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

[Jean-Pierre Desrochers]

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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