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9. TROUBLESHOOTING
H
um in the speakers
Possible cause: multiple grounds
Hum in an audio system is often caused by several system components having their own
separate grounds. It is particularly common in systems containing tuners, VCRs or satellite
receivers, as these components are connected to an aerial. Because aerials and cables are
always grounded, ground loops can form between the aerial connection and other grounded
equipment. Other equipment that is normally grounded may include PCs with sound cards, and
some CD/DVD players and DACs.
Although the RE 290 is grounded, its signal ground is a "floating" ground, which means that the
RE 290 cannot itself create ground loops. Hum can only be caused when it is connected to
other items of equipment.
To fix the problem
Before trying to fix the problem, find out which of your system components is responsible for
generating the hum.
Procedure:
- Unplug all source equipment, including any equalizer if used, from the RE 290, leaving only
the loudspeakers connected.
-
Reconnect the components back to the RE 290 one at a time. As soon as the hum
reappears, you have two grounded components connected to the RE 290 via the preamp.
Consult a technician if there is a possibility to eliminate this grounding problem.
Tube faults caused by output tubes
There are 3 different symptoms indicating a faulty output tube:
1. Broken heater filament: the tube stops glowing.
2. Defective cathode layer: the tube glows, but no current can flow. You can confirm this fault
using the bias display LEDs - no amount of adjustment will be able to extinguish the minus
LED.
3. Internal short circuit: Normally, the electronic protection cuts in and the red "off" LED
illuminates, or the tube will not respond to BIAS adjustment (the display keeps jumping
from plus to minus or vice versa).
With faults 1 and 2, the amplifier will still operate, although the channel containing the faulty
tube will be quieter than normal. At low listening levels, the fault may not be obvious, but
distortion will become evident at higher listening levels.
If fault 3 occurs, the protection circuits will normally switch off the amplifier. You may also hear
loud background noises just before it switches off, although these will not harm the amplifier.
You can find the faulty tube by removing one tube at a time. Operating the power amplifier with
just a single output tube is allowed for test purposes and will not damage the power amplifier.
Only a qualified technician must carry out this test.
Tube faults caused by driver tubes
Increased noise in one channel.
Normally the noise level of a driver tube is stable during its lifetime. Depending on the
production and type of tube, the noise can noticeably increase over time. The most sensitive
tube in the RE 290 in this regard is the input tube V6.
