Update and finalize Rotel RB970BX post
This commit is contained in:
parent
02e7fa982e
commit
f2b6db48eb
|
@ -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
|
||||
|
||||
|
|
BIN
static/img/rotel-rb970bx-bottom-repair-annotated.jpg
Normal file
BIN
static/img/rotel-rb970bx-bottom-repair-annotated.jpg
Normal file
Binary file not shown.
After Width: | Height: | Size: 2.1 MiB |
BIN
static/img/rotel-rb970bx-bottom-repair.jpg
Normal file
BIN
static/img/rotel-rb970bx-bottom-repair.jpg
Normal file
Binary file not shown.
After Width: | Height: | Size: 2.1 MiB |
BIN
static/img/rotel-rb970bx-npn-defect.jpg
Normal file
BIN
static/img/rotel-rb970bx-npn-defect.jpg
Normal file
Binary file not shown.
After Width: | Height: | Size: 1.9 MiB |
BIN
static/img/rotel-rb970bx-npn-okay.jpg
Normal file
BIN
static/img/rotel-rb970bx-npn-okay.jpg
Normal file
Binary file not shown.
After Width: | Height: | Size: 1.9 MiB |
Loading…
Reference in New Issue
Block a user