Yamaha MOX8 with no power

Synth Repair
1

Hello!

I recently picked up a Yamaha MOX8 that won’t power up at a steep discount hoping I could resurrect it.

So far, no luck. Has anyone ever worked on one of these? I’ve been testing the voltages on the various paths and so far it looks like the power board is okay, and the 3 volt path seems to make it pretty far (before I stopped tracing it). But the 5v path seems to end around IC311. I’m just not sure if it’s something earlier that’s actually not triggering since IC311 is a “soft switch”.

I’m really hoping the CPU isn’t bad since that would mean I either need to spend nearly $500 more to get a new mother board, or part this thing out and call it a day.

If anyone has worked on one of these or knows someone who has who could shed some light into what else I can test, that would be great.

2

Check the power on/off circuitry all these voltages have to be here for it to do anything - incl trace/cables to the power on/off switch

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3

Thank you! I know I checked some (and I know the 9v works out of the power board as well as the 5v lines), but I hadn’t checked that much, so this gives me more to work with.

4

So far it looks like the power board is okay. Nothing out of the ordinary as I trace voltages, and CN501 has the right voltage on the pins as labeled in the PDF. I’ll keep tracing the DM board next and see what I can figure out.

5

Key is PIN 3 (PWR) on CN501 - that signal is HIGH if the CPU is running.

6

high as in high voltage? It’s pulling 3.275v right now on pin 3. If that’s too low, then you’re on to something.
Here’s the other oddity (see images). There’s a blue wire going from IC501 pin 4 (I think it’s pin 4) to the other side of the board onto one of the pins of the power switch-only it’s NOT on any board I see online for sale for the MOX8.

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IMG_19691920×2560 349 KB

IMG_1970
IMG_19701920×2560 273 KB

7

To follow up, still no power, but I did test and pin 3 is only hot when everything is wired up. If nothing is plugged into CN501 then pin 3 is dead. That seems to fit with what you mentioned earlier.

Update, I did a few more tests. On the DM board, it appears that part of IC 6 isn’t functioning unless it’s supposed to be a ground, but it says “Long Reset” for CPU/JTAG. Here’s the voltages:

IC3
R14 - 3.5 both sides

IC6
1 - 3.5
2 - 0 - R17 0 both sides
3 - 3.5
4 - 0 - R15/D2 - 0 both sides
5 - 0
6 - 3.5
7 - 0
8 - 3.5 - R16 3.5 both sides
9 - 3.5
10 - 0 - R18 - 3.5 both sides
11 - 0
12 - 3.5
13 - 0
14 - 3.5

8

I took a chance and ordered a replacement power board and…

The power board is not the issue. Still nothing when I try to power it up with the new board. So unless I can figure out what to replace on the motherboard, I basically have an expensive paperweight. I suppose I could part it out and make some of my money back. Total bummer.

9

I had a similar problem with my MOX8, it would not power on. I started troubleshooting by checking the voltages on the power supply. All voltages (3V, 5V, 9V) would briefly come up upon pressing the power switch, but within a second all of them would begin rapidly descending toward 0V.

I believe the IC on the PS board causes this to happen when it senses too much current draw. I suspected the JK (Jack) board because I have a bad habit of connecting inputs to outputs and vice-versa.

I found that when I disconnected CN101 on the JK board, the keyboard would boot up normally, and all voltages on the PS board were normal. I had no analog output, of course, but USB MIDI worked fine (that’s how I use it these days anyway).

However, I wanted the keyboard to be fully functional. I used to be a tech for Ensoniq so I know my way around analog circuits and I did some in-circuit testing. It turned out that C018 (220uf, 16V blue electrolytic) appeared to be completely shorted, which was verified by removing it from the circuit.

Once the capacitor was replaced, I reconnected CN101 to the JK board and everything was fine.

So, if anyone sees their power supply voltages briefly come up and then fall to nothing, removing CN101 from the JK board appears to be a safe way to see if a component on that board is the cause, and it will boot and be usable as a USB MIDI controller in that state.

I’m sure that there are a lot of other failures that could cause the power supply to behave that way, probably even the power supply itself. In my case though, it was a capacitor on the JK board, which is mostly (if not completely) analog.

BTW.. The JK board schematic is on page 148 of the service manual.