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FAT Freebass / MAM MB33 Problems + Midi channel settings

Started by Circuitbenders, January 23, 2007, 07:39:02 PM

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Gordonjcp

#45
March 17, 2009, 02:22:41 AM
It's probably worth cleaning up and resoldering the pins on the MIDI socket.  I cured the intermittent MIDI problems on my mate's Sequential Sixtrak by doing that.  There was no visible dry joint, but somehow the pins were oxidised and not sticking to the solder.
If at first you don't succeed, stick it through a fuzzbox.

Circuitbenders

#46
March 17, 2009, 05:18:39 PM
Same goes for the midi channel DIP switches. As i say, i've fixed problems on a couple of these machines just by replacing the DIP switches.

You might get away just with using some contact cleaner on them but otherwise just replace or resolder the switches, if resoldering the midi plug doesn't work.
i am not paid to listen to this drivel, you are a terminal fool

chamelerus

#47
March 21, 2009, 09:59:57 AM
thx very much guys! i opened the mb33 and saw that 3 pins of the midi connector were 'lose'. it seems it got a punch sometime.. dunno. so i went to the store, got myself a cheap soldering iron and did the job. it's fully functioning again! :-)

Gordonjcp

#48
March 21, 2009, 06:46:25 PM
Quote from: chamelerus on March 21, 2009, 09:59:57 AM
it's fully functioning again! :-)

Very satisfying feeling, isn't it?
If at first you don't succeed, stick it through a fuzzbox.

DOXXOD

#49
March 31, 2009, 09:41:43 PM
Any one got any details on the FM mod? still not worked out how to do it on mine, i'm pretty luck ive still not fried it ( or myself!)

dirtycircuits

#50
April 05, 2009, 01:58:55 PM
Hi folks, I've been modding my fb-383 for the last few days. I'm lucky enough to have a fully functional machine, with 12v ac/ac adapter.

Firstly, thanks for sharing the midi switch key. It's saved me alot of hassle.

The filter - when I plug in an external audio source (drum machine) , nothing happens until I hit play on cubase and the 383 is playing a sequence. Then the sound from my drum machine can then be heard with some envelope.
Is this normal? Would it be possible to feed my audio source straight into the filter, bypassing the envelope section?

I'm also thinking about adding modular outputs for cv/gate and also the accent + porta, so that I can midi-fy an mc-202 on the cheap. Has anyone tried this yet? Are there any issues I should be aware of before try this?

As usual, thanks for advice and help . . . . .  ;)

Circuitbenders

#51
April 17, 2009, 02:14:41 PM Last Edit: February 23, 2013, 05:56:29 PM by Circuitbenders
The filter never stays open on an Freebass. You have to trigger it with a midi note and you will get the envelope on it every time.

As i said further up this thread, a while back i had a completely destroyed freebass that i rebuilt into a filter rack. On that one i managed to find the filter trigger input and feed it a constant 5v so that it stayed permanently open and you could control the cutoff with the knobs. Unfortunately i lost my notes on how to do it and i've just managed to kill another freebass in an attempt to redo that mod.

On this occassion the probelm is definately with the firmware EPROM as i stuck in another one from a working machine and it works fine. I've taken a dump from the working firmware chip which is a 27C256 but i'm having a bit of difficultly burning it successfully to a new EPROM. Is anyone able to check out the dump i've got, or even better get a dump of the EPROM in their machine.

My OS dump is at:
http://www.mediafire.com/?u4kkx9659e9tk6r

UPDATE:

Problem solved. That OS dump works fine, i just had my programmer set up wrong. I've erased and reprogrammed the EPROM on the non-working freebass and now it works fine, so theres something else to look at if you machine is misbehaving.

On another note. From messing around with the filter triggering it appears that the midi LED on the front panel doesn't actually represent midi activity at all. In fact it appears to represent the envelope opening and closing. If you trigger the filter/envelope without using midi the LED comes on while the envelope is active and then goes off when its completely closed again. It takes about 5 seconds to completely decay.
i am not paid to listen to this drivel, you are a terminal fool

Gordonjcp

#52
April 17, 2009, 06:46:30 PM
It shouldn't be hard to figure out what's going on inside the Freebass.  I doubt if it would even take long to draw out the complete circuit.  Most of it will be much the same as the TB-303 circuit anyway...
If at first you don't succeed, stick it through a fuzzbox.

Circuitbenders

#53
April 19, 2009, 02:49:31 AM
Parts of it certainly seem very similar indeed. I think the main differences is the cheaper more easily obtainable components such as the 2N3904 and 2N3906's instead of the 2SA733P and 2SC536F's you find in the TB303 and the x0xb0x. I'm wondering if it would be worthwhile swapping out any transisters in the VCO and filter with original TB components and seeing if it sounded any more 303-like. The filter on the Freebass certainly isn't all that it should be.

I just had another look at exactly whats in there and there appear to be a whole load of original 303 parts including a   2SK30, a 2SC2291 and two  2SC1583's which are all supposedly rare parts according to the x0xb0x BOM.
i am not paid to listen to this drivel, you are a terminal fool

Gordonjcp

#54
April 19, 2009, 04:28:33 PM
IIRC those are listed as equivalents.  I doubt that swapping the trannies for the "proper" ones would make much of a difference - the tolerances are huge anyway, so the difference between two different units is probably greater.
If at first you don't succeed, stick it through a fuzzbox.

Circuitbenders

#55
July 17, 2009, 03:00:13 AM
well, i finally tried swapping some transistors in the VCO and it does make a fairly big difference to the sound of the squarewave.

You can make the squarewave less 'hollow' sounding and possibly a little more 303 like by swapping the 2SK30AGR in the VCO for a 2SK30AY as found in a real 303 VCO but to be honest i prefer the sound of the original.  :-\
i am not paid to listen to this drivel, you are a terminal fool

Circuitbenders

#56
March 11, 2010, 02:49:15 PM
Resurrecting the longest Freebass / MB33 thread on the net once again  ::)

Seeing as schematics for any MAM device are stupidly rare i thought i'd upload the set i have for the MB33 mkII that i found the other day. Take a look here to download them:

http://www.mediafire.com/?fomzjmzqnyq

Now who wants to post the mkI schematics, they must be out there somewhere.
i am not paid to listen to this drivel, you are a terminal fool

omarko

#57
March 17, 2010, 02:26:15 AM
Hi All, interesting topic - my fb383 midi's stopped working :( im going to try the tips about resoldering the joints (esp around the midi connector), its strange! The midi light wont indicate a midi signal is present.

Anyone worked out the FM Mods for it yet?

Circuitbenders

#58
March 17, 2010, 01:37:13 AM
If it starts up fine and does the autotune correctly, what i'd try is:

1: Resolder the midi socket
2: Clean or replace the Midi channel selection DIP switches.
3: Test that the voltage regulators are working properly
4: Burn the firmware onto a new EPROM and try replacing the one in the Freebass.

i am not paid to listen to this drivel, you are a terminal fool

omarko

#59
May 19, 2010, 09:37:21 AM
Hi All, Ive managed to fix my fb383  :D

I think the problem was due to a dodgy dip switch.

Thanks for the help, much apreciated.

I was thinking of adding a filter trigger pot to the unit - has any body any idea on what value (ohms) the pot should be?
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