A couple of questions about the LFOs

[marfoski]

Hallo Yves and all,
I've just finished to build my second VCLFO (type 2). Calibrated OK, all the waveforms are perfect, the notorious glitch is visible on the scope but completely transparent to the ear.
I have just a couple of small issues :
- It seems that the minimum rate on the 1.0X range cannot go beyond 6-7 sec.
- I've had to try three different 10uF tantalum caps to avoid a freezing of the oscillator on the 0.1X range, occurring on the lower half (or even 3/4) of the pot range. Even now that freezing seems to surface again on the longest rates (the oscillation reaches the top of the curve and it just sits there).
That's a bit strange because the parts I used (BC557B, caps, ICs) are from the same batch from which I've built my first LFO, that works flawlessly.
The LFO is perfectly useful as is but nonetheless someone could help me to pinpoint even these small issues? Thanks in advance.

Marco

[marfoski]

A little update: I noticed an excess of flux on the PCB and I removed it. Now the ratio between the two ranges it's OK and both are a little too fast (5 sec at minimum at 1X and 50s at 0.1X). On the wiper of T1 I read 0V.

[marfoski]

Well, FWIW I post my own solution : the BC557B must be carefully selected. In the small bunch I had, the best performer gave me a cycle of 8s/80s.
And that's OK for me now.

[iopop3]

I'm about to build my second LFO, this time the LFO#1 panel design. Can you explain a little bit more about the careful selection of the BC557?

In regards of what, hfe , Vbe or did you just change transistor until you had a long period?

[marfoski]

Well, I installed a socket and tried all the BC557B I had. In another topic Yves suggests that one may try even a BC557A to get longer times but I didn't have any of these. By all means one must avoid a BC557C.