[sdiy] Working with the ASM1 VCO.. weird MAT-02 behaviour...

[Ian Fritz]

Have you checked for hi freq oscillations?

Ian


> On Jan 16, 2020, at 9:48 AM, Jean-Pierre Desrochers <jpdesroc at oricom.ca> wrote:
> 
> Hi list,
>  
> I'm working on the Elby-Design ASM-1 VCO
> https://www.elby-designs.com/webtek/asm-1/vco/vco-asm1-cct.pdf
> Im want to use it as a 1v/oct LFO (.05hz to 20hz)
> I first mounted the circuit on my protoboard with success.
> at first I used a matched pair of 2N3904 for Q1A & Q1B
> instead of the MAT02 shown in the schematic.
> I could reach very low frequencies (as low as .04hz).
> But to do so I needed to replace R9(56k) for a 33k temporarily. Fine.
> This morning I replaced both 2N3904 with a genuine MAT-02
> and started the same frequency measurements as yesterday.
> I bumped into something weird..
>  
> Here are the actual measurements I first did using 2x2N3904:
>  
> mVolt at 1k TEMPCO resistor vs Sawtooth period
> 158.0mV => 0.050sec
> 176.0mV => 0.100sec
> 193.8mV => 0.200sec
> 212.0mV => 0.400sec
> 229.8mV => 0.800sec
> 247.5mV => 1.6sec
> 265.5mV => 3.2sec
> 283.2mV => 6.4sec
> 301.7mV => 12.8sec
> 319.7mV => 25.6sec
> (18mV steps, nice exponential response!)
>  
> Now here are the actual measurements with a MAT-02:
> mVolt at 1k TEMPCO resistor vs Sawtooth period
> 157.0mV => 0.050sec
> 175.0mV => 0.100sec
> 193.0mV => 0.200sec
> 211.0mV => 0.400sec
> 229.0mV => 0.800sec
> 247.0mV => 1.6sec
> 271.0mV => 3.2sec
> *** Now trying to go further increasing the TEMPCO mVolts
> *** the oscillator starts backing up and re-increase its frequencies !!!
> *** I could not go further lowering the frequency.
>  
> I tried 3 different MAT-02's and even LM394's
> and they all behave the same.
> Using different matched pairs of 2N3904 worked perfectly..
> I suspect  MAT-02’s minimum base to emitter current reached (???)
>  
> What am-I missing here ??
> By the way, all the IC's I'm using here are genuine one's
> (TL082, MAT-02, CA3140, LM311).
>  
> JP
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