PAiA 9700 MIDI2CV8, 9720 VCO

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PAiA 9700 MIDI2CV8, 9720 VCO

Post by PAiA-Scott »

Hello Paolo.

The mod you describe for regulating the positive and negative dc supplies to the MIDI2CV8 parts should be OK. I am curious though, does your board have the update with the trace to pin 7 of IC13 cut, and a piece of thin wire added to link pins 7 and 8 of this IC? Sorry, but I am at the disadvantage of being at a remote location to the offices from where the kits are sent and don't have a match for the date kits started being sent with this change. This is much simpler than the addition of voltage regulators.

True, there is ripple voltage on the V+ and V- supplies but it normally does not get through to the cv being output. I think it depends upon the maker of the 4051IC at IC13 but some run hot when the supplies are more than 12v. With the pin7 disconnected from V- and tied to ground (pin 8), the pin 16 voltage can be up to 22v without a problem (and heat at the part).

About getting the VCO tracking, there are some things to check or try before going for parts changes. First, double-check to be sure there aren't any parts that have been installed in each others place (ie the 10k Symmetry Trim in place of the 1K Scale trim, or some other fixed resistor in place of another) and the soldering for the parts related to the circuit sections including the scale trims all looks good. The LFO/Norm switch must be set to Norm or VCO A won't properly scale. If pw modulation to VCOB causes any pitch change to the other waveforms from this section, a change of resistor R59 is needed (replace the 91k with a 27k). Always wait a minute or two after power has been applied before making the scale adjustments so the parts on the board have had a chance to make it to a normal operating temperature. Power-up the MIDI2CV8 while holding the lowest note on the connected MIDI controller (releasing this key signals to the MIDI2CV8 that it should output zero volts for this key). This puts the pitch cv in a desired range. It should be set for 1v changes for octave spaced key presses and the setting is most accurate when the octaves pressed for the adjustment don't include the lowest 'zero' volt key. Listen for the vco octaves for the scale adjustments for these modules with them sounding for pitch cvs produced from the adjusted MIDI2CV8 (w/low-key transpose asserted) in the first four octave range where it's easiest to pick out the octaves and unisons and beat frequencies which result as the tones are compared.

I haven't recognized any trends with the VCOs not tracking but if there is any part that might affect this, I'd say the matched pair transistors would be a spot where variation between multiple VCOs could be introduced. They are matched in hfe tests to be within 5 of each other, but the pairs from one to another could have more than this difference. If you continue to have trouble getting them tracking, let me know and I'll match some sets of pairs and mail them to you to try.
Thank you.

Sincerely, Scott Lee

Vallebona Paolo wrote:
> Hello Scott,
> thank you very much for the info you sent me
> I'll try the suggested tuning procedures
> I think I will try this mod:
> cut trace from R51/C20 to +V (the big one, on solder side of board) and insert a 12V voltage regulator 7812 with a capacitor on its output;
> cut trace from R52/C22 to -V (component side, going to the (-) pad for the V/Oct board) and insert a -12V voltage regulator 7912 with a capacitor on its output. In short, have a regulated ±12V supply for all the analog part of the board. This would get rid of the overvoltage and ripple that I saw on IC13 pin16. This would also let me use the 9700 alone or in a P9700S system (with 9700 supplying power to 9720, 9710, 9730) without any further change.
> Your opinion and/or suggestions would be welcome
> Talking about 9720 VCOs, is there any known simple mod or adjustment I can do in order to improve their tracking ? e.g. try substitute the matched transistors, or the 13700, or anything else to find the best ones, or change some values...
> Best regards
> Paolo Vallebona
> -----Messaggio originale-----
> Da: Scott Lee [] Inviato: giovedì 15 gennaio 2009 20.50
> A: Vallebona Paolo
> Oggetto: Re: Support
> Hello Paolo.
> I have attached a pdf file with details about power for the 9700K which you might find useful. It boils down to the transformer for a MIDI2CV8 in a P9700S being a bit strong for the MIDI2CV8 on its own. Now, we modify the boards so the IC13 doesn't get so much voltage and this prevents any possible trouble (open the circuit to pin 7 and link pins 7 and 8).
> Then, below, I have copied a method that should work to get the trims set on the MIDI2CV8 and the 9720VCO for the most accurate tuning.
> Here's a way to get them by ear... First set the MIDI2CV8 to be
> outputting 0V for the low key pressed on the controller, by powering
> the 9700K while holding the low key on the controller so that
> releasing the key will cause the first message input to the MIDI2CV8
> to be a note-off (it interprets this as a signal to output zero
> volts for this note number). A dvm can now be used to check that
> octave spaced key presses are outputting a 1v pitch cv change (down
> to a hundredth of a volt or so) as set by the trim on the MIDI2CV8. The most accurate setting will be obtained if you press octaves that
> start a few notes above the lowest key, ie if your low key is a C,
> press D or higher for the lowest octave reading.
> The usual tuned setting for the VCO scale trims is about a 1:00
> setting for the pointer of there disk which covers a cw range from
> about 7:00 (ccw) to 5:00 (cw). Start with the trim at 1 o'clock.
> After having been powered for a minute or two patch the low-key
> transposed pitch CV over to the VCOA P2 input. Set the two pitch
> controls to they're both in-tune at unison at about a mid-rotation
> setting for the low key pressed on the controller. Play an octave
> higher and the adjust scaleA for an octave relationship with B. Go
> back to the low note and adjust the A panel pitch control for unison
> and then, again, press an octave higher and adjust the scaleA for an
> ocative relationship with B. After going back and forth like this
> another time or two, you should find the scale is about as close as
> it can get. Then, do this all again pressing some note a few keys
> higher than the lowest key (with VCOB adjusted to match at this new
> low pitch) and confirm/tweak the scale to match for higher pressed
> octave relationships with B (using the A panel pitch to realign A
> with B after adjusting the scale). When the scaleA adjustment is
> complete, move the pitch control over to VCOB P2 and make the panel
> Pitch and scaleB trim adjustments to get unison and octaves with the
> VCOA pitch (adjusted to match B for the low key pressed and pitch CV
> sent to it).
> After the scaleB is set, move the Pitch CV over to the VCOA P1 input
> and listen to the two vcos tracking across the keyboard range (again
> with the low-key transpose asserted) after aligning their pitch
> while a higher note is pressed, say the third or fourth octave. Confirm they track for a higher octave pressed and hold as you press
> keys going down towards the low key. You should be able to turn up
> the sound of the controller or some other MIDI Sound Module and hear
> that the tones made from the VCOs can be aligned using the panel
> pitch controls and follow as you play different ranges on the
> keyboard (make the alignment between the two when a key pressed in
> the middle C, A 440 range is pressed--its easier to hear the beating
> slow as they are tuned).
> Thank you.
> Sincerely, Scott Lee
> ps It helps to keep the VCO Modulation control at minimum so there
> isn't any possible pw modulation which can make the VCOB pitch
> change slightly (often, a change of R59 from 91k to 27k prevents
> this interaction).
> Thank you and please let me know if you have any questions about any of this.
> Sincerley, Scott Lee
Scott Lee