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Digital Audio Workstation & MIDI => Audio & MIDI Hardware => Topic started by: davorin on April 10, 2016, 01:43:51 AM

Title: Motu 828 distorted inputs
Post by: davorin on April 10, 2016, 01:43:51 AM

ANyone having a Motu 828 and also a voltmeter here? (o;

Have a broken 828 here where each input has a 625kHz signal overlayed on the - inputs of the 2115 OP amps....

Just curious which voltage you have between GND and pin 4/8 on those input amplifiers.

Title: Re: Motu 828 distorted inputs
Post by: davorin on April 10, 2016, 06:51:40 AM

A little further (o;

Cut open the +9V power rail and injected the 9V from an external HP 6632B power supply...

Seems all input circuits are tending to oscillate heavily when even touching  with a DSO probe at the input....and the power consumption goes over 0.3A.....currently it is noise free and draws around 0.19A @ 9V with 2 inputs recording...

Might be some stains from the soldering process causing leakage current which triggers then oscillation and more current consumption....

Title: Re: Motu 828 distorted inputs
Post by: davorin on April 13, 2016, 05:49:00 AM

Distortion came back...*bugger...

Removed the PCB...and looks someone already added some additional diodes on the bottom side for the rectifier on the top side...

So now for replacing the rectifier and also the capacitors...as there might be capacitance loss due to time and heat....

Title: Re: Motu 828 distorted inputs
Post by: Protools5LEGuy on April 13, 2016, 05:58:24 AM

To my eyes, someone connected a 125 Volt to XLR to the input.

I have a friend with a 828 mk2 with a broken PRE, but he has no time to open his unit. He use it daily for mixing/mastering.

Maybe we can learn that MOTU has "delicate" inputs...

Title: Re: Motu 828 distorted inputs
Post by: davorin on April 13, 2016, 06:10:25 AM

Well...when I fed the positive 9V rail from my HP power supply...the distortion was after a while gone....on all inputs.
Don't think someone applied 125VAC to the inputs...would be more 230V here in Europe (o;

Have only drawn some part of the input circuit yet....would need a second unit to unsolder everything (o;

Doing now some load tests with the just the transformer feeding an external rectifier/capacitor circuit...which feeds my HP power supply (it can also be a remote programmable load ;o).

Title: Re: Motu 828 distorted inputs
Post by: Philgood on April 13, 2016, 09:12:09 AM

Hi.
Where do you think that 625kHz is coming from.
PowerSupply (switching type), Clocking signals...?

Just curious as i'm very interested in electronics.

Title: Re: Motu 828 distorted inputs
Post by: davorin on April 13, 2016, 09:23:47 AM

Not sure where it comes from....though must come from some clock source as it is stable...

Hmm...no clue what power rating the transformer has....but would expect some more power as in the load diagrams below (o;

Negative rail must be less consuming current, as it is around -8.9V....but positive rail draws up to 0.3A when inputs are distorted...

Title: Re: Motu 828 distorted inputs
Post by: davorin on April 13, 2016, 09:45:15 AM

Well stupid me (o;

The positive rail also powers all +5V components, like CPU, FW controller/transceiver, Codecs and more...

Title: Re: Motu 828 distorted inputs
Post by: davorin on April 16, 2016, 03:37:54 AM

Well....seems to be somewhere else....powered the 5V directly from my HP power supply....positive rail stays at around +8.8V...same as the negative rail with -8.8V....and still distortion on all inputs....

Also immune to temperature when using a cooling spray....