FAT Freebass / MAM MB33 Problems + Midi channel settings

[leeroylee]

hello, i had a problem with my freebass, when it started up the midi led would stay on and i woundnt get any sound, so i bought  some electronic cleaner and sprayed the midi port contacts and the midi dip switch, it now works ok again.

[benergy]

Hello every one, new member here....
love my fb383, it's one of the few items thats come around the world with me.
I was wondering if anyone can shed some light on a small issue
whenever vca is triggered, an audible 'pop' happens regardless how how low the filter is
i want to put a resistor or cap in to soften the instant attack just enough to solve this .
does anyone know a starting point, schematics seem a little thin on the ground

regards
B

[Gordonjcp]

Well, it's supposed to be a clone of the TB303, isn't it?  So I'd be trying to figure out from the board what the equivalent of the TB303's IC15 and friends is.  On the real 303 circuit, it *looks like* (I could be wrong) the envelope attack is set by how quickly Q32 can discharge C42 through R130 and R132.  This is getting into the realms of conjecture because I haven't played with building this part of the circuit yet, but it looks like Q32 is pulsed on briefly to discharge C42, which (once Q32 turns off again) slowly charges up through R123.  This gradually turns off Q31 (PNP, so the base goes positive to the same potential as the emitter), reducing the voltage on the gain control pin of the VCA chip.

[Circuitbenders]

You'd probably have more luck with the x0xb0x schematic as they are a very similar circuit but a lot easier to read than the TB303 diagrams.

On an x0xb0x i had that clicked like a bastard i stuck a 86nF cap between the junction of R131 & R133, and ground. I also shorted D35. These part numbers are from the x0xb0x schematic but you can probably relate this to the FB383 circuit with a bit of comparison. This pretty much got rid of the click but didn't remove it completely. You can try different values for the cap but it seemed that anything higher than 86nF started to effect the decay.

The Freebass really shouldn't click at all, at least not enough to be an issue.

[Gordonjcp]

I wonder if clicking is a symptom of a dodgy VCA chip, or possibly DC leaking through one of the capacitors in the signal path.

[AndyF]

Just found this thread and website via google..

Really interesting reading, luckily my FB383 Freebass is working fine. Nice to know what to look out for though.

Im actually posting because ive borrowed a MB33 mk1 (black/silver) from a mate and have been comparing it to my FB (orange/silver)

It seems the filters are different.. (or at least calibrated differently)

The FB filter does not reach as far down and doesnt open as much as the MB.. resulting in it sounding duller with no resonance set.

But the MB resonance doesnt go as high and filter envelope doesnt open as much as on the FB.
With full resonance the FB sounds much brighter and filter envelope snaps more aggressively.

Im just wondering if anyone else has noticed this?
Does anyone know if these things can be changed by tuning pots inside?
Or do they use a few different components?

Im more curious than anything

[Circuitbenders]

The later Freebass and MB33 boards are identical, in fact if you open up a later Freebass it actually has MB33 printed on the circuitboard. Some of the early Freebass' have one sided brown circuitboards rather than the green double sided boards you see on later models and MB33's. They also have much better steel sheet cases rather than the normal aluminium, along with better quality sealed pots. These earlier ones do seem to sound slightly fatter and more organic to my ears but i haven't done a direct compariison of components to find out why. They do have supposedly 50K pots instead of 47K ones, though i can't see that making much audble difference.

There are two trimmer pots behind the cutoff front panel pot on the green board models, if i recall correctly they both affect the cutoff width and range to some extent but i haven't fiddled with them for a while. They will probably be around the same area on the brown board version.

[heretic]

any schematics available for the MB33 mkI?

[Circuitbenders]

Finally got a FAT Freebass manual and made it into a PDF if anyone wants a copy.

http://www.circuitbenders.co.uk/hostedstuff/Freebass-manual.pdf

[01012k7]

Its not actually a problem with the power supplies as it turns out it doesn't actually matter if you use 12v AC or 15v DC. Once its gone through the circuitry and voltage regulators in the power supply area of the board its all the same anyway.

We get at least two or three emails every month saying exactly what you have just said and i'm afraid to say that at the moment i don't actually know how to fix it. This appears to be a very common fault with all versions of the freebass. I have had 2 freebasses and an MB33 that displayed the same symptoms i.e you turn it on, the midi LED comes on and it makes a noise like its going to do the autotune start up it normally does but the midi LED never goes off, the autotune doesn't happen and the VCO doesn't respond to any midi messages.

The problem isn't with the firmware EPROM as i swapped the OS chip with one from a working Freebass and the MB33 still didn't work. I've been through the circuit looking for dead components a few times and have come to the conclusion that its most probably something corrupted in the big Philips PCF80C31BH-3 12P microcontroller chip near the power input, but have yet to get around to trying to replace it as its a bit of a monster.

