devroom.io/content/projects/repair-rotel-rb-970bx/index.md

+++
date = "2017-11-13"
title = "Repair: Rotel RB-970BX"
tags = ["rotel", "repair", "audiorepair"]
categories = [ "Audio Repair" ]
summary = "Another lucky find: Rotel RB-970BX Power Amplifier with one defective channel. Let's fix this!"
aliases = [
  "/2017/11/13/repair-rotel-rb-970bx/"
]
+++

![Featured](featured.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 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 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)

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.

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. 

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. Right was close to 0V whereas the
left channel had a few mV of DC. Might be nothing, might be something. 

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. 

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. 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 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. 

![Bottom view during repair](/img/rotel-rb970bx-bottom-repair-annotated.jpg)

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.

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

[Rotel RB-970BX Datasheet at Hifiengine](https://www.hifiengine.com/manual_library/rotel/rb-970.shtml)