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Old 03-18-2020, 05:03 AM   #1
Andrealikescaps
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Default RME Octamic II input circuit anyone

Hello Badcappers.
I just finished wasting time contacting RME for any useful information on the Octamic II preamp. We have been repairing these for some years (with no help from RME). These are expensive devices with cheap unreliable components inside. Caveat emptor.
Anyway, we are managing to repair them all the same, usually by replacing the dodgy switchmode power supply with something reliable, but recently I thought it would be nice to actually repair one of these devices, rather than replacing boards in them. A likely candidate came along with just one channel out of 8 showing no output. Surprisingly, this one actually had a working power supply, which was nice.
Measuring voltages I could see 9VDC on the negative side of most of the caps in the faulty channel. Obviously not meant to be there, especially when compared with the other channels.
Anyway RME wouldn't even drop a hint as to how their input is wired and the attached guess is just a guess. So cut a long story short, anyone ever mapped out the input to ANY similar RME devices as I'd bet they all are the same?
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File Type: jpg typical input.jpg (13.8 KB, 12 views)
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Old 03-20-2020, 10:56 AM   #2
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Default Re: RME Octamic II input circuit anyone

Short answer, yes, but there are several variations. You could consider starting with some well-lit (ie. not dark, but no flash-glare either) and well-focused photos of one channel.
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Old 03-23-2020, 05:51 AM   #3
Andrealikescaps
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Default Re: RME Octamic II input circuit anyone

Thank you Khron, is this photo ok?
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File Type: jpg IMG_20200323_194651.jpg (365.8 KB, 12 views)
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Old 03-23-2020, 10:14 AM   #4
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Default Re: RME Octamic II input circuit anyone

probably some transistor went south. what is the marking on the 3 legged SMD part? is there anything on the bottom of that circuit board?
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Old 03-23-2020, 12:50 PM   #5
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Default Re: RME Octamic II input circuit anyone

That's a bit on the dark side, but anyway... So you're not seeing a signal on the ADAT channel 8 (where the ch8 preamp is routed)? Or the analog output 8 is dead?
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Old 03-23-2020, 07:40 PM   #6
Andrealikescaps
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Default Re: RME Octamic II input circuit anyone

Thanks for the suggestions. I've uploaded more photos. No signal on ADAT or analog output on channel 4. All the rest OK.
All the caps except the largest and smallest on the left have around +9VDC on the can, as pictured.
Disconnecting the 48V power feed to the board does not make the stray 9V go away, so not caused by those diodes. Removing the ADAT board makes no difference either.

To answer Capleakers sensible question, there is nothing under the board as pictured. The 3 legged device is marked "1 GM" followed by what could be a sideways "N" or a squashed "Z". Photo attached.
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File Type: jpg IMG_20200324_090159.jpg (360.4 KB, 6 views)
File Type: jpg IMG_20200324_090226.jpg (372.4 KB, 4 views)
File Type: jpg IMG_20200324_090255.jpg (407.5 KB, 4 views)
File Type: jpg IMG_20200324_090619.jpg (376.3 KB, 8 views)
File Type: jpg IMG_20200324_092802.jpg (685.0 KB, 4 views)
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Old 03-24-2020, 08:04 AM   #7
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Default Re: RME Octamic II input circuit anyone

The can is not necessarily connected to anything, so... I'm not 100% sure what exactly you're measuring. For reference, i just measured a couple Rubycon electrolytics i had on the bench here, and my meter's showing 1.8-2Mohms (that's mega-ohms, millions) between the can and either leg.

"1G" seems to correlate with a number of different NPN transistors - my guess would be, they switch the phantom-power on and off for that respective channel.

Any chance you can get enough glare-less lighting on the board where you wouldn't need to use your phone's / camera's flash?

I wouldn't remove the various DC-blocking electrolytics from suspicion. But some more "overall" photos of the board might help with at least getting a feel for the structure of the preamps.

In a pinch, assuming you've got a signal source you could set to 100Hz or so, you could try tracing the path of the signal with your multimeter - the AC range should be capable of measuring 100-120Hz frequencies with some degree of accuracy.
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Old 03-24-2020, 08:27 PM   #8
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Default Re: RME Octamic II input circuit anyone

Thanks again. I have measured the same voltages at the board as on the Caps can. I know there isn't necessarily a hard connection to the can, but it makes an easy measuring point, besides for a high impedance voltmeter, 1Mohm is no obstacle to measurement.
The stray 9V seems to be coming from the IDS connector pictured, 5th pin from the top right.
I'll pull the front panel board, also pictured, and follow this stray DC voltage in.

You are correct. The 1G transistor is 48V power feed and since that is disconnected, not relevant to this problem.

Audio does seem to push through the DC, past the diodes, at an attenuated level. I'm pretty sure once the DC is removed, the audio will behave normally.

A circuit would help this a great deal. It's a shame RME are so insecure about their products that they won't share service material.
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File Type: jpg IMG_20200325_100814.jpg (580.7 KB, 2 views)
File Type: jpg IMG_20200325_100914.jpg (313.0 KB, 3 views)
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Old 03-24-2020, 10:30 PM   #9
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Default Re: RME Octamic II input circuit anyone

Those big 6.3V caps in and around a mic preamp start to take my thoughts towards a pretty classic mic preamp design.

https://sound-au.com/project66.htm

Could one or several of the input transistors have become electrically leaky and "produce" that 9V offset?

And/or it might be interesting to know what voltages the analog circuitry's powered with.
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Old 03-25-2020, 11:24 AM   #10
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Default Re: RME Octamic II input circuit anyone

measuring voltage on the cap lid like shown in the first 2 pictures in post #6 is wrong. you need to measure the legs of the cap.
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Old 03-25-2020, 07:10 PM   #11
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Default Re: RME Octamic II input circuit anyone

It looks like a vanilla mic pre-amp front end. It's common for these to get zapped by transients from the phantom power or ESD. Note OP's pic has four components not populated which are likely protection diodes.

I would check the 470uF 63V input coupling caps have not shorted as a side effect.
When phantom power is on, they see 48V on (+) and 0V on the other (op-amp) side, which is normal.
With phantom power off, 0V on the (+) and with a failed op-amp say -9V, which is reverse polarity for them.

The 3,300 6.3V cap is for the mic pre-amp op-amp (gain control) so the voltage on it will tell a lot, if the IC is OK.

Measuring voltage touching the can of an electrolytic cap doesn't work- it did 40 years ago with can-type electrolytics, but now you're just measuring leakage to the can which can be higher than the multimeter's input resistance and inconsistent readings. It just leads you on a goose chase.
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