Behringer GMX212 sevice & mods !

A month ago I bought this Behringer GMX 212 electric guitar combo amplifier used. It was really cheap (around 120Euros) for a 2 X 12" combo with Jensen/Bugera speakers. Also, I thought I could use it as a cab anyway even if its tone was not that good. At first glance, the amp was in a good condition and everything worked. Some models had an issue with the digital FX unit but mine was good. There's actually a way to bypass the on-board digital effects should you encounter this issue:  http://davidegironi.blogspot.gr/2012/01/behringer-v-tone-gmx212-repair-bypass.html#.Vxcs2lSUPIk

After testing the amp a little bit, I opened it up and noticed there was a lot of carbon build up inside the amp. This was something I 've never seen before. It wasn't like some burnt component had created all this carbon dust. Then I realized that this amp was stored in a basement near the boiler room for many years... So I cleaned all the boards and inspected the amp for bad components. All is good! It was time for some mods!

The amp sounds really good in its stock form as the bugera/Jensen-copy speakers are loud with plenty on punch and round mids. The digital effects are not all so convincing but there are a few very tasty stereo effects such delays and reverbs. The pitch shifters are not that good. There's also a family of "mini combo amp" effects that simulate the sound of a small amplifier which is cool for soloing as the mids of these effects really cut through the mix. The poweramp is a 2X60W "gainclone" unit using to LM-family chip-amps. That's really good as you don't have to deal with ultra cheap class B or Class D custom circuits that don't make sense in some modelling amps. The modelling preamp is all-analog (the digital fx unit is placed near the FX loop and it's not producing the clean and OD tones). The preamp is basically a Tech-21 sansamp copy. Both channels are identical so you can choose between clean/OD, clean/clean and OD/OD channels using the provided footswitch. That's really cool as you may want to have a couple of OD tones rather that clean ones for your setlist.
The amp has no built in noise gate though, which is something I would like it to have. I am using a Behringer NR300 noise gate pedal instead which works really well for single coil and noisy pickups. Also, the input stage buffer has a weird design that sucks pickup output and treble. Therefore I modified it to provide a bit of extra gain and punch.

The schematic of the gmx212 combo (and the gmx1200H head) can be sourced here:
http://elektrotanya.com/behringer_gmx1200h_gmx212_sch.pdf/download.html

Modification:

The easiest mod one can perform on this amplifier is for the input stage. There's a FET buffer/booster on the input jack separate board that can be easily modified. To remove the input jack board, simply undo the single bolt that holds the metal shield in place and then remove the input jack nut. Disconnect the cable going to the main board and you're ready.
The input buffer is designed in a weird way, more like a DI box rather than a buffer. The output impedance of this circuit is really low, leading to some treble roll-off and pickup output reduction which makes the clean channel a bit life-less and the OD channels muddy. To add some brightness and gain to your tone (the clean channel remains clean), you can simply re-arrange a few things on this little input jack board. Here's the original schematic for this board:

Here's the Modified schematic of this board:

So, all you have to do is to lift the C53 capacitor and connect its leg to the 3rd pin of the ribbon cable (or simply, the ground) instead of the 2nd pin. The 2nd pin of the cable can be connected to a 22nF capacitor and the other leg of this capacitor will be connected the the drain pin of the J112 Fet. It's done! You now have an input stage with a higher gain factor and more treble. A simpler mod would be to remove the C53 capacitor completely while connecting the 22nF cap as stated above (this reduces the treble boost effect for less feedback/hiss noise).

Cheers,
Thanos