KORG M1 - No sound / Low battery indication. SOLVED!

[mellotron]

Moderator: I am posting here because of a "no sound" issue. We do NOT get a Low Battery indication. If I should post this elsewhere, please feel free to move my post.

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Hi all; first time poster here.

I just replaced all electrolytic caps, switches and backup battery in an M1. The original switches were flaky, but the synth had no other issues. I replaced caps due to age.

After reassembling, I returned it to its owner, who said he would perform the full restore from a backup he had created before leaving it with me. Now that he has it back, he finds that he cannot restore the factory sounds. He has a couple cards, but he can only get sounds for single selections, there is no sound from combo selections. The screen displays the expected sound but he hears nothing.

Any thoughts on where I should begin troubleshooting?

[mellotron]

Updates:

I reflowed the solder on all caps I replace on the Main Motherboard and the DAC board, then plugged everything back in.

I remembered that some of the glue from a cap on the power supply board flowed over top one resistor, R20. The glue was rather brown there but the resistor *looks* fine. After this latest reassembly, I measured the DC voltages coming from the power supply under full load.

On the MOBO, I located the 4-pin connector coming from the PS. At that connector (easy access from the face-up, non-component side of the board), I get +5.06VDC at both sets of pins (pins 1 & 2, and pins 3 & 4).

Likewise, having identified the four pins on the back of the DAC board, I get +12.00VDC (from the pair of pins with the gray and red wires) and -12.07VDC (from the pair of pins with the remaining two red wires).

While the brown glue concerned me, voltages do appear to be within spec.

I have reloaded from a recent Full Backup (to/from a MIDI-connected floppy in an Alesis DataDisk), as well as the M1_Preload (M1preld.syx) file from korg.com, pushed from the PC using MIDI-OX.

At this time, patch I00 reads "c". I01 is blank. I02 something like ".*Nx ) V" etc.

Trying a fresh M1preld.syx push.

[mellotron]

This latest problem has been SOLVED.

Apparently a Factory Reset is required before reloading the M1_Preload file. After the reset, I was able to successfully restore the factory sounds. All is well.

[belzrebuth]

I have a similar problem with mine.
The sound cuts off for a sec and comes back again.
Little after this problem started I saw smoke.
When I opened it up I saw a blown tantalum cap next to the R80 ( in the DAC board).
The R80 was also discolored but measured fine.
These are in the +5V rail which is used for the multiplexers that switch audio signals around.
I replaced the cap and the resistor but the issue remains.
I may need to recap the power supply as it seems cause the +12V is around 10.8 and -12 about 11 which seemed reasonable at first but after reading this thread I wouldn't trust the power supply caps.
The 5V and -5V rails seem fine but I may need to monitor all four voltages (+-12 and +-5) when the cut off happens to check if it's power related.
The sound comes back almost immediately after being cut off so it looks like be a power issue of sorts.

I'll update when I have some news..

[Yekuku]

I used to have a similar problem (intermittent sound) with a M1 and the culprit was a 4053 after the DAC.
Good luck !

[belzrebuth]

Τhanks for your reply, Yekuku.
That's what I replaced as well.
In fact I replaced all three in the DAC board just to be sure.
The intermittent sound problem stopped so it must have been one of these 3.

So I screwed the back cover in order to test it and I noticed there's some DC spikes going on now when I play the keys, just like if the gates of the keyboard leak to the speakers.
I then hit the keys a bit harder to see if the clicks are contact related (which are not) and I also managed to short it out (lost all the sounds) when I applied a little more force.

Since I lost all the sounds it must have been the battery that was shorted to the metal chassis so I put some plastic spacers all over the mainboard and the DAC boards since the cover has enough flex to maybe short things out. (and that's maybe the source of of shorted tantalums in the DAC boards)

After putting the spacers I screwed the machine back once again and tested it and sure enough the glitches came back!
Now there's something in the keyboard assembly that's shorting out since the main and DAC boards are well insulated from the plastic spacers.

If I place the whole keyboard vertically against the wall I can play for hours without any problems.
It's when it's sitting on its bottom cover and actually closer to the grounded chassis that does goes crazy.
So now I'm on the hunt for a mechanical problem which are the worst since I have to constantly screw and unscrew the machine to check it out.

I think the synth is actually okay as far as the electronics are concerned but this is now a far more frustrating problem..

Any ideas are welcome of course.

[Yekuku]

Hi buddy,
did you figured it out ?
I had a similar problem with a D50, when I was screwing back together it was malfunctioning, the culprit was a crack in the pcb . Maybe applying pressure with a stick on the pcbs while the unit is on , could help to identify the problem.
good luck!

[belzrebuth]

Yeah I've found hairline PCB cracks in old Casios (FZ and HS line) two times actually, those phenolic PCBs are very prone to cracking under mechanical stress but it was not it (although close).

My issue was the flex cable from DAC board to mainboard.
This seems to have fixed it but I need to test it a lot more before coming to a conclusion.
So far no freezes and it sounds good.

The problem was that one the wires had been detached from the flex cable edge and was slightly bent towards its adjacent pin. This has definitely happened due to excessive stress from someone trying to insert the cable at the PCB connector (at an angle maybe).
It should have taken a photo of the problem cause it's quite interesting what one can do with that extra little force in these fragile cables if not careful.
I haven't messed with the main board so that must be the previous owner who opened the machine regularly to "clean" it and also regularly "reseating" the cables.

Now, why this thing was okay when in vertical position vs horizontal I'm not exactly sure.
If I have to guess it's because at a vertical position the weight of the flex cable should have moved that pin a tiny bit further from the one next to it?
I really don't know..
Since these cables are a bit fragile I thought about rewiring the whole thing with a ribbon or something to future-proof or cut this pin off the connector and just bodge a wire from the DAC board to the mainboard but I did neither of those.
I'll try the synth a few days before call it a "fix" maybe carry out around the house or gently tap it in various places to put it under a bit of mechanical stress and see if this happens again.
If someone really likes this synth I think that the tactile switches, the electrolytic caps and these flex cables should definitely get replaced or at least checked closely since they are very prone to fail and manage to freeze the whole thing.

Another note : C11 is very important although it's common knowledge that the primary capacitor rarely fails.
I replaced it last and the power supply noise definitely reduced so replacing ALL electrolytics in the power supply is a very good idea.
I'll post back if another problem arises, thanks for the help Yekuku.

[cake100]

Hello,
After about 2 years not using it, I dusted it off, turned it on and there were no sounds (a click in the headphones when you power up or turn off on full volume), and even though something showed up, the screen was essentially blank as in the photo below.


I opened it up and changed the battery (multimeter read it as 1.3V) and also noticed what looks like a leakage as in the photos below:




I assume this is probably the reason the synth is not working?
I guess the leaking capacitors need replacing, along with the resistors with leakage engulfing them?
Anyone recommend which type I should buy?

Thanks.