FAT Freebass / MAM MB33 Problems + Midi channel settings

[spk]

crusty paul u have the PM!

[DOXXOD]

Hello chaps and chapesses, i'm a forum newbie here. Just got hold of a FAT Freebass off a colleage, and after replacing a few doddgy pots am ready to do some bending. Already had a poke around with my wet finger tips n found some ineresting points, one that seems to by pass the decay n turn the tones almost SIDchip style. Will be posting some details as soon as i have some pots n switches hooked up. Any one got any pointers of what area to start looking for the FM mod or the VCO/VCF input switch mods?

Any help would be much appreciated, i dont want to blow this little beast!

Thanks in advance for any replys

Matt.

[Circuitbenders]

bugger, sorry people, ii completely forgot about this, everything has been a bit hectic over the past couple of weeks.

I've just shipped off the only MB33 i had already modded so i can't put up any photos but i'll dig out my notes asap and see what i can do.

From memory, the switch that cuts the VCO from the input when you plug a jack into the VCF input is the middle switching connector on the VCF input jack. If you cut the switching lever off you can connect a switch across the gap to turn the VCO on and off. I usually use a DPDT switch wired so that when its in the ON position the connection is made across the gap but when its OFF the switching lever side is connected to ground.

[Circuitbenders]

Already had a poke around with my wet finger tips n found some ineresting points,

Not something i would advise on a Freebass. You may have one of the versions that isn't prone to dying or you may just be very lucky, but thats asking for it.

[bliprock]

  yeha thanks circuit benders for the only place i can find my mb33 dipswitch settings. i copied it and went straight to my mb33 and found that it seems upside down. No drama cane farmer though... just read it upside down........i wonder if they put the dip switch in upside down?? or is it just different on the mb33 mk2..anyways keep up the good work and thanks again....

[superset]

Hello all... this is my first post but hopefully not my last.

Like most of you I have a non-functional FB383.  I bought it off of ebay for $10 and figured I'd be able at least, to utilize the filter section and maybe wire up the oscillators for external control.

I've documented my work so far which includes  three mods your all familiar with... the Resonance Boost mod, the filter input switch mod, and a Filter trigger mod which triggers the filter section allowing it to be used with external signals.

But what I think will be most helpful to all is a detailed circuit diagram which outlines signal flow through all components of the FB383 and MB33.  I've also detailed ground, the supply rail, the 5VDC lines and jumpers throughout.  Take note that the resistors with one black line are zero ohm resistors and are used liberally as jumpers.  You'll also see overlays on most of the ICs so you can quickly identify the multiplexers, op amps, D/A converters, etc.

What I'm working on now is a schematic of the oscillator section for you all to look at, f with, and bend to your delight.  So far I'm having luck using pin 10 of the TLC2741N quad op amp as a CV input but the circuit begins to oscillate out of control... which some of  you may like, but a bit more stability would be useful in revamping these machines.  I'll post more as I have something functional.

Anyway... you can find all the pics and documentation to the mods on the Flickr Collection here:

http://www.flickr.com/photos/97633426@N00/collections/72157606341720299/

and the high res circuit diagram is here:

http://www.supersetsounds.com/images/FB383/freebass_signal_flow_final.jpg

[Circuitbenders]

I'd completely missed this post! Did you ever get any further with this? I could still really do with a schematic for the freebass / MB33.

Nice job on the filter input trigger pics, at least some Freebasses will probably be salvaged to some extent with that information. Good job on the board layout as well, although It should probably be noted that later models have a far better quality double sided green board

[gimedici]

Hi, I have the same problem with my MB33, midi light stays always on and nothing happen. So I bought a second one to try to repair the first; but the second after some tests died as the first.
But I found that during self tuning, sawtooth wave starts at 550Hz roughly and frequency is lowered to 17-19 Hz, then midi light goes off and MB33 is working. In the same time there is a variable tension at pins 1,2, 14,15 of IC 4053. This voltage starts with 5V roughly at power up, and decays to 0.2-0.3 volts during self tuning; when reaches this value midi light goes off and MB33 is working.
But i was not able to understand how this tension controls the 555 oscillator frequency, and where it comes from.
After some tests (voltage measuring, wave finding and measuring) the second MB 33 died, so now I'm desperate.
Could anyone help me please?

