Floor Monitor Hum

[euanjappy93]

Hello,

I was wondering if anyone could give me some advice. I have a SubZero 200W 10" Active Floor Monitor which has a constant hum when switched on. This hum is apparent even when there are no inputs/outputs plugged into the unit and doesn’t change when the settings are altered. I have an oscilloscope which I have connected to the monitor which shows some harmonics on the output of the monitor. At first I thought it was the filtering capacitors on the mains input but now I am not so sure.

Cheers,
Euan

[Jazz P Bass]

Disconnect the speaker and check for Volts dc on the output.
If you have a gross reading, it sounds like the output section is fried.

[Jon Snell]

I expect that is noise from the class D amplifier and perfectly normal as loudspeakers are not very efficient at that frequency.
Personally I would look for main smoothing capacitor issues, not mains borne interference.

[euanjappy93]

Hello,

Sorry for the delayed reply.

Jon Snell I have removed the main smoothing capacitors from the pcb and tested them with a capacitance meter. The capacitors seem ok because the reading I was getting was 6500uf for a 6800uf capacitor.

Jazz P Bass do you mean the output of the amplifier chip. The amplifier chips are made by st microelectronics, part no. TDA7294. I have measured the voltage across the subwoofer cable and it is around 4/5v.

Any more help with this would be much appreciated.

Thank you,
Euan

[euanjappy93]

Does anyone know if there is circuit diagrams available for this make and model of floor monitor.

[Jazz P Bass]

No schematic that I have.
But...
4-5 Vdc out of the TDA 7294 indicates that the ic itself is probably blown.
You have to prove that the voltage IN TO the ic is not Vdc.

[Jon Snell]

A TDA7294 will shut down if there is a DC level on the output, they are very robust. If the IC was blown it would give no audio.
Possibly there is goo on the pcboard, as most do now coming out of China and the goo gets conductive with moisture, like all of the Rockit range that are written off when trying to remove the goo.

[euanjappy93]

Jon Snell are you referring to the silicon type adhesive they have used between the tda7294 and it’s heat sink, or is it something else.

Jazz P Bass I will need to check that and get back to you.

[Jon Snell]

Some manufacturers use a black/off white goo that they stick the components to the board in an effort to stop any vibrations from breaking them, cheaper than fixing them properly. This goo becomes porous and then conductive in time. Possibly to ensure a failure so another sale will be lined up.
The silicon paste between the heatsink and the output components is perfectly OK and that is not the issue.
An internal photo may help.

[Jon Snell]

This is what I mean by goo found in the rockit series of Chinesium amplifiers;

[euanjappy93]

Hi

the website won’t let me add pictures for some reason but there is adhesive like this on the board.

[Jon Snell]

Upload the photo as an attachment.