[sdiy] pining for old oscilloscope (weird LFO's)

[KA4HJH]

>Today I'm really needing to look at LFO waveforms on my scope and this 60
>MHz doesn't want to do it. I mean it CAN do it - it just seems liek the
>phosphors are way to fast to be of much use. At around 8 Hz I can see a
>square wave and sine wave shape quite plainly albeit faintly. I think my old
>Heathkit would have been FAB for this.

This is why  I never throw a scope away.

>I'm looking at various stages of the LFO waveform of my DOD R-870
>Flanger/Doubler. Tweaking this unit is becoming problematic because the LFO
>seems to be a sinewave ultimately derived by a squarewave oscillator which
>shapes it with a big cap and a resistor (integrator).

It's not a bug, it's a feature. Someday people will spend hours programming
a DSP to sound just like it.

>The problem is that trying to use this as a normal flanger is very
>unsatisfactory because it doesn't make long sweeps. It just doesn't sound
>"right". And this being a rack unit I have a ton of parametrs to tweak and
>none of them sound right when trying to gert a flanger sound. However,
>chorus and vibrato are superb. But I've been having bad luck getting the LFO
>speed to go faster. At first I had a problem with the LFO ticking, but I
>matched some resistor pairs for a couple of voltage dividers related to this
>circuit (and the regen) and that fixed several problems.

Seriously, you need a decent LFO. Build one with a simple expo convertor on
the output for expo flanger sweeps as a bonus feature.

Another thing you could try is a fixed slew rate LFO for the flanger. This
is an idea I mentioned here a long time ago but I haven't seen it done yet.
Instead of rate/depth controls, you have a slew rate and a depth control.
When you vary the depth of modulation the slew rate doesn't change. When
you vary the slew rate, the depth of the modulation doesn't change. Why
would you want this? Because it makes the pitch-bend constant no matter how
much you change the depth control. I also suspect it would feel more
natural and you'd probably never want to go back.

>I actualy think it's probably pointless to bother with the flanger part
>because this unit has an MN3005 for the doubler part and that's where I
>spend most of my time. It's like having a Boss DM2 with all the controls of
>a Boss BF2 and more (or maybe I should say like a DOD 585 and a DOD 575...).
>I like the analog delay sound so much more than digital (and I really really
>like the way my Digitech PDS2000 sounds). I just wish it had 2 or 3 MN3005's
>instead of just one.
>
>The way the clock works is kinda screwy too. I think using an SAD512 and an
>MN3005 together is a bad idea. They realised that they should send the
>single-phase clock to the SAD512 before it gets divided by 2 for the MN3005,
>but the SAD512 divides its clock by 2 as well. So really although it looks
>like a 20kHz-400kHz clock is going to the 512 it's really only a
>20kHz-200kHz clock. That + the sine waveform = not a good flanger. I
>wouldn't gripe except I don't have a usable flanger that I like. I have been
>considering making a Bad Stone clone because stackign up a lot of BBD
>effects almost always results in clocks interfering no matter how hard I
>try.
>
>BUT what would be a good way to get this kind of LFO to go faster AND make
>smoother slow sweeps?

-- 

Terry Bowman, KA4HJH
"The Mac Doctor"