MOTU 828 MK2 potentiometer problem

[MarkusBass]

I'm looking for some help regarding noise on Input1 in 828 MK2. There is noise without any signal source plugged in. I checked that the instrumental preamp in channel 1 has incorrect voltages on the output (4 Volts). I tracked the problem and I found out that the noise and incorrect voltages and caused by somehow failed trimmer pot in channel 1. The potentiometer used there is quite unusual - it is marked as C20K/C2K. It is double pot; one is 20K and the other 2K. The pot has 7 pins but only 4 of them are used: hot pin and the slider. I assume that one pot is used for mic preamp gain and the other for instrumental preamp gain. The potentiometers should be electrically isolated. This is the case of channel 2 trimmer. But the pots in channel 1 are somehow shorted internally and this causes the noise. I could replace the trimmer from channel 1 with the trimmer from channel 2 to make sure that the noise will move to channel 2. But since this is a lot of soldering I would rather buy the potentiometer. It seems like custom made by Alpha or Bourns but since it has two different values, it is not available from these suppliers. Can I purchase it directly from Motu?

Mark

[torrentg]

I see no reason why they would not sell a pot to you, if you ask them to. Go ahead and try. Good luck and tell us if the operation turns out to be successful.

I think they will.

[MarkusBass]

Yes, I will try to do it. The problem is that in order to start any conversation they require product registration. And I'm not the owner of this MOTU 828 - I only fix it. I don't want to register the product under my name. I even don't know the owner - I work for audio equipment service.
In the past I fixed several MOTU 828 but this time I need parts from MOTU.
I sent them an e-mail with the description of the problem. We'll see if they can help.

Mark

[MarkusBass]

I decided to desolder the pot without waiting for the answer from MOTU. It's because the input was bad and it couldn't be used anyway.
After desoldering I found out that the pot is OK. Also the PC board was OK. It means that there was rather a short circuit on the board (between the pot's pads) and it disappeared when I desoldered the pot. It is now assembled back and all the problems are gone. But I still would like to hear the answer from MOTU about the possibility of buy the pot from them. For future repairs of course .

Mark

[mecano]

Hello Markus, did you get any response from MOTU?
Just got a secondhand 896HD and one pot is giving me approx. +1db and knob is broken.
Part ref. is the same C20K C2K with a very long shaft and it looks like the knob is moulded (didn't remove faceplate to check properly yet).
Just opened a tech note with MOTU.
Regards

[mecano]

Got a kind chat with MOTU through "tech note", that was fast. They do not sell replacement parts but can service the unit (896HD in this case) and routed me to my local distributor (I'm in Europe). Good to know this unit is still serviceable, I took the chance opened the unit, clean pots, see that the back line/+4/mic selector on channel 8 was behaving, fixed it, put all things in place and tadaam! Uh? What is that popcorn smell!? Turned the unit off, opened it again and in my haste noticed I did screw/cut a PSU wire with the case One PSU resistor burned… changed it, put all screws back and tadaam! No more scratches on first two channels and channel 8 is now behaving correctly. Pfffuu was lucky…

[mertkara]

@mecano, they generally don’t sell parts separately as that’s the whole point of their business to build available designs and then charge people for repairs. Anyways, the pot in question in a multiple pot since it can control different output by just changing one trimmer knob. These pots are hard to come by since they are not used in most circuits but I think you could’ve easily got one off farnell and soldering it isn’t that difficult. Could’ve saved you some bucks.