[sdiy] VCO that goes SHARP in highest octaves??

Phil Macphail phil.macphail at liivatera.com
Sat Jun 18 14:14:48 CEST 2022


I think this is the most likely cause suggested so far. If the expo transistor is driving an LM13700 then there is around 10 volts across one transistor, and 0.7 across the other. SMT transistor pairs are usually not monolithic so differential heating is very likely. Placing a resistor in series from the transistor to OTA will significantly reduce the thermal mis-match,


Phil

Sent from my iPhone

> On 17. Jun 2022, at 21:11, Ian Fritz via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Something else you might try: reduce the values of the integrating cap and the reference current by a factor of two or so. This could possibly reveal any self-heating in the expo transistors.
> 
> Ian
> 
>> On Jun 17, 2022, at 11:41 AM, Neil Harper via Synth-diy <synth-diy at synth-diy.org> wrote:
>> 
>> 
>> 
>>>> On 6/16/22 02:48, Maarten Halmans wrote:
>>> Hello Neil,
>>> 
>>> Are you sure the problem is in the higher octaves, didn't you compensate
>>> for a problem on the low end resulting in the oscillator going sharp in
>>> the higher octaves? I had this twice with a Jupiter-8 oscillator, once
>>> it was caused by flux and the other time by a leaky FET.
>>> 
>>> Best,
>>> 
>>> Maarten
>> 
>> 
>> Hi maarten, I am doing nothing to compensate for the low end. the
>> oscillator design just has a scale adjust and a high-frequency trim
>> adjustment.
>> 
>> interesting about the flux though.. my thru-hole version was
>> water-soluble flux that I cleaned off manually. the SMT board was done
>> by the assembly place using a no-clean flux.
>> 
>> --
>> /// Neil Harper
>> /// Every Wave is New Until it Breaks
>> 
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> 
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