AN749?

[Martin Czech]

> Hi transistor gurus.
> 
> My Doepfer PMS VCOs use a transistor marked AN749 to discharge the timing cap. 
> This works fine, but if I try any other transistor the sawthooth wave vill not 
> be linear at medium to lower frequencies. Increasing the base resistor (to 1M 
> from 680k) improves linearity somewhat, but not enough. Even the AN749 gives 
> reduced linearity at very low frequencies, but it is still much better. 
> Anyone know what's so special about the AN749? I have a couple of VCO boards 
> that are unpopulated, so I need the components, but the AN749 is nowhere to be 
> found. Well, at least Farnell and a couple of others don't stock it.
> 
I'm no guru. 

This looks like a leakage problem. AN... a germanium tranny or a kind
of scr ?  If it is the normal type of sawtooth vco with comparator, any
Si small signal transistor should do. Formant uses BC107 (from my head,
check !), but : leakage varies very much, you have too measure it. For
this purpose you could use a low leakage integrator (polystyrene cap,
CA3140, tranny socket), load cap resistor to some voltage (expected
voltage in application) and then switch resistor off. (check first
without tranny, to see the cap and buffer leakage), and see how long it
takes for the tranny leakage to discharge the cap (obviously a grounded cap
integrator, do not use a integrator with cap in feedback path, this
won't work). This way you can measure very small currents with cheap
equipment.

m.c.

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