Good morning, Anyone here more familiar with LC based VCO clock circuits ? I've finally got this Roland SDE2000 unit up and running after numerous issues and now the only thing that is left is the VCO calibration for the sampling clock. Adjustment is via tuning L1 however turning L1 clockwise at almost one end stop of the coil I get too quick a pulse and the remaining 85% of the coil there is no clock at all. No adjustment range. I was of the understanding that clockwise was increasing the inductance which should reduce the resonant freq but the clock speeds up. I'm kinda assuming that the coil is OK as it does have some adjustment but only a very small amount right at one end and certainly nothing in the middle ? The transistors, diodes, KV1 and tants all seem ok. A replacement coil is proving impossible to source (to test) as its a 7.5mm mini sized one. I'm not sure if it is the coil or something else. Is it possible that either any of the DC blocking caps could be causing this coil adjustment issue. or is there a way to reduce the resonant frequency clockspeed by changing/adding either capacitance or inductance somewhere else in the circuit? Essentially the unit is overclocking, so it has an extra 100ms (ish) of delay sampling time (yay) but it isnt happy as it drops out and distorts when in this region (not yay). Any thoughts from someone would be greatly appreciated.
Problematic LC VCO adjustment
- Thread starter studiog
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Mmm thanks. Interestingly out of all the components i checked off board that C7 tant was reading 1uf but off the scale ESR so i replaced that one hoping that was the issue…but same problem. I didnt think about changing its value though so may try that
I cannot thank you enough for taking the time to look and reply to me KhRon. You spotted what I had already dismissed and you were spot on. Ive just tried wiring a 4.7uf tant in parallel on the backside of the C7 and hey presto !!! Coil has full adjustment now. Clock can bet set as per calibration. I thank you again for sharing your wisdom. All i need to do now is fabricate a fittable led barograph input display then put it a back together and listen. Best of regards.
Oh, great to hear, and glad i could help! Wasn't a whole lot more than a half-educated guess though 
I only noticed that polarized capacitor there, and assumed it was an aluminium electrolytic, which have a limited lifetime. And since the device itself is no spring chicken, it was a safe-ish assumption that it may well have lost some capacitance, and thus boggled up the working of that oscillator.
I didn't consider altering the value of that cap though; i only thought of increasing or decreasing the value of that 50pF cap that's in parallel with that inductor, to shift the range towards the "right" one, but great that the solution was simpler than that
And i don't really trust tantalums all that much, as a matter of course...
I only noticed that polarized capacitor there, and assumed it was an aluminium electrolytic, which have a limited lifetime. And since the device itself is no spring chicken, it was a safe-ish assumption that it may well have lost some capacitance, and thus boggled up the working of that oscillator.
I didn't consider altering the value of that cap though; i only thought of increasing or decreasing the value of that 50pF cap that's in parallel with that inductor, to shift the range towards the "right" one, but great that the solution was simpler than that
And i don't really trust tantalums all that much, as a matter of course...
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Oh, great to hear, and glad i could help! Wasn't a whole lot more than a half-educated guess though
Oh, great to hear, and glad i could help! Wasn't a whole lot more than a half-educated guess though
Now quit typing and go make music with that thing! 
