X0XB0X a go go

We have a bit of experience with the X0XB0X, we rebuilt one of these that was bought secondhand on E-Bay and in a non-working condition. We got the unit along with a box of spare components, thousands of resistors, the original LED’s from the kit.
As the kit comes from China, the solder supplied with it is Lead free, which is OK to solder with but an absolute pig to remove with a solder sucker and iron, it always requires more Lead solder and flux to get it to reflow so components can be removed.
The X0XB0X is a great piece of design, it behaves and sounds very similar to the original, it has MIDI in and out, and stores far more patterns than the original, as well as MIDI to CV conversion built in.
The one we rebuilt was sloppily assembled, the resistors were all at sixes and sevens, the switches were unevenly inserted, and many of the connections of the ribbon cables had broken connections.
The X0XB0X is a complicated piece of electronics, while the LadyAda website does have very thorough instructions, you do need a fair bit of Electronics savvy to make sense of them, and to make it happen successfully.

We have just got a text from someone who has 2 faulty X0XB0X’s which will be arriving this week, and another client with a completely unstarted kit from Technology Transplant.
This version of the kit did generate a bit of controversy at the time of its release, due to it having no financial or production connections with the original LadyAda version.

The complete strip and rebuild of the original X0XB0X took 3 days of work, mainly due to the problems of the Lead free solder, our estimates are that a kit can be built in about 12 hours from scratch, and with normal solder, but we guess this week we will see.

Even though we are adding this bit on the July blog, just for continuity reasons, the current work was completed in August and early September.

The two faulty X0XB0X’s have been repaired (or more accurately rebuilt), apart from electronic faults such as broken pcb tracks, shorted capacitors due to poor assembly, faulty encoders, and a still unresolved fault with MIDI in on one of the units, the main issue was the 17 resistors in the DAC which converts the signals from MIDI or the onboard keyboard into a CV for the Oscillator. The original Roland TB303 manual says that they should be matched in resistance value to within 0.1% and this sounds fairly tight, and you might think a little unnecessary, until you try to play or tune a X0XB0X where all the 1% resistors in the DAC are within tolerance but every note in an octave is out of tune and any pretense at using it as a musical instrument becomes a vain quest.
We bought a hundred high quality branded metal film resistors, with a very low Tempco (50 ppm) and measured them with a 5 and a half digit  Ohmeter and grouped them according to their resistance, marking the bandolier of resistors with different colour permanent markers to indicate different bands of resistance from the ideal.
With the TB303 or X0XB0X DAC, it doesn’t matter what the resistance value is, just as long as they are closely matched to each other.
We had no problem matching groups of resistors to within 0.01%, and so far have reDAC’ed three X0X’s from the same original 100 resistors, and we have another group matched for a fourth.
The other upside of such extreme attention to matching is that the CV output becomes useable, another analogue synth attached to the CV output is definitely playable and useable and it turns the X0XB0X into a very accurate MIDI/CV converter, the only slight downside being that 0V is not achieveable, so any other synth attached will be an octave up, but that can be dealt with easily enough.
As a matter of interest, we measured the unused 200k DAC resistors from the kit, along with all the resistors removed from the pre-built kits to see if a well matched set of resistors could be achieved, and we couldn’t even get close, most of the resistors were well within 0.5% but they were half and half more than desired value and less than desired value, and no attention was paid to matching even these poor components to make the Most Significant Bits of the DAC accurate in the prebuilt X0X’s.

As part of a deal with the owner of the two faulty pre-built X0X’s, we got one to keep, more as a piece of workbench test equipment than as a synthesiser, and with its rebuilt DAC, it is a very accurate MIDI/CV converter but the big surprise to us was that the X0X can play polyphonically in keyboard play mode. We had it attached to an Oberheim Xpander and it played all six voices from the X0X keyboard, (the chord produced wasn’t the most musical in the world but it demonstrated the possible).
As the Xpander’s remaining problem was a strange one involving just one voice, we just used the CV and gate out of the X0X to trigger a single voice of the Xpander through it’s CV/Gate inputs.
The end result was good, the Xpander is fine now, and the X0X can take some of the credit for speeding up the repair. It is slightly ironic that the ultimate acid bassline clone should be primarily used as a piece of test gear, but we would like to think that LadyAda would in some way approve. It also sounds very good in its own right,
but as ex musicians, we are not going to get too much mileage out of that.