+++
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. The right channel was working just fine, but the left channel 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)