[Circuitbenders]

Bizarrely i've rescued three Freebass / MB33's simply buy replacing the DIP switches for the midi channel selection so you might want to try that first.

I'm starting to think there might be various causes for these faults. Stangely i've had two Frebasses sent to me that the owners claimed had exactly the fault you described, but when they arrived i couldn't find anything wrong with them. This would suggest to me that its either something mechanical such as the DIP switches or a dry joint which has been jogged back into place in transit, or its something to do with the power supplies people are using. I'm thinking the first option might be more likely.

[Gordonjcp]

or its something to do with the power supplies people are using.

Now it's funny you should say that.  Are the power supplies the original ones?  Did they come with switch-mode power supplies or linear ones?

I had a lot of problems a couple of years ago with little Alvarion wireless bridges that would keep rebooting every ten seconds.  You'd power them up, and it would do the boot countdown on the status LEDs, then start all over again.  Turned out that the switch-mode power supplies they were supplied with partially failed after a while, and sagged badly when the radio was turned on - the last thing the unit did after booting!  This would trigger a reset.

If the PSU isn't up to the job of running the Freebass then it might have all sorts of weird problems, like never coming out of reset.  Have you 'scoped the supply rails?

[Circuitbenders]

All the PSU's i've ever got with freebasses or MB33's are linear, either 12V AC/AC or more rarely 15V AC/DC

[gimedici]

My PSU are linear too, if you mean that they produce a sinus wave 50Hz at deisred voltage (12V).

Now it's happening something new, but random (or I didn't understand why); sometimes the MB33 2 goes out from the standby condition and CPU and MIDI works because you can play and hear it, but oscillator is out of tune and doesn't articulate the note (only one note). It's not CV sensitive.
This happens when I go testing around the LM723 and the two suplly transistors (BD137 for +12v & BD138 for -12V). I don't see anithing wrong scoping and testing anyway.

The most times  nothing happens and the MB33 remains with midi light on; but i noted that the 555 oscillator doesn't emit the sawtooth wave, that usually emits; better,  it emits the sawtooth in the first seconds after powerup, then tries to tune (lowering pitch), and then dies.

I thint that something goes wrong whith CV, but I can't demonstrate.

Is there someone in internet or in the world could help us?
I hope that some MAM engineer tells us something........Please!

[gimedici]

I FOUND AND SOLVED THE PROBLEM

I'm tallking about MAM MB 33 2 (which transmits potentiometer movements as control change MIDI messages)

The problem was a faulty 4051 mux/demux.
In the board we have two of these ICs, one to multiplex the 6 signals coming from potentiometers (env pwm, cutoff, reso, env mod, accent, decay, distortion) before converting it in digital (thru the ADC0804) and sending it to the micorcontroller (8051), the other to demultiplex 8 signals coming out form the 8051 and translated in analog thru the TLC7524 digital to analog converters. These 8 signals are the previous 6 plus two new used to drive the pitch CV.

During statrtup the 8051 lowers the pitch loweing these two voltages, and monitors the pitch through the square wave produced by the LM393.  (this square wave is obtained from the 555 oscillator sawtooth wave compared with a reference voltage in the LM 393 that is a dual voltage comparator; one side for our ears the other for the microprocessor pin 2)

When the 4051 demultiplexer (near MIDI connectors side) is broken, the CPU can not perform the tuning at powerup, so stays forever with MIDI light on doing nothing.

So, do replace the 4051 (both is better), and your MAM MB 33 will work again (in the most cases)

[Gordonjcp]

The problem was a faulty 4051 mux/demux.

Quelle surprise.  These are the first things I go for in an analogue synth, those and TL072s.  They are in one of two states - failed, or just about to fail.

Well done on getting it running!

[chamelerus]

hi. i have no idea about electronics or circuit bending, i just came here searching for info about the mb33.

i own a mk2 and a mk1, both run with 12V AC, although on the back of the mk1 it says 15V DC. (i got it with the AC)

now the mk1 isn't working correctly anymore and i really thought i can never operate it again. it shows the same symptoms as described above and coincidentally i found out that if i switch one of the midi-jumpers and re-switch it again, the mb33 starts to work again. and here comes the curious: as soon as i touch the midi-cable or the midi-plug, the sound stops. when i do the midi-jumper-trick, it works again until the midi-cable is moved a bit..

what do you think? is it just a defective contact that can be repaired easily?