asm problems.

[Magnus Danielson]

> 
> Hey diyers,
> 
> I am working on my ASM-1 and I thought I would just throw out the problems
> I am having so maybe somebody could give me some advice.
> 
> Both of my oscillators work wrong in the same way.  I have checked the circuit
> and I know that I am getting an expodential current source off of the MAT-02
> when I sweep the control voltage.  On the outputs I get an unstable, high
> pitched tone that I change with the control voltage knobs, but the change
> is erratic.  For example, I will sweep from -15 to +15 and the tone will go
> from medium to high, then crackle out then come in high again and go medium.
> I can't get any low tones out of the oscillator.  I have replaced several
> components (like the 3140) and it still does the same thing.  Both oscillators
> do this.
> 
> Whats interesting is that I hooked up the VCF, which works, except I get
> that same high pitched tone out of my VCF output... and I don't even have
> the oscillators mixed in anywhere (I am using an external oscillator).  I
> can tweak the ASM's VCOs control voltage and it effects the high-pitched
> output on the VCF.  I can tweak the VCO's control voltage so that it goes
> away on the VCF.
> 
> So, I beleive that this high-pitched tone is not even necissarily coming from
> the oscillator.  I scoped out a bunch of points on the VCO and I got a lot
> of garbage.
> 
> Any suggestions are welcome.

Well, I had my first ASM-1 up recently (earlier this week) and had VCO 
problems.
I had a HF signal only and it was tweaked by the touching of various components
but not (much) by the CV trimmer... so I hooked a scope to it, and then I could
see the HF at the CA3140 output, but when moving the probe to the other side of
the 15k resistor to the LM311 input I suddenly see an waveform... a second 
probe
at the CA3140 reveal the same waveform put only as long as the probe at the
LM311 input is still attached.... hmm.... so, I hooked a 1 Meg resistor in...
No sucess, and then a 30 pF cap (to even further simulate the probe) and voila!

The 18 pF cap is too small for the FETs that I use (2N4391) but an extra 30 pF
did the trick. What it actually do is to slow down the "click" which occurs
when the LM311 first pulls so that the FET empties the 2.2 nF cap but then
flips back as the voltage of the cap falls. Slowing down the flipback will 
allow
for the FET to fully empty the cap at the expense of frequency exactness at 
high
frequency (say above 20 kHz or so :).

I recommend you to verify the signals of the output of the CA3140, the LM311 as
well as the input of the LM311. An oscillioscope is really a necessity here...
This is the only tricky part of the ASM-1...

Altougth I did have a carefull look at the schematics before building it was 
not
until after first building it and debuging it that I fully learnt the curcuit.

You seem to suffer the same problem as I did, but not as drastically...

Note: The Linear FM inputs have NEGATIVE polarity... an positive voltage will
decreese the frequency... 

PS. Insert a 100k trimmer in series with the 1.5 MOhm resistor connected to the
collector of the first MAT-02 transistor (Q1). Use this trimmer to trim the
frequency of the oscillator to a A5 (880 Hz) when the base of the Q1 is at 0V.
Then will the scale and offset trimming be much more straigth forward.
An 25 turn trimmer is strongly recommended. The "unmodulated" frequency of the
oscillator is about 910 Hz, so trimming to an A5 of 880 Hz is natural.
Then use a 1 V signal to pitch up for scale trimming and then use any note as
reference for trimming the note offset.

Cheers,
Magnus