[mekanoplastik]

I am measuring the dc offset of my China hybrid amp and all I am getting are "dancing numbers" the result of the multimeter will not stop in one number..it will fluctuate all the way from -40.0 mV to 180.0 mV

(the multimeter has 200mV reading), while it is a cheap tool I am using

, I know it works ok because there are some test points on the board where it will clearly stop at one value and not move at all

(also into the mV range)

I am measuring with no input pluged, using the output board holes and the preamp out as test points and i am getting the same results on both channels -actually there is a small diference in top and lower values but both fluctuate like crazy-

what do you guys think?

 

[Sovkiller]

It looks like AC......

 

[Nisbeth]

Did you short the input to ground?


/U.

 

[tangent]

You're not trying to test with music playing, are you? The LF content in music will confuse a DC meter. If there is no music, there's probably still some LF AC content, perhaps a form of oscillation. You'd have to scope it to get a good idea of what is happening.

 

[mekanoplastik]

Did you short the input to ground?

no..inputs are ok

You're not trying to test with music playing, are you? The LF content in music will confuse a DC meter. If there is no music, there's probably still some LF AC content, perhaps a form of oscillation. You'd have to scope it to get a good idea of what is happening.

no..no music, no scope either

where can i read about oscillation? just to get possible causes and troubleshooting ideas...

thank you guys..any possible test i can run?...(without scope)..the amp works and plays fine..but this can not be good so i have to take care of it..

 

[tangent]

Quote:

inputs are ok

So short them, then. If there's nothing connected to the inputs, the drifty signal you may be measuring could be interference picked up by the inputs. Shorting the inputs ensures that any DC offsets you're measuring are generated within the amp.

Quote:

where can i read about oscillation?

Do a search.

The books and app notes you will read are very theoretical. The things you'll pick up by searching tend to be more practical -- how to fix it if you have it, rather than the reason it is ultimately caused. Knowing the theory is helpful for design work, but if you are building an existing design the practical bits are what you need to know now.

Quote:

any possible test i can run?

There are various scope programs that use your PC's sound card for input. These are no replacement for a real scope, but for your purpose it should be okay.

Also consider downloading Spectrum Laboratory (not to be confused with SpecraLab). Set it up to show you only the lower 100Hz or so of the spectrum and see if you get some spikes.