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  #1  
Old 01-23-2022, 11:42 AM
scottie_uk scottie_uk is offline
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Default MBox 3 (not pro or mini) Input Two Stopped Working

Background:

Dear all, I have an MBox 3 (not the pro or the mini), and I have been using it now for a couple of months, connected to a small Alan and Heath mixer for my collection of five Synthesizers. It has been working well since installing it a couple of months ago until just recently.



Issue:

Recently, I temporarily repurposed the MBox 3 as a microphone amplifier on channel one (with 48v phantom power the mic needed) and it worked well; I used it with OBS Studio to narrate a video. Upon attempting to put it back to use on my synthesizers, I noticed channel two is no longer working properly. Now channel two barely picks up sound at all, and only if the input is on full (with soft instrument selected) I get a terribly distorted and intermittent signal at best and more commonly mostly nothing.


The MBox software confirms this. Chanel 2 if connected to microphone with phantom power using an XLR cable, or being connected to a synthesizer via a jack plug exhibits the aforementioned problems while channel one works just fine.



I'm running Windows 10, I have an intel 4790K quad core i7, and I have the most recent drivers installed (it says package 1.110 with firmware 1.0.8). I have the monitor outs going to a stereo amplifier and had the digital outs and ins going to a Lexicon FX Processor.



Thanks for reading. Help.
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  #2  
Old 01-23-2022, 12:14 PM
Darryl Ramm Darryl Ramm is offline
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

hi Welcome to DUC.

What exact instrument/synth etc. are you connecting and what output does it provide? What exact port are you plugging in with what type connector? Does the instrument need a DI input or is it line level?

Make sure the #2 gain knob is not pulled out and engaging the -20dB attenuator. Pull it out and in several times and see that it's cutting the level as expected even if it's already too low.

If you are using the DI input (on the front panel) then you should see changing the preamp gain adjust the level ... do you? I just want clarity on that since if the gain is turned down or the gain pot is faulty on channel 2 etc. then that might also explain what you are seeing. Even if things are way off does changing that input 2 gain knob change anything? kidna work as expected?

Yes I'm looking for a faulty gain control knob here. Can you make things intermittent by wiggling that knob or any of the input cables/jacks?

It may well be that the input mic pre/DI circuit is damaged. If so then unless you are handy with electronics repairs yourself it's not worth dealing with and these are really beyond end of support life... and there are lot of *great* interfaces available (although S/PDIF out and in limits choices, but Moto has 'em and is a good vendor).

What happens if you try the rear panel line inputs with a line level input signal? (on these units that line in is *not* behind the preamp circuit, it that also goes though the preamp hardware just with a preset low gain. So that might give some diagnostic clue).

Not sure what you mean by "soft instrumental". The Sft-lmt button means soft limit. It's just a simple limiting compressor on the input, you can try with that on and off to see if it's part of the problem. But there is no universe in which driving these low-end preamps hard with or without soft limiting helps stuff sound better.
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Old 01-23-2022, 12:35 PM
scottie_uk scottie_uk is offline
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

Thanks for the response. I have wired the input of channel 2 on the back to the R output of a Roland JV2080 rack based synthesizer, and the front to an Alesis DM5 drum brain which I assume to be line level. Both use a TRS jack plug.
I have also wired the back connector to a condenser microphone that needs 48v phantom power using an XLR cable (with phantom power turned on).


Moving the volume knob for channel 2 does indeed make the sound go louder and quieter but before the 4 oclock position almost nothing is heard, when it does come on it like hit hits some threshold and the sound breaks though, but is it always very distorted.



I have tried with the knobs in and out for that gain attenuation. I have also tried moving that knob in and out and up and down several times too see if the problem is related to a poor /dirty connection in the pot. It does not seem that way, the behavior is pretty consistent. For channel 2 on max, all I get is distorted but not loud sound.


If there are known issues with capacitors drying out ETC I'm ok with that. I would also be ok with disassembly and deoxit, but I am not convinced that this is the problem as the issue is not intermittent and came on all of a sudden rather than gradually over time.

Last edited by scottie_uk; 01-23-2022 at 01:52 PM.
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Old 01-23-2022, 02:04 PM
Darryl Ramm Darryl Ramm is offline
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

I don't think there are any commonly known issues with these interfaces, and certainly nothing that causes one channel to fail. I'm not optimistic that aging electrolytics would be the suspect here, maybe, but I'm expecting (but not sure) that these use a single chip two channel input circuitry and at least the power supply related electrics are common to both. By all means poke around if you are handy, but I would not spend too much time on something that is likely worth ~ $100... I expect you could pick up a used one for around that price. But I'd personally looking for a new interface with current support from a vendor like RME, Motu, Focusrite, Presonus, etc. But the need for S/PDIF out and in will keep you out of those vendors lowest cost models.
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Old 01-23-2022, 02:10 PM
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

Sounds very much like that Mbox input is busted.
A quick temporary fix could be to connect your mixer outputs to the Lexicon and use it in bypass and use the digital inputs for audio from the A&H. You still also have one input left on the front of Mbox.
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Old 01-23-2022, 02:55 PM
scottie_uk scottie_uk is offline
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

Ben, good idea. My plan is to use the main outs to connect them to the analog input on the Lexicon, and then feed the lexicon back into one spare input on the mixer. My mixer (Alan and Heath Zed 10FX) is limited in that its external FX send is mono). My amp is then connected to the monitor outputs on the desk.


It's no biggie, my employer sold me the Avid MBox 3 cheaply as surplus anyway.
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Old 01-23-2022, 04:30 PM
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

Quote:
Originally Posted by scottie_uk View Post
My plan is to use the main outs to connect them to the analog input on the Lexicon, and then feed the lexicon back into one spare input on the mixer.
What I meant was, if one of the analog inputs on the mbox is broken, then you can use the digital spdif inputs, it's already connected, right? just plug your mixer into the lex and set it to dry, and select the digital inputs in your daw.
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Old 01-23-2022, 05:08 PM
scottie_uk scottie_uk is offline
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

I see what you are saying, and I think that's a good idea.
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Old 01-23-2022, 08:24 PM
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

Darryl makes mention of no known common issues with the Mboxes. I would say an uncommonly known issue is that the preamp design does not tolerate extreme clipping at all. It takes almost no time to fry the input if it gets hit with too strong a signal. I once loaned an Mbox to a friend to transfer cassette recordings into Pro Tools. He had the gain on both channels too high and he overdrove the inputs for 3-4 seconds. That was all it took to fry both channels.
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  #10  
Old 01-29-2022, 08:49 AM
scottie_uk scottie_uk is offline
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Default Re: MBox 3 (not pro or mini) Input Two Stopped Working

Thanks for all your suggestions and points.The box was fried. I found another unested on Ebay for $25 and was going to use it for parts, but it worked fine. I may have a poke arround in the old one but from what the last poster said, I assume some IC has gone and I'm not good with surface mount stuff.
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