.
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This amp looks really clean and lightweight. There's not a single IC in there! I checked for
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any obvious failures, like buldging capacitors, fried resistors or burn marks on the PCB.
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Nothing looks out of ordinary, so that means there's a possible issue with any one of the
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transistors on the board.
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The power transistors, those mounted on the heatsinks, are prime candidates for failure due
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to the thermal stress they get. With my multimeter I did a quick continuity test to see if
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any of these were shorted out - all appear to be fine.
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Another quick test I performed is verifying the bias resistors (the large white ones). These
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are high power, low resistance. All measured in spec at around 0.22Ω - so no open
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or short circuits there either.
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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. Right was close to 0V whereas the
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left channel had a few mV of DC. Might be nothing, might be something.
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Next I hooked up a dummy load (8Ω 200W in power resistors) and applied a 1kHz sine
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wave to the input. With my oscilloscope I quickly determined that the right channel was
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in working order. Left was out completely.
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Since I already checked the major culprits of a dead channel, I continued to check the
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other transistors on the left channel circuit. The first one I tested (Q615, a 2SC2910
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NPN transisor) showed a short between _emitter_ and _base_.
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A quick probe around showed no other damaged parts. Resistors showed their correct values
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or I would get values that indicate a capacitor getting charged. All other transistors
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checked out ok as well.
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After taking out Q615 I put it in my component tester and instead of an NPN transistor, it
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showed up as two diodes.
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This is what a working 2SC2910 transistors looks like on my tester:
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## The Repair
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The shorted transistors was quickly replaced (a local supplier has the exact model in stock).
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This model has a removable bottom panel that gives easy access to the bottom of the PCB.
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While I had the unit opened up on my bench I also to the opportunity to clean the insides using
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some rubbing alcohol and compressed air.
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After verifying correct operation with a dummy load and my scope I hooked up pair of speakers and
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got to enjoy the beautiful sound of this Rotel amp.
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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) |