A while back someone got in touch with me saying the same thing and said that they had somehow got in contact with an original MAM engineer who claimed they could fix this problem but wanted to charge more than a secondhand freebass to do so!

If we could actually work out whats wrong with these things we could save a whole lot of machines from the scrap heap.

hi guys i am new here i just wanted to ask i have a freebass fb383 which has this problem but i have open it up and found it is a Philips PCF80C31BH-3 16p microcontroller chip  40 pin job any ideas if you can still get them ...

[Circuitbenders]

i assume you've read the rest of this thread?

Its most likely the DIP switches for the midi channel selection.

Don't go near that micro-controller unless its as a last resort. You can still get them from ebay but i've only ended up having to replace one of those on two occasions, and one of those was when someone else had tried to do it previously and had put in an incorrect replacement.

If nothing else, removing that chip without a proper heated desoldering tool will be one of the least fun things you'll ever do.   

[01012k7]

i assume you've read the rest of this thread?

Its most likely the DIP switches for the midi channel selection.

Don't go near that micro-controller unless its as a last resort. You can still get them from eBay but I've only ended up having to replace one of those on two occasions, and one of those was when someone else had tried to do it previously and had put in an incorrect replacement.

If nothing else, removing that chip without a proper heated desoldering tool will be one of the least fun things you'll ever do.   

Tbh thxs for your  help i  did read this topic and sort of read most of it but start to fall a sleep lol
i got it from ebay fs383 did not understand the problems i would be facing but when i got it was like midi port look like a cable had been pulled out the wrong way it had damage the pins so took the midi socket out as i could see some track had come away so i decided to get my angle grinder to it ( only joking) i got some fine wet and dry rubbed the track to show the copper then i bridged the track to the midi connection then i came here .. i order a new dip switch as one was not making a connection also was like loose when you switch it .. but i did see one of the track was lifted so done a temp job till i have time which will be over the weekend ..it sort of works but i need to do a better job also want to do some mods my self the leds and the power switch will be first as keep having pull the plug out is a pain

[01012k7]

Hi thinking of options as it work for like 10 mins and its given up again so think of things i can do with it

I have order a Meeblip micro kit which should be here in a week or 2  so my ideas which i am not shure will work is maybe link the meeblip micro up with the freebass filter ( not shure if this will work) too is just user the rack for the mee blip micro and then sell the bits on or may keep the bits just in case thing change on repairing it

parts that have been change on the free 383 so far

dip switch
cyn-17
power cap 470uf to 470 uf 25 volts at a temp job
Done some track work also repaird the midi ssocket  but may order a new one ..i have per order both  5 v and 12 v power reg think it if it done't work then go with a option here

[01012k7]

Its not actually a problem with the power supplies as it turns out it doesn't actually matter if you use 12v AC or 15v DC. Once its gone through the circuitry and voltage regulators in the power supply area of the board its all the same anyway.

We get at least two or three emails every month saying exactly what you have just said and i'm afraid to say that at the moment i don't actually know how to fix it. This appears to be a very common fault with all versions of the freebass. I have had 2 freebasses and an MB33 that displayed the same symptoms i.e you turn it on, the midi LED comes on and it makes a noise like its going to do the autotune start up it normally does but the midi LED never goes off, the autotune doesn't happen and the VCO doesn't respond to any midi messages.

The problem isn't with the firmware EPROM as i swapped the OS chip with one from a working Freebass and the MB33 still didn't work. I've been through the circuit looking for dead components a few times and have come to the conclusion that its most probably something corrupted in the big Philips PCF80C31BH-3 12P microcontroller chip near the power input, but have yet to get around to trying to replace it as its a bit of a monster.

A while back someone got in touch with me saying the same thing and said that they had somehow got in contact with an original MAM engineer who claimed they could fix this problem but wanted to charge more than a secondhand freebass to do so!

If we could actually work out whats wrong with these things we could save a whole lot of machines from the scrap heap.

hi is the Philips PCF80C31BH-2 12P  the same i look at the data sheets same pin out also i have a PCF80C31BH-3 16P just looking at options at the moment ...

[Circuitbenders]

If you have one already and you've decided you're going to attempt to remove the old one, just give it a go, its not going to hurt the freebass, unless you make a complete mess of removing it.

I've just got a load of PCB80C31BH-3-16P's from somewhere in the US, and they certainly work, so i don't see why a PCF80C31BH-3-16P wouldn't. Its seems to be the 80C31 part thats the important bit. The  12P / 16P bit seems to indicate the maximum mhz speed, but i'd imagine that higher than the original would be ok, but lower wouldn't.