Loud hum when I sample

Hello,

I have an Akai MPC3000 that is having noise issues when I sample.

My MPC is very quiet when in playback mode. However, when I try to sample something I get a very loud hum in the sample. It sounds like the noise I used to get in the headphones before I did OldGearGuy's headphone ground mod.

When I try to sample the Akai will be silent until I plug in an RCA cables or 1/4" audio cables. The cables don't have to be connected to anything, the hum starts as soon as they are plugged in. If you take the cables out, the MPC is quiet again. If I sample "silence" with the hum and then play back the sample, it will playback the hum as part of the recording.

People have mentioned the inverter, but I don't think this is the issue. There is a very small inverter noise when I sample, but that stops when I turn the backlight off.

I've gone so far as to open up the MPC and disconnect the two pin inverter cable from the filter PCB as well as the three pin cable next to the two pin cable. I've tried to sample while it is disconnected and the noise is still there when I insert cables into the audio in jacks on the MPC3000.

What do you guys think my next troubleshooting step should be?

Is the grounding of your turntable (?) OK? You didn't mention that.
And it definitely isn't the display inverter leaking into the sound.

The external grounding is not the issue. This happens when using any kind of audio cable across multiple devices.

-It happens when I sample multiple different keyboards with 1/4" cables
-It happens when I sample audio with RCA cables
-It happens when a cable is inserted into the audio in jack with or without anything attached to the other end of the cable.

IOW when you put a cable into that jack, some kind of ground noise is coming into the audio path.

Would replacing the caps around C103, C104, C105, and C106 make any change to the grounding issues?

Also, as an FYI:

If I am sampling monitoring in stereo, if nothing is in the jacks, both sides are silent.

If there is a cable only in the right jack, I only get noise on the right channel. The left channel is quiet.

If there is a cable only in the left jack, I only get noise on the left channel. The right channel is quiet.

When noise is present, the noise is of an equal volume in both channels.

Would the various caps around the edges of the AD/DA PCB that hit analog ground have anything to do with this? IE C214/C217, C213/C190, C200/C212, C219/215, C221/C220, C216?

Wal Martian wrote:What is the gain switch on the back of the 3K set to?

This issue happens at all three gain settings. My 3k had a different kind of noise with the preamp before this issue. It was not the headphone buzz sound, but a weird digital crackling when I sampled in med or high gain modes. My solution was to just make sure I took care of the gain staging outside of the MPC and hit it hot on the way in so that I did not have to rely on the noisy MPC sampler preamp.

The loud hum was not a ground issue, it was actually bleed from the SMPTE PCB. I disconnected the ribbon cable from the SMPTE I/O board and the CPU board and magically my issue when away.

I still have a little noise when I crank the preamp, but nothing that can't be worked around by adjusting the gainstaging before I hit the MPC.

It's really nice to have this working once again.