Motu 828 mkii Phantom Power Switches

[zephyrmic]

I have a Motu 828 mkii in which both phantom power switches have failed mechanically, for what reason I don't know. The unit has not been used all that much over the years, so I have to surmise that these switches were not the most reliable in the first place.
On removing the top cover, I found the red on/off toggle switches are PCB mount ones. Looking from the rear of the switches they have three pins going to the board. The centre one is the 48v. feed. The right one, looking from the rear, presumably goes to the TRRS/XLR socket on the front panel. The LEFT pin, looking from the rear, is soldered to the earth plane on the board, which leads me to assume that there is no internal connection in the switch, otherwise the 48v. would be shorted to ground when the switch is OFF. Having no schematic to consult, I have had to use my observation.
In looking through catalogues available to me, I can find no equivalents in Australia. I have some same sized toggle switches that could be used as replacements. They have three pins on the back of the switch. They are ON/OFF/ON SPDT as to function.
I assume that if I press these into service, that I will need to make sure that the left pin looking from the rear is not connected to ground. That would short 48v. to ground in the extreme ON position (or OFF in the Motu switch).  Am I correct here?
Any advice re these switches, or where I can buy the Motu equivalent toggle switches in Aust. would be greatly appreciated. Thanks.
Kindest regards,
zephyrmic

 

[radardoug]

Pictures! Pictures!

 

[zephyrmic]

Okay. I will take the lid off and try to get some pics. There is nothing on the exterior ahat indicates switch failure, except that the toggle refuses to move.
zephyrmic

 

[zephyrmic]

Here are two pics of the P48 switches. Next one in next post.
zephyrmic

 

[zephyrmic]

Pic with trim board removed.
zepphyrmic

 

[radardoug]

looks like a standard C&K switch, sold by Digikey, Mouser and others. Go to their site and do a search. You only need an ON ON switch.

 

[radardoug]

Also RS Components and Element14 who both have branches in Australia.

 

[Khron]

The centre one is the 48v. feed. The right one, looking from the rear, presumably goes to the TRRS/XLR socket on the front panel. The LEFT pin, looking from the rear, is soldered to the earth plane on the board, which leads me to assume that there is no internal connection in the switch, otherwise the 48v. would be shorted to ground when the switch is OFF.

Close, but no cigar. If you take a look at The ch2 switch in the last photo you posted, you'll note a trace going along the front edge of the pcb, and going to the right-most pin on both switches - THAT is the 48v feed coming from the power supply. The center pin (the "common") is the "output", goes through that 1k ("102" marking) SMD resistor, gets filtered with that pair of electrolytics, and fed through the pair of blue-bodied 6.8k axial / through-hole resistors to the XLR pins 2 and 3.

True, when they're off, the switches do short the phantom power to ground, but only the "output", in order to discharge those last filter caps (via those series 1k resistors).

 

[Bo Deadly]

Are you sure the 48V supply isn't dead? It's kinda strange that both switches would fail like that. Can you measure 48V on the pins on the right?

 

[zephyrmic]

@ squarewave: Checked out phantom supply. All good, measures 50v. on my DMM. Taken from the right pin of the switches looking from the rear. The problem is mechanical failure of the switches, I am sure, which is rather surprising. Have not encountered toggle switch failure before.
@Khron: Thanks for that explanation. So you are saying that when P48 is switched off, and the unit is still powered up, the 48v. continues to be shorted to ground through the two 6.8/1K resistors etc.? I guess if that is the case, no drama because of the current limit through those resistors.
Kindest regards,
zephyrmic

 

[Bo Deadly]

So you are saying that when P48 is switched off, and the unit is still powered up, the 48v. continues to be shorted to ground through the two 6.8/1K resistors etc.? I guess if that is the case, no drama because of the current limit through those resistors.

Not quite. The common is switched from the 48V to ground so no 48V is sourced. It drains the caps through the 1K which will run 50mA through the switches very briefly but that should be no problem for 1A switches.

The 1K in series with the 50V source is a little high. If a mic draws 10mA, that's a drop of an extra 10V. Something like 220 would be more appropriate. But for this circuit that would run 230mA through the switches.

I'm still intrigued that both switches failed. Maybe the unit lived in a harsh environment for a while and the contacts are shot. But just to be really sure, did you measure continuity at the pins as opposed to the pads? Is it possible that the solder joints cracked up from heat?

 

[zephyrmic]

All solder joints are fine, and also even in the PS section, there is no evidence of undue heating of the board,etc, which is good. I measured continuity of the switches through the pins, and it is clear that the contacts have been degraded in switch on input #1. And the toggle is stuck in OFF position, and will not move from there, and also there is no continuity between middle pin and LH pin that goes to earth. On input 2, the switch still operates, but will not stay in the ON position, but goes back to OFF by itself. From the look of the board etc in the case, there is virtually no dust or dirt, and everything looks really clean.
The only other repair I have had to do is to replace the SMD LED backlighting the main panel. Fiddly job, done some years ago. When I bought the unit, this LED had already failed.