Some other possibilities if the condition just started to develop in one day, would be to look for the presence of flourescent lights, or large motorized machinery. The flourescent light ballast can sometimes induce hum in stereo equipment, as well as develop line noise in the mains. Machinery will often create strong magnetic fields, but only when in operation. Was a light dimmer, or other lighting control added into the area?
My personal instinct is to check the grounding first. It may nothurt to run an additional ground wire or two from your main grounding connection- be it a water pipe, or the ground reference in the wall mains outlet. You may find the offending component when you touch it, because the quality of the hum will change.
If the components are stacked, it is easy enough to simply take one long length of wire, and starting from the top of the stack, work your way down the stack, attaching that wire to each component's chassis ground, and then finally to a portion of the wall outlet that is grounded. All requisite cautions apply when dealing with anything near an exposed outlet. This method avavoids loops that may be the cause of the hum.
: i have a bang and olufsen beomaster 1900 which has started humming, it is a little better when the voltage selector is set to 220V instead of the usual 240V... it is not the source or cables, but is affected by the tone controls, any advice appreciated
When i push FM1, FM2, FM3 and so on the light in front of each key doesn't remove on .In this case i hear the switch on relay but this relay doesn't remove on.
If you know this kind of failure any advice appreciated or tell me where is it possible to get a shematic diagram of this appliance.
By advance thank you for your cooperation
E.THOMAS
I have a beomaster 1900 that has started doing exactly the same thing, though the hum disappears gradually when the machine is left switched on for an hour or so. Although I have no circuit diagram (B & O used to place a folded circuit diagram inside a lot of their equipment, but apparently not this model), I suspect an electrolytic capacitor. In integrated amplifiers, such as in this, the main power supply is generally unregulated, and there is additional smoothing for the preamplifier stages using one or more resistors/capacitors. I have not had time to investigate yet, but will post a further message if I discover the source of the trouble.
I had the same problem and last weekend tracked it down to electrolytic C66 on the main board being open circuit. (10uf/63v)I don't have any service info but this appears to be decoupling the base of a voltage regulating transistor driving the 27v rail. Note that the varicap tuning voltage seems to be derived from this rail and changing the capacitor altered the preset tuning settings slightly. Whilst the unit was disassembled I also took the opportunity to replace the bass, treble and balance film strips that operate the panel indicator strips (you know what I mean) as these were breaking down quite badly. I made new ones by measuring and drawing them up on the PC and printing to acetate. They seem to work very well. I still have the file if you need it.
Hope you find this useful. Regards, Mick.