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Behringer Eurorack MX802A modifications

Make the "Phones" volume control independent of the "Main mix" fader

Make all the AUX sends pre-fader

Bad power supply capacitors

One might think I do this headphone volume modification for a living... This is the third mixer I've modified this way! That's the result of upgrading and upgrading and upgrading whenever a nice mixer comes my way with a good price tag. If you wish to read a preamble as to why I want these mods, or how the mods work on a general level, please see my pages on the t.Mix MIX 802 and the Behringer UB502. On this page I'll just get straight to the details of how these mods are performed.

So far, Behringer mixers have been especially nice to modify, because you can easily find online both the user manuals and the complete schematics for most mixers. However, what I found inside this MX802A (with the main board marked "MX802A/01" and "PCB908039REVG") absolutely did not correspond to the schematic I found—which did say only "MX802" in its title block, without the "A" suffix. Probably the function of the mixer follows the schematic, but component values might differ, and their reference designators definitely do!

Therefore, please do not blindly trust any component reference designators that I mention on my web pages! It is entirely possible that they do not correspond to the circuit board inside your device! First follow the traces on the board to ensure the component in question is the correct one in relation to other components! The designators I mention in the following are what I found on my board.

Opening the mixer

If you want to remove the main circuit board, you will firstly need to pull off all the potentiometer knobs except the "Gain" knobs on the four Mic inputs. Yes, there's 46 knobs to remove regardless. They're only friction fit, but some can be rather tight. Make note, which color knobs go where. Then you'll need to remove the retaining nuts on all 19 of the 6.3 mm jacks (and don't lose track of the associated 19 washers either!), as well as two retaining screws each on all four XLR jacks, and the single screw in the middle of the silly RCA jacks labeled "Tape in" and "Tape out". However, do not remove the nine screws located in the midst of all the potentiometers.

Then, to get inside the mixer, remove the six screws protruding from the sides of the chassis (three on either side), but not the two screws recessed into the plastic bits at the very front. Next remove the six smaller screws from the bottom, three at the back and three in the groove near the front, but not the two bigger screws in their own separate recesses. Now you can remove the bottom cover—but be careful not to yank the wires between the main board and the small power supply board, which is mounted on the bottom cover.

Then remove the nine screws holding the main circuit board in place. The board is separated from the chassis by stand-offs, which will most likely remain with the chassis. If not, then go ahead and remove that screw from amidst all the potentiometers on the top panel, which I earlier told you not to remove. After you've done that, you can remove the main circuit board, which will still be tangled with the wires going to the power supply board. Just as well not to try to disentangle those.

Oh by the way, you did keep track which screws go where, right? Right?

The headphone volume modification

This modification is exactly the same as the ones I did to my t.Mix MIX 802 and Behringer UB502 mixers. There is also a Behringer MX802A Monitoring Modification on the net, which does essentially the same thing, but leaves the "2TK to Control Room" switch nonfunctional. My modification does not—you can still route the "Tape In" signal to the headphones or "Ctrl R" output if you like. The only side effect of my modification is that the level indicator LEDs are now pre-fader also, so they keep blinking even if you adjust the main output to zero. So they're not useful for adjusting the output level when mastering a mix, but if that's what you want to do, then you're probably not doing this modification anyway.

As in the UB502, the headphone amplifier gets its signal via the "2TK TO CTRL ROOM" switch, which is SW6A/SW6B in the schematic and on the board. I wanted to isolate it from the signal coming from the main fader, then connect it to the signal line going into into the main fader, as shown for the right channel in this excerpt from the schematic. Starting from an easily identifiable component such as the RCA jacks, and using a multimeter to find zero-resistance paths from component to component, I was able to identify the necessary components on the board. Their reference designators on the circuit board are marked here in red.
Click for a bigger picture of the board's back side. I've labeled the places where I cut traces. Cut "3" is missing from the picture, as that's on the front side of the board. The black markings on the board are my own scribblings from while I was trying to decipher its components.

The switch terminals to isolate on SW6 are the ones furthest away from the board's bottom edge. To isolate the left-hand one (which is the right channel) I cut the trace leading downwards from it at a suitable place where there's room to work with a Dremel tool's milling bit (the cut labeled "1" in the photo, click for a closeup) and then verified with the multimeter that R339 and C166 (R345 and C259 in the schematic) had been isolated from the switch. However, R59 and R60 (R20 and R37 in the schematic) were still connected, so I had to make another cut (labeled "2", click for a closeup) to isolate those. Now the switch terminal is isolated from all of them, however R339 and C166 are now also isolated from R59 and R60, which is not the intention. So I will have to wire those together again later.

Click for a bigger picture of the front side.

