K+H O 110 high-pitched tone - no signal applied

[thriley]

Hi there everyone,

I'm having a problem with my monitors and I'm posting here in the hope that someone might be able to help. Sorry to make my first post a question!

Some time ago now, I was living overseas and bought a pair of Klein and Hummel O 110 monitors, with 120V power supplies. When I moved back to a country with 240V mains power, I purchased a step-down transformer to use with them. The step-down transformer has a rating of 600VA. The monitors worked well for a number of years, until I had to put them into storage when I moved from one city to another. It was some time before I was able to get them unpacked and set up again.

When I did get them unpacked and set up, I immediately noticed a pronounced tone from both speakers, at-or-about 2kHz. The tone eventually settled to a point where it was only audible from one speaker - but still very definitely there.

The chain goes like this: 240V mains > Step-down transformer > 120V Furman power conditioner > 120V K+H O110 (x 2).

The tone is present without anything connected to the monitors other than AC, which makes me think that the problem has something to do with the step-down transformer, the mains, or the power supplies in the monitors.

So, in summary, I'd appreciate any opinions or advice on where the tone might be coming from and how best to fix it. I'm willing to have the power supplies replaced, but if the problem was able to be fixed with a new step-down transformer then that would save me some cash!

Thanks for reading,

Tom

[resistorman]

Well, start by taking the Furman out of the chain. If problem continues, try another step-down device.

[thriley]

Thanks, will do!

[Hugh Robjohns]

Very unlikely to be an external mains issue. These monitors are getting on a bit and have been unused for a while, so my bet is that electrolytic capacitors have started to dry out and some of the circuitry has become unstable. They are probably going to need a trip to a Neumann doctor.

However, one possibility is that the amps are picking up some external interference. Is the whistle still there if you connect the inputs via XLR cables to a known good (active) monitor controller or mixing console monitor output.

[thriley]

Thanks Hugh - I've tried setting them up with balanced connections to a few different mixers (all known to be working correctly), and in different locations around the house, but the whistle is still stubbornly there.

Off to the repair shop, then!

Thanks again.