Friday, June 06, 2008

Synth tech tests VC 730 proto type

Paul posted this extensive 'report' of the test and it's functionality.

I have finished my initial testing of the production-ready (almost, see below) pc board set for the MOTM-730 VC Pulse Divider.

The board set consists of:

a) vertical-mounted jack board (uses new Switchcraft RoHS vertical jacks)
This connects to CPU board via 34-pin ribbon cable

b) pot board, uses 2 of the Frac-style pots/knobs, connects to CPU via ribbon cable

c) switch board, has 4 NKK pcb mounted toggles, connects to CPU via header pins

d) CPU card, a 4-layer pcb ~ 3.5 " x 3.5", mixture of SMT and thru-hole.
Contain s 2 PIC processors, 1 drives the 3-digit 0.56" LED display, the other does all of the physical counting/dividing.

e) LED display card, contains the 3-digit display, connects to CPU via header.

The CPU requires the 6-pin MOTM-950-type power connection, as +5V at ~ 65ma is required in addition to the +-15V (at ~ 10ma). I will not offer stuffed/tested CPU cards until after I have sold 100 modules :)

I have spent ~ 20 hours checking this thing out over and over. I need to add 3 10K pullup resistors on pins of the PIC controller, that were on the prototype but someone (me? [cough]) had crossed them out and wrote 'Not needed' but guess what? NEEDED! The part has open-drain pins and needs a physical pullup.

Other than that, it seems to work flawlessly. Input frequency can go up to around 15Khz before 'glitching', although the primary application is firing off multiple EGs when driven by LFOs or low audio range (say 100Hz CLK IN). The outputs are 0 to +5V square waves (or as close as we can get to square, depends on the math) so they can be directly connected to EGs as a GATE input. The outputs are driven by a 74HC244 HCMOS buffer and a 100ohm series resistor.

The '730 has 11 fixed divider outputs and 1 variable divider output. There is toggle switch to select a '1/2 divide', meaning all of the divisors are scaled by 2. Yes, this means *fractional division*, like dividing by 1.5, 2.5, 3.5, etc. When the toggle switch is in the 'HALF' position, *all* divisors are affected.

The fixed divisor ratios are:
2, 3, 4, 5, 6, 7, 8, 9, 11, 12 and 16 in 'FULL'

The variable (in 'FULL') goes from 2 to 32. There is an 'INITIAL DIVIDE' pot, and a CV input scaled for -5V to +5V full range, and a CV MOD attenuator.

There is an input jack called RUN/STOP and when pulled *low* (ie shorted to
ground) the unit resets. Can be used as a sync, but this input is *level sensitive*, not edge.

Lastly, there is a "fun" output called STEPPED CV. This is a moderately accurate (1%) 3-bit DAC connected to a 3-bit up-counter. This will generate a positive-going stepped 'sawtooth' waveform. The step size can be 1 of 3 switch-selected options: 4th, 5ths or Octave.
This output is *untrimmed*, but good enough for messing around. You can always use an input attenuator to tweak in closer. The clock the counter uses is switch selectable between the input clock or the divide-by-N output.

The 4th switch selects the positive or negative-edge to trigger the divider.

I already have production quantities of the display board, and will order
the switch, pot and jack production boards in ~ 1 week. This weekend I will add the 3
missing pullups and run the unit 72 hours straight at full speed to see if there are other issues I missed. I will then order production CPU boards.

I am making 100 modules up, in 2 batches of 50. If these sell out, I will make more.
I can only afford parts for the first 100 right now (I just spent $4200 on more parts yesterday and $1350 today).

I still need to design the front panel, and get a few prototypes made to be sure I did not screw up mounting holes, the LED bezel cutout, etc. It is a *tight* fit, as 3 of the boards stack up and they have to be perfectly aligned for the header pins to mate.

The cost of this module will be $399 and I am looking at a Oct 1st date for
the first 50. These will be *in stock, packed in boxes* the day they are announced.

OK, I'm pooped. Back to kitting and shipping this weekend. The MOTM-410 and MOTM-120 kits are all done. Moving on to MOTM-420 and MOTM-440 this weekend. I also have the pc boards in to make ALL of the MOTM-910 mults.

Paul S.


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