Chosen Solution

For the better half of my night tonight, I tried googling/youtubing/askingjeeves (before attempting any teardown) of the inside of a yamaha mg12xu (or even 16xu for that matter). I can not find any guide or information, just two videos showing the same problem I have. Apparently either a PFL circuit has failed or the buttons went bad and are miss-pressing. I just got this last night (ebay), so I don’t want to open it up yet. (I may open it up before i take it to a authorized yamaha repair center and open it again to see what they fixed) so I may end up making a guide/before and after; if I i don’t get a refund. If there is anyone out there with any experience on these devices (mixers in general) and how to go about fixing this particular problem; please let me know. I’m a repair tech for a CPR but I do small electronics, I have no idea where to start with this. Oh, here’s a (not my) video to the exact problem, apparently it’s a known thing: https://www.youtube.com/watch?v=AGXQ13hL…

Hello! ICM02 is the marking on the schematic diagram of that NJM4565M (TE1). The case type is SOP8 and it contains two OP-Amps. First one is with 1 (Output), 2 (invert. Input) and 3 (direct Input). For beginning I added resistor 91kOhm (or 100kOhm) between bus PFLC and pin 2 (invert. Input) of ICM02. Then grounded PFLC bus thru 6.8kohm resistor. It wasn’t enough like it turned out. Second time I added 300kOhms resistor between -15VDC and pin 2 (invert. Input) to give additional shift to it. All connections are provided within ICM02. Princip is following: if there is no pushed any PFL switches - pin 1 (output) has to be with zero volts. If any of PFL switches is activated - pin 1 (output) changes to negative and crosses triggering level (-3VDC) of the following comparator stage (pins 7, 6 and 5 of ICM02 respectively). Due to temperature drift and may-be something else (like induced noise) sometimes it’s possible to trigger the comparator without switching on any PFL switches. 300kOhms resistor between -15VDC and pin 2 (invert. Input) gives additional drift to not achieve that easily triggering level for comparator. Upper side of schematic on picture: resistor RM14 is connected to +15VDC (sorry, my mistake - didn’t fit on the picture)

This is a factory default bug from circuit design. I didn’t fix it yet (I have only MG16XU SERVICE MANUAL but sure I’ll get it also for MG12XU). Common bus (second) from PFL switches goes directly to OP-amp input. +15VDC thru 56 kOhms resistor goes to first PFL switches bus. It would be great to have 10 kOhms resistor from second bus to OP-amp input. And second bus is better to ground through 100…330 kOhm resistor. Default second bus is too sensitive. With hotter days OP-amp’s temperature drift causes triggering PFL (IMHO).

Hi! I’ll try to upload pictures from it. I used ceramic capacitors, non-polar, for 50 volts. I had one more such issue this summer - with extreme high temperature (about 30 degrees C). Then it was right thing to mount also switch I had to use. Leakage current to the bus connected to the OP-Amp inverted input is non-stabile, floating in time. Therefore You never know when it will be in bothering area. Using all resistors with lower resistance will improve it but not guarantee. Good luck to Your work!

An MG16XU came for the same reason to my shop…with so service data, was really thinking but I think I solved the issue by putting a 200k resistor between pin 2 and pin 4 of IC 602. The random PFL flashing seems to have stopped and is stable for now… will keep under check and update.

. Pic 1 - Schematic diagram: in Red color are shown added components - 2 resistors. Shown 10 and 100 kOhms, really I put (what I had): 6,8 and 91 kOhms. May-be it’s not enough to put 300 kOhm Resistor from -15VDC to common point for other resistors (please look at last picture for schematic). In this Summer with hot weather I had PFL flash again! Instead of 300 kOhm You have to put in may-be 200 kOhm or less. I haven’t pics about 300 kOhm Resistor in place nor I don’t remember even I put it in also. As pics have here no names I can’t say exactly what pic is for what. On one pic is shown Earth Connection Point. It was easier to find it in some other place on PC. Also may-be better to disable PFL with switch (look at last pic, blue color).

My pictures didn’t reached here. I’ll try once more… but if I choose above “Insert an image” icon it shows all my uploaded pictures. Where are they?

An alternative to these solutions is to remove ICM02 from the PCB.This will disable PFL entirely - but for me that’s not an issue as I never use it.