Reviews and questions about the entry-level MPC500
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By DjDiskmachine Sat Jan 05, 2019 4:54 pm
Bad luck you managed to pinch the cable, I recommend shrinkwrap or electrical tape to fix that!

Hmm what caps did you replace again? I'm not having any problems with my inputs, they used to have a bad noise problem but now they are pretty much transparent.
I can't see any evidence that the line in caps should need replacement, the ones I see in the Service Manual are only there for DC blocking and perform no filtering.
The line in circuitry uses a different OP amp than the outputs (IC19 as opposed to IC11 for outs). It uses the same rail (5V) as the one supplying the output amplification circuitry. Are you sure you replaced all the caps? Are you sure you got the right values in the right place?

The only thing different between the input OP and the output OP is they have their own cap for HF filtering (on the input OP that's C46, seen way at the bottom right of the last page in the Service Manual).
This doesn't seem like a likely cause of the problem to me, but can't be ruled out of course since I can't perform any measurements ;)

This sentence got my attention now when I read your post again, can you elaborate, please?
NearTao wrote:"If a sound is playing it is okay and the noise floor seems to cut out"

Do you mean that the noise is supressed, like when doing compression -eg the louder sound "pushes" the quieter noise down then the noise is louder again after the sound stops playing?

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By NearTao Tue Jan 15, 2019 1:33 pm
The noise you quoted would best be described as ducking I suppose. There is a low level noise that goes nearly silent while playing a sample.

One thing I believe I have narrowed down the problem to seems to be that when plugged into the outlet I have tons of noise issues, but when on battery power they all appear to go away. I suspect the next piece of the puzzle is to get a new power pack, the one I have is Sino-American model:A31250. I'll try to keep narrowing down the issues :)

As to the capacitors I used Nichicon ULD1C101MED1TD/493-14722-1-ND and UVY1C221MED1TD/493-12895-1-ND that I ordered from DigiKey.
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By DjDiskmachine Wed Jan 16, 2019 9:11 am
OK I guess that a badly filtered PSU might be the cause of this problem, get one that's a little less cheap and report your results! :)
Please double-check your Cap values for C907 & C910, these are the ones that perform filtering for the rail Mainpow +5V in the service manual and they should be able to take care of this.
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By NearTao Thu Jan 17, 2019 4:39 pm
I have to give it to you... you know more about electronics than I do... which is probably why I went into management :)

Anyways... to clarify for future readers... the PSU was *NOT* the problem. I realized this when it started acting up with the batteries in it. I stripped the 500 back down to check the solder joints (all look good), and the caps (all look good)... and was left with that tweaked wire, and an odd behavior when I would mess with the volume knob. I re-taped the battery line in case that was causing a problem and rerouted it so it wouldn't get pinched again.

I ended up finding that if I flexed the volume/q-link board a little while it was powered that the hum would change. I ended up reseating all of the cables, which helped, but the noise instantly went away when I just put the board back where it was supposed to be. I'm not sure if it needs a circuit to the case, but on the bottom of the board there is some exposed copper on one of the posts for the case. It's possible I hadn't screwed this down well enough to the case, and the reason the noise was going away when I pushed on the volume knob was that it was re-establishing the circuit.

So... I've screwed everything down tight and will continue to give this thing a workout for the next few days.

EDIT: Also going to add, still definitely getting line noise. I thought I could hear it ducking, but normalizing and playing back some recordings it is definitely there. I tried powered, battery, as well as line and mic levels. It is there and not going anywhere.

EDIT 2: Oddly the record gain doesn't change the sound at all, but the main volume does control how loud it is... strangely, recording with the record gain down, normalizing the sound you can definitely hear it... whatever it is :/
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By DjDiskmachine Tue Jan 22, 2019 2:33 pm
Hehe.. I'm not trying to show off mah skillz here or anything, I'm just attempting to make it a little more clear for anyone that's debugging their hardware and to simplify the process of reading schematics and problem solving.

I'm impressed with your tenacity, good job on finding the broken circuit to the ground plane!

I'm curious about this comment:
"Oddly the record gain doesn't change the sound at all, but the main volume does control how loud it is..."
Do you mean that sampling with the master volume down produces a sample that is less noisy?
Do you hear this noise when auditioning before actually sampling?

Together with a colleague we're leaning towards a problem with the VMID reference, which is a common "ground" of the audio codec which handles all audio I/O before the CPU.
This should be approximately 1,65 V reference to ground. If you have a multimeter, this can be measured between the battery ground and pin 1 on the rec gain pot.
I'm pretty sure that one or both of the DC-blocking Caps from the Output OP have gone bad. These are C19 & C38 of 22uF, the two slimmer ones slightly to the left of this pic:

Might have to update the guide thanks to you! ;D If you can find the time and energy to investigate further, please report your results!