Odyssey, CV at last!
\o/ \o/ Got the control voltage issue sorted out! \o/ \o/
Just to recap things from my earlier post, when triggering notes from the keyboard the CV output at pin 7 of opamp A1 was not driven higher than 4,5V. After swapping Q8 this was corrected (A1 pin 7 going to approx. 13V), but when measured from the junction of R26 and Q8 gate (keyboard CV out, interconnect pin 8) the voltage was still not driven past 5V. So, when playing the highest note on the keyboard (7V), there was a voltage drop of about 8 volts over R26.. Definitely some current leaking, but where?!
All this time, I had had the interconnection board removed/disconnected so there was no load on the CV out pin. Still, when measured against ground this junction showed a resistance of 1kOhms; a thing, that should’ve pointed me direction of what to check next. Unfortunately I just wasn’t thinking far enough at this point.
What eventually pointed the way, was me holding board A on its place from the pitch bend knob whilst measuring other points from the CV circuitry. Whilst doing so, I managed to accidentally move the pitch bend slightly and this in turn caused a voltage change on the CV output when a key from the upper end of the keyboard was held down. Definitely, the pitch bend control shouldn’t affect pitch CV at this point of the circuit, so there had to be some kind of a short-circuit somewhere between CV and PB outputs..
Indeed, looking at the schematics (see pic above), the output of the pitch bend circuitry has a 1k pulldown resistor. A quick measure check between pins 8 and 9 on the interconnect connector and (surprise surprise) it reads clean zero ohms. D’OH!! What had happened was that at some point after swapping Q8 and cleaning away some of the old solder, I had accidentally wiped a trace of old solder (as thin as it could probably be) that short-circuited the Keyboard CV and Pitch Bend traces near R15. What a simple fuckup, but it sure had me stuck for quite some time. Sure goes to show how “blind” you can get when looking at a single board for hours on end.
For a while I was also stuck with neither one of the CV outputs not doing anything, but this was just me plugging the portamento cable connector to J3 the wrong way around while putting the board back in to the case at some point. A quick resistance measurement from pin 1 of A1 against ground showed zero ohms.. double-D’OH! :)
And _finally_.. Not much else to it than perform calibration for the last time and the Odyssey is ready to rock! The headphone jacks I chose as replacement parts didn’t have a metal case, so they needed a separate ground wire. I took this from the power connector on board A (see picture below, it’s the single brown wire). Having separate audio outputs for low/high levels seemed silly, so I routed the both signals to the low level jack through a switch. This way, you just flick it for the desired output level, no need to swap cables! While at it, I also preinstalled a mode selection switch for the dual VCF.
The 3,5mm jack is just a alternate and temporary output, no way I’m gonna leave it like in the picture above! This just allows me to use both jack sizes without adapters, easier for certain testing setups. On to the dual VCF board then, reckon it’ll have to wait a while or three while I test drive the synth in its stock configuration.
HAWT REPAIR TIP: Stuck with a fault that seems evade your repair attempts no matter what? Sleep on it or even give it several days off!