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Rotel RB-870BX, hum in left channel.
Rotel RB-870BX, hum in left channel.
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Old 15th December 2016, 09:00 AM   #1
tenta is offline tenta  Sweden
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Default Rotel RB-870BX, hum in left channel.

Hi,
Iíve had a Rotel RB-870BX since new that Iíve been very pleased with. Recently I bought a second one, but unfortunately this one is slightly noisier and has a hum in the left channel.

Compared the inside of both amplifiers and they are absolutely identical and there are no signs in the ďnewĒ one that it has ever been fixed, repaired or modified in any way.

I guess the large 10000uF power supply capacitors for the left channel could be likely suspects?
But actually any suggestions on where to start tracking down and hopefully fixing the hum issue would be greatly appreciated.

Link to the service manual: http://bwgroupsupport.com/downloads/...B-870BX-TM.pdf
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Old 15th December 2016, 09:19 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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What is the hum frequency? 50/60 HZ is mains hum caused by a bad ground connection and 100/120 HZ is power supply related.
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Old 15th December 2016, 09:33 AM   #3
sgrossklass is offline sgrossklass  Germany
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This unit has separate bridge rectifiers and smoothing caps for both channels. I'd check the rectifiers for shorted diodes, if those are fine a bad smoothing cap does seem reasonably likely.

The unit being more noisy is interesting, as noise of these should be negligible vs. most preamps. Maybe the bridge mode switch and/or relay have contact issues and would appreciate a bit of a workout.

Of course you might've gotten unlucky and the unit contracted a hairline crack in the PCB during shipping, which in turn could cause all kinds of funny issues - definitely worth checking. How well was it packaged?

Last edited by sgrossklass; 15th December 2016 at 09:37 AM.
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Old 15th December 2016, 09:34 AM   #4
tenta is offline tenta  Sweden
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It's 100Hz.
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Old 15th December 2016, 10:37 AM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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100HZ means smoothing issues. Possibly the smoothing caps on the affected channel. If you bridge the large caps with some 4k7u caps and it improves, I would replace the old capacitors.
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Old 20th January 2017, 03:53 PM   #6
tenta is offline tenta  Sweden
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Thanks for the good suggestions!

It was shipped in the original box and the box wasn't that badly bruised in transport, so I doubt there are any cracks in the PCBs or broken solder joints. The difference in noise level is really,really minor, so that no big deal. But the hum can be heard 1 meter from the speaker when the room is silent.

I finally got some time to investigate the issue some more, and the rectifier is OK and the capacitors appear OK too.
The ripple voltage on the positive supply is around 12mV p-p and 10mV p-p on the negative supply, guess that is OK so the problem probably is somewhere else?

I dug up my old analog oscilloscope from storage and powered it on for the first time in 15 years to check the output signal to the speaker and this is the result, (sorry for the lousy picture).
The time is 5ms/div and 5mV/div which is on the edge of what this scope can detect.

Any ideas what might cause this "error" in the output signal? And/or where to start a deeper investigation?
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File Type: jpg Output_signal.jpg (82.4 KB, 90 views)
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Old 20th January 2017, 05:09 PM   #7
Mooly is offline Mooly  United Kingdom
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Rotel RB-870BX, hum in left channel.
That looks like the kind of problem caused by an imperfect grounding scheme. Those pulses are the reservoir cap charging currents entering the audio path.

If its like this with the inputs shorted and nothing else connected then its probably as good as its going to get without major diagnostics to look at where the problem is... problem being down to the design rather than a faulty part.
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Old 21st January 2017, 01:05 PM   #8
jaycee is offline jaycee  United Kingdom
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it's a dual mono design so it's a bit strange.
Perhaps the signal ground should be isolated from the chassis, and for some reason the left hand channel's signal ground is shorted to the chassis ?
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Old 21st January 2017, 08:37 PM   #9
Welcome is offline Welcome  France
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Try a different set of RCA inputs. They might be broken off the PCB. It commonly happens on bottom of the barrel quality amplifiers like Rotel, NAD and Harman Kardon.
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Last edited by Welcome; 21st January 2017 at 08:39 PM.
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Old 21st January 2017, 09:19 PM   #10
tenta is offline tenta  Sweden
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Since this was -at the time- Rotels top of the line amplifier I would be a bit surprised if the issue was because of bad design.
The amplifier has no mains ground (/safety earth) connection and both of the amplifier PCBs are connected to the chassis at one single point.

I will remove all connections to the left amplifier circuit board except the cables to the power transformer and the speaker terminals and short the input pins on the PCB. If that doesn't have any effect on the hum at least I know that the issue is somewhere on the PCB.
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