Share your knowledge on these two classic MPCs
By MPCHunter Thu May 20, 2010 5:41 pm
thanks for the update and upload oldgearguy. I owe you a beer or two the next time you are out this way...

I believe FG is Frame Ground -- which is the same as chassis ground.

It looks like the caps there connecting the FG, AG and digital ground are probably there to eliminate some RF noise caused some design flaw somewhere...

Do you measure any continuity between the Digital ground and the chassis or analog ground and the chassis?

Out of curiosity, does the MPC behavior differ when you are missing the ground prong on the AC cord?
By oldgearguy Fri May 21, 2010 12:00 pm
MPCHunter wrote:thanks for the update and upload oldgearguy. I owe you a beer or two the next time you are out this way...

I believe FG is Frame Ground -- which is the same as chassis ground.

It looks like the caps there connecting the FG, AG and digital ground are probably there to eliminate some RF noise caused some design flaw somewhere...

Do you measure any continuity between the Digital ground and the chassis or analog ground and the chassis?

Out of curiosity, does the MPC behavior differ when you are missing the ground prong on the AC cord?


Glad to help.

Frame ground. Hmm, hadn't considered that possibility. Looking at the other boards mounted to the back panel, I see they also have FG so that makes perfect sense. I'll have a few minutes this afternoon to measure continuity between the grounds. Going from memory (I'll update later today), I didn't see that noise on the GND or AG pins of the connector and once again - if I physically disconnect the jack board frame from the MPC, the problem goes away.

However, looking at the AD/DA schematic (page 24 of the PDF) I see that the headphone jack ground is tied directly to FG and the audio jacks have a cap between them and FG. From an earlier poster's comment, Akai Japan indicated a problem with the headphone ground, so maybe their fix is to add a cap between the headphone gound connection and FG. To me, that's a possibility, but it doesn't address the reason as to why the problem is just surfacing instead of being there from day 1.

I didn't try a ground lift adapter, but that is something else to try. It may be that the noise I see on FG is supposed to be there, but somehow it is now shorted directly to the frame instead of passing through those caps to AG and GND. What puzzles me is that if you follow the sync out line from the connector, it goes through a resistor to FG and also through a second resistor and cap to FG, so it looks like there is going to be some junk on FG. The two 0.1 caps (518 and 519) are probably still blocking any noise on FG from getting into the system grounds, but I see nothing from blocking any noise on FG from getting directly into the headphone ground.
By oldgearguy Mon Jun 07, 2010 2:18 pm
Since there seems to be more people experiencing this problem, I have a question -- what is the impedence rating of your headphones?

I was talking to a knowledgeable EE friend of mine and he was reviewing the whole grounding scheme of the MPC 3000 and had some concerns. Since this stuff used to kind of work in the past, I wonder if newer low impedence headphones are contributing to the issue. I'm going to try some experiments this evening.

For reference, here's his quick analysis of the schematics only (he doesn't have an MPC in front of him to test):

*******************
From what I see on the schematic there is no DC connection anywhere between the frame ground and the analog ground.

On page 36, the SYNC OUT jack, it referenced directly to frame ground. Which is to say, to nothing.
Notice C512, R524, R525 and C511. That is a DC blocking circuit, will let AC through, down to very low freqs, and filter out some high freq components. Notice, it uses frame ground.

Since frame ground isn't grounded, R525 basically puts some of that signal onto frame ground.

All the boards seemed to be slathered with these 0.1uF caps between ground, analog ground, and frame ground.
Together, in parallel these form a somewhat larger capacitance to ground, which connect all these grounds together for AC. But not well for the end freq end of audio.

OK, back to page 24.
The headphones are connected to frame ground.
The op-amp drivers for the left and right sides of the phones are referenced to analog ground.

So right there the thing can't work. The left and right can drive through each other's "common" point between them. (which is the frame ground) So you'd be listening to the difference between the two sides.

The ground point of headphone is also being supplied with the sync out, per my explanation above.

