Steinberg MR816x distorted sound

[RoadrunnerOZ]

Looking back on service notes for these units I’ve replaced the NJM2068 in tandem with replacing the resistors - possibility is the burnout is due to failing IC’s, (but then the failing IC’s can also be a result of the burnt out resistors). Safest is to do both.

 

[RoadrunnerOZ]

Just had a look inside the combo connector - the tongue spring connector for the tip is folded back on itself - in other words the tip of the jack has not ridden over the tongue as it should but got caught on it and dragged it over on top of itself. Testing a Mutron BiPhase with the customers lead - never again - should have used my cable tester instead of trying to take a shortcut!!

 

[NoisyIndividual]

Muton Biphase, that's the thing where the pots support the PCB via long wires that inevitably break?
Stupid mechanical design but sounds rather nice.

 

[RoadrunnerOZ]

Yeah I’ve repaired a lot of Mutron stuff - Mike Beigel is always very helpful as is Richard Lingenberg

 

[camarada78]

I have an MR816 and had a similar problem - it turned out to be 27KΩ smd resistors on the front and back of the main board in the area of the gain pots, the op-amps immediately following. It’s easy to see with a magnifier if they’re burnt but replace them anyway.
Ch1 & Ch2 the IC to look for is just behind Ch2 combo input socket, it’s IC602, replace resistors R747 &R748 - these are the feedback resistors for IC602 which may also be faulty. I replaced all 4 IC’s immediately following the gain pots and the smd feedback resistors all of which were cooked.
You need to get at the underside of the board next - you’ll need to remove all the screws holding down the DM board, take off the front plate, remove the 8 pot nuts and front combo connector clamps, the two front panel ribbon cables, the insert connector and two power supply cables - leave the rest connected.
On the underside of the board in between each pair of pots 3-4, 5-6, & 7-8 is a set of 4 rows of smd resistors and capacitors for each pair of pots - the resistor numbers are all in little boxes adjacent to their location. Replace R755 R756, R1041 R1042, R1043 R1044 - I just tacked little metal film resistors to the parallel smd capacitors. Their associated IC’s are directly above on the top of the board - these are IC’s 603, 650 & 651 - NJM2068

Thanks for the comment. I will investigate it.

 

[RoadrunnerOZ]

Check the op-amps as well

 

[camarada78]

I have an MR816 and had a similar problem - it turned out to be 27KΩ smd resistors on the front and back of the main board in the area of the gain pots, the op-amps immediately following. It’s easy to see with a magnifier if they’re burnt but replace them anyway.
Ch1 & Ch2 the IC to look for is just behind Ch2 combo input socket, it’s IC602, replace resistors R747 &R748 - these are the feedback resistors for IC602 which may also be faulty. I replaced all 4 IC’s immediately following the gain pots and the smd feedback resistors all of which were cooked.
You need to get at the underside of the board next - you’ll need to remove all the screws holding down the DM board, take off the front plate, remove the 8 pot nuts and front combo connector clamps, the two front panel ribbon cables, the insert connector and two power supply cables - leave the rest connected.
On the underside of the board in between each pair of pots 3-4, 5-6, & 7-8 is a set of 4 rows of smd resistors and capacitors for each pair of pots - the resistor numbers are all in little boxes adjacent to their location. Replace R755 R756, R1041 R1042, R1043 R1044 - I just tacked little metal film resistors to the parallel smd capacitors. Their associated IC’s are directly above on the top of the board - these are IC’s 603, 650 & 651 - NJM2068

My friend, thanks a lot for these insights. I started looking around there and found weird values also. So I changed both 27k resistors in each channel and the noise was completely gone. Changed even on the working channels with great success.

edit: Luckly the OPAMPs were fine.

Thansk a lot.

 

[RoadrunnerOZ]

My friend, thanks a lot for these insights. I started looking around there and found weird values also. So I changed both 27k resistors in each channel and the noise was completely gone. Changed even on the working channels with great success.

edit: Luckly the OPAMPs were fine.

Thansk a lot.

No problem.
Just another point to note is the phantom supply can go noisy and if engaged can introduce noise and distortion to the “phantomed” channel with a condenser mic. This is either one of the two (or both) 220uF/63v electrolytics or the FET HAT2256R IC FT850 or the N-Channel controller LM3478 IC852 - DM diagram page 013, top left corner K-M/3-4. If you get noise the first thing is to replace C1622 & C1625 the electrolytics (and C1621 - 0.1uF just in case) and measure the two 4K7 resistors - R1623/1624 and resoldering all the joins around the inverter circuit.
You can see from the tiny channel meters in the MR Editor if a little low level noise appears when you engage phantom with no mic connected - if this is present it’s best to fix it before the phantom fails altogether. The inverter runs at very high frequency which is controlled by R1615 39K - runs from pin 7 of the controller to ground - if this resistance goes dodgy you’ll get erratic behaviour from the inverter.