Buzz/Hum from Audiosector LM3876 board itself - no buzz in speakers

[dakku]

I have a distinct buzzing/hum from the actual amp board. Its like the hardware is making the buzz, but I do not hear it in the speakers, just directly from the amp..

For the starground I have joined the ground wires from the following to a single point (not connected to the chassis):

Mains Ground
PG1+ and PG- (rectifier)
Left amp OG
Right amp OG

Any ideas?

 

[dakku]

I disconnected the speakers, hum is still there
I disconnected the RCA in, hum is still there

 

[AndrewT]

Do you have a mains bulb tester?
If not, build one and use it.

Does the bulb flash, or glow, or light up bright, when you start up?

Have you measured the voltages at the output?
Have you shorted the input?

 

[danielwritesbac]

The concept:

image by antiqueradio.org
The bulb is incandescent, both standard and halogen work, and 25W fridge bulbs and 40W clear oven bulbs remain widely available.

 

[Leglandu]

my 2 cents (of euro)...

... Its like the hardware is making the buzz... Any ideas?

Hi dakku... as you already disconnected the speakers... some ideas :

- transformer vibration (toroidal no tight enough, DC component on the 120 or 230VAC...) ?
- toroidal incorrectly mounted on metalic surface with metal screw creating a kind of secondary short-circuit ?
- defective chemical cap or cap enclosure in contact with a rigid structure ?
- inverted polarity or alternative current (bad diode) on cap...