The traces going to the right-hand switch terminal (left channel) are a bit more difficult to follow. One cut was more convenient to make on the component side of the board, which I had already removed in order to read the capacitor reference designators—I just had to make double certain that I follow the trace correctly from one side of the board to the other. (I could also have ground out the via right next to R53 on the back side.) I finally ended up cutting at a position marked "3" on the front side (click for a closeup), and at a position marked "4" on the back side (click for a closeup). As before, this also isolated C167 and R338 (C258 and R344 in the schematic) from R53 and R54 (R19 and R18 in the schematic), so those too will have to be reconnected together again.

Click for a bigger picture of the wiring. It is all on the back side of the board.

To reconnect the circuits which I had unintentionally cut in half in order to to isolate the switch terminals, I soldered jumper wires from the lower end of R59 to the positive end of C166, and from the lower end of R53 to positive end of C167 (see a closeup of the capacitors, at the bottom edge of the board, C166 is on the right, C167 on the left). The two resistors are SMD versions, but entirely possible to solder to regardless. I put a blob of hot glue at the midpoint in the wires, just to help keep them in place. At this point, the modifications I've done have isolated the two switch terminals, and nothing else.

I decided to take the pre-fader signal from the negative ends of C161 (right channel, labeled C253 in the schematic) and C154 (left channel, labeled C251 in the schematic, see a closeup of the capacitors, at the left-hand edge of the board, C161 is the lower one, C154 the upper). I soldered jumper wires from them to the now isolated left-hand terminal and right-hand terminal of the switch, respectively. These are trivial to solder, as they are all through-hole components. This completes the modification.

The AUX send modification

The Behringer Eurorack MX802A has two AUX sends for each input channel. AUX1 is pre-fader, AUX2 is post-fader. I wanted both AUX sends to be pre-fader. I explain why on my t.Mix MIX 802 page—that mixer had just a single post-fader AUX send, which I wanted to convert to pre-fader.

The MX802A has provisions to convert AUX2 also to pre-fader on the four mono mic channels (the modification is explained in the user manual on page 11). However, the AUX2 sends on the two stereo channels do not have this provision, and I wanted to convert them to pre-fader as well! Here's how I did that.

To isolate the AUX2 send of stereo channel 5/6, I cut the trace leading from VR35 towards R295 and R296 (labeled R283 and R309 in the schematic). The cut is labeled "5" on the photo (click for a closeup). This was very obvious and easy. Likewise, to isolate the AUX2 send of stereo channel 7/8, I cut the exact same trace leading from VR36 (labeled "6"). Both stereo channels occupy their own strip of circuit board, and those strips have exactly the same architecture and layout (likewise for all the mono channels as well), so any channel-specific modification is easily replicated to other, identical channels.
Click for a bigger picture.

To reconnect the AUX2 send for stereo channel 5/6, I soldered two 10 kΩ resistors to the middle leg of VR35. I soldered their free ends to the negative terminals of C146 and C147 (labeled C220 and C221 in the schematic), which are where the corresponding resistors leading to AUX1 are connected as well. For stereo channel 7/8, I soldered two 10 kΩ resistors in the corresponding positions, from the middle leg of VR36 to the negative terminals of C148 and C149 (C222 and C223 in the schematic, click for a closeup of the resistors.)

Bad power supply capacitors

Oh, by the way, before you reassemble your mixer, take a look at the capacitors on the power supply board (the small PCB attached to the bottom of the case). See any brown gunk oozing from underneath the plastic shell near their tops like I did? Or are they actually swollen, like they were in my Behringer UB502? Geez, how cheap electrolytics does Behringer use in their power supplies??? While you have the thing open, consider replacing them as well, to avoid another service a year or two later!

However, there's very little extra room inside the mixer, so the replacement caps must not be any taller then the originals (which are 21 mm tall). I actually could not find suitable ones anywhere, which is why I had to place them so helter-skelter. (I also couldn't immediately source any 100 μF 100 V caps, so I substituted 2×47 μF in parallel.)

Come to think of it, might this explain why the caps fail in the first place? Some poor sap designed the mixer's chassis too small, forcing Behringer to use some sub-spec capacitors because they're the only ones that will fit? I mean, caps fail in SMPSs every other day, but to fail in a linear power supply, they really have to be bad!

If you do choose to replace them (yes, it will be a pain in this case), use good-quality 105°C ones of the same capacitance and same or higher voltage rating. I generally use low-ESR for pretty much everything—they're not ridiculously expensive, so I see no reason to keep "ordinary" ones in shelf, although they'd be just fine in this case. Generally low-ESR ones may be slightly bigger then ordinary ones, but I just couldn't find physically small enough caps of any type!

A word of caution

There's nothing especially difficult about these modifications, but if you do manage to break something regardless, you now have a broken mixer, congratulations! So don't attempt this modification unless you know what you are doing and are willing to take that risk, and in any case don't blame me for anything bad that happens! (You did read the bit in the beginning, about component reference designators I found on the circuit board not matching those I found on the schematic, right? Right?)


Antti J. Niskanen <uuki@iki.fi>