So all this conspires to form weird grounding and the fact you hear the sync output in the headphones doesn't surprise me.
By oldgearguy Tue Jun 15, 2010 11:21 pm
OK, thanks to my friend Bill who helped me understand how Akai wired this thing up, I fixed my 'noise in the headphone jack' issue.

I'm attaching a couple pictures to hopefully make it clearer (sorry for the slight blur in the 2nd one).

Anyway - the summary is I cut the headphone jack ground away from frame ground and jumpered it to analog ground. That gave me no background whine, no tape sync noise in the headphones, no thumping in the main L/R. It also gave me a very clear, clean, and loud headphone signal into my AKG 240DF headphones.

If you want to do this yourself, I make no claims as to whether your problem is exactly the same as mine nor whether these same steps will actually fix your MPC. I also warn you not to stick your hands inside an MPC while it is plugged in.

with that out of the way...

Open your MPC, unplug the 5 cables connecting the top output board to the rest of your MPC (don't forget that thin 2 pin connector).

Unscrew the 2 screws inside holding the output board to the indivudual 8 outs board.

Remove the 3 screws holding the output board to the back panel (don't forget the screw above the digital jack).

Flip the board upside down and locate the headphone jack and the ground pin of the headphone jack. Notice that the ground pin is soldered in the middle of a small square that is connected to the larger silver section by 3 tiny bridges.

Using an Exacto knife, a box cutter, your Aunt's sharpest paring knife, old rusty tweezers, or anything else you can think of, carefully cut those small metal bridges.

Yes, I know you were careful, pressed hard, and scraped away for a while, but I still don't trust you. Use a multi-meter to verify you have no continuity between the ground post and the surrounding area. It's very easy to push those metal bridges around and not actually get them completely out.

Now take a small piece of wire (even use part of a resistor leg or capacitor leg) and carefully solder it to the ground pin of the headphone jack and then to any analog ground point on the board. Be careful that solder doesn't reconnect the areas you just tried to separate. Recheck with your meter!! I happened to choose the analog ground side of capacitor C200, but you can choose any suitable point.

Check your work, reassemble, and test.
Let me know if it works for you.

Tom


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By Rey305 Fri Jun 18, 2010 7:37 pm
Oldgearguy: Thank you for your follow up and solution to this particular problem...I was able to follow your logic and understood what you did(especially with the pics)...very simple and functional fix...although at the moment I don't have a 3000(I have a 60) little tidbits of info like this go a long way, especially for me since I am looking into getting a 3000...and thank you for the service manual as well...
By TSR Fri Sep 03, 2010 6:31 pm
oldgearguy wrote:Anyway - the summary is I cut the headphone jack ground away from frame ground and jumpered it to analog ground. That gave me no background whine, no tape sync noise in the headphones, no thumping in the main L/R.



Thank you very much! I just fixed my MPC3000 headphone-out problem!

Best, TSR
By daicehawk Thu Sep 23, 2010 3:21 pm
Needs to be sticky!!!
Got mine a couple of days ago and that problem occured immediately. Cured it this way.
Still the foil in the LCD makes ratatat when backlight is on plus the MP stops and the sound stutters in some time from powering on. Seems to be a grounding problem as Rohan said. BTW, how do you guys that have a 110 V version in a 220 V country power the MP?
The voltage converter I have got has no grounding neither the wall socket has. Talking Russia here :cry:
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By Shostakovich Thu Sep 23, 2010 6:36 pm
daicehawk wrote:Needs to be sticky!!!
Got mine a couple of days ago and that problem occured immediately. Cured it this way.
Still the foil in the LCD makes ratatat when backlight is on plus the MP stops and the sound stutters in some time from powering on. Seems to be a grounding problem as Rohan said. BTW, how do you guys that have a 110 V version in a 220 V country power the MP?
The voltage converter I have got has no grounding neither the wall socket has. Talking Russia here :cry:


My MPC is 220-230V, 50Hz
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By b.read Tue Jan 24, 2012 1:30 am
Looks like I'm going to have to give this a go soon, mine is now makin' noise on me. :?

Thank you oldgearguy for the write up/pics! Much appreciated... :D