LFO Troubles

[bree9217]

So my LFO works perfectly until I remove it from power and start it back up again. When I come back to it and turn the power on, it won't be working (no signal at all, or a very faint distorted sound if strummed hard) until the depth knob is turned all the way down. Then it comes to life again and the depth knob and everything else will work as intended after that. I've searched everywhere I could think of but can't find anyone with the same problem. I'm also not the most savvy when it comes to LFO's.

I'm fairly certain that everything is positioned appropriately and polarities are correct. It is on a trustworthy breadboard so there's no solder joints to worry about. I've tried swapping parts and pots, including different values with no success.

If I place a fixed resistor, the same thing happens. When I remove the depth pot and join pins 6 and 7 on the TL022, it will work fine but the depth is obviously fixed at one point. That setting does sound okay but I'd really like to keep the depth control. I am just stumped on this one

Any assistance/suggestions would be greatly appreciated!

[Mark Hammer]

All LFOs work on the principle of the time it takes to charge up one or more capacitors.  Faster charge-up time = faster oscillation.  If it takes a while to fully charge up the relevant cap, then the first fwump may take a moment or two or three to start.  Try reducing C16's value and see what that does.

[PRR]

U6.2 has way more gain than it can use. Try R55 as 47k.

I'm not sure what the point is, cascading DEPTH gain and RANGE loss. While they have different DC references, they seem redundant.

[bree9217]

All LFOs work on the principle of the time it takes to charge up one or more capacitors.  Faster charge-up time = faster oscillation.  If it takes a while to fully charge up the relevant cap, then the first fwump may take a moment or two or three to start.  Try reducing C16's value and see what that does.

so this didn't fix the issue. It still is unresponsive completely when the power is turned back on unless the depth knob is all the way down to begin with or turned all the way down after powering up. Changing c16 really only affected the overall speed range.

U6.2 has way more gain than it can use. Try R55 as 47k.

I'm not sure what the point is, cascading DEPTH gain and RANGE loss. While they have different DC references, they seem redundant.

R55 as 47k gives zero response or distorted signal no matter where the pots are turned at.
This is pretty much the LFO from Rick Holt's Causality 4 mkII phaser and the depth(or width) controls the intensity of the sweep while the range seems to be more of the frequency range of the sweep

[DrAlx]

When you power the circuit back on have you got varying voltage at pin 1 of the TL022 ?
I can't tell from your description if the LFO is oscillating at all (i.e. problem with U1.1) or if the problem is just how the oscillatiing voltage at pin 1 is being converted to the voltage at pin 7 using U2.2.

[bree9217]

When you power the circuit back on have you got varying voltage at pin 1 of the TL022 ?
I can't tell from your description if the LFO is oscillating at all (i.e. problem with U1.1) or if the problem is just how the oscillatiing voltage at pin 1 is being converted to the voltage at pin 7 using U2.2.

Yeah I'm getting varying voltage at pin 1 when power is back on between 3 and 7V and a stationary 8.44V at pin 7. After the depth knob is turned all the way down, pin 1 jumps anywhere from .6 to 8.6V and pin 7 will vary between 1 and 8.5V.

[ElectricDruid]

R52 was 330K in the original. Does that make any difference?

http://i210.photobucket.com/albums/bb292/frequencycentral/Causality4MkII.jpg

[bree9217]

R52 was 330K in the original. Does that make any difference?

http://i210.photobucket.com/albums/bb292/frequencycentral/Causality4MkII.jpg

Not a difference that solves the problem. It really just affects the symmetry of the sweep

[DrAlx]

Original schematic had a single "Depth" pot on the output of the second opamp (it's a simple buffer).

https://www.diystompboxes.com/smfforum/index.php?topic=80456.msg665400#msg665400

and I second what Paul (PRR) is saying.  It's odd including both Depth and Range pots.  They are almost doing the same thing in that they scale the amplitude of the LFO wave.  It's just that one of them changes the centre point of the wave also.  The gain provided by that second op-amp looks excessive anyway.  There are better ways of getting an LFO wave that has independently controllable amplitude and offset.

If residual charge on C16 is the problem (i.e.  try short out C16 to discharge it before you do the power-up and see if the problem goes away) then I would try either

1) Adding a massive R in parallel to C16 to discharge it.

or

2) Replace C16 with 2 back-to-back 22 uF caps in series (to make something that looks like a non-polarised electrolytic) so that
you can then connect C16 to Vb instead of ground.  BTW, I assume what you call Vb in your schematic is the exact same voltage as pin 3 on U1 (i.e. direct connection to it). If not then it probably should be.

[ElectricDruid]

I don't understand how the Depth pot affects the circuit at all. It's on "the other side" of a op-amp that would usually be regarded as a high impedance buffer. So changing the Depth pot shouldn't change anything in the LFO circuit at all.

Are you *sure* your breadboard follows that schematic? I think we might be looking for a snarl-up.

[bree9217]

Original schematic had a single "Depth" pot on the output of the second opamp (it's a simple buffer).

https://www.diystompboxes.com/smfforum/index.php?topic=80456.msg665400#

Yeah and it works well but it isn't really a depth control to my ears. So I was mostly looking at the MKII version which renamed the depth control to range and added a width control which acts more as a depth control.

and I second what Paul (PRR) is saying.  It's odd including both Depth and Range pots.  They are almost doing the same thing in that they scale the amplitude of the LFO wave.  It's just that one of them changes the centre point of the wave also.  The gain provided by that second op-amp looks excessive anyway.  There are better ways of getting an LFO wave that has independently controllable amplitude and offset.

If residual charge on C16 is the problem (i.e.  try short out C16 to discharge it before you do the power-up and see if the problem goes away) then I would try either

1) Adding a massive R in parallel to C16 to discharge it.

or

2) Replace C16 with 2 back-to-back 22 uF caps in series (to make something that looks like a non-polarised electrolytic) so that
you can then connect C16 to Vb instead of ground.  BTW, I assume what you call Vb in your schematic is the exact same voltage as pin 3 on U1 (i.e. direct connection to it). If not then it probably should be.

I appreciate all the input! I'll give these suggestions a try later today

I don't understand how the Depth pot affects the circuit at all. It's on "the other side" of a op-amp that would usually be regarded as a high impedance buffer. So changing the Depth pot shouldn't change anything in the LFO circuit at all.

Are you *sure* your breadboard follows that schematic? I think we might be looking for a snarl-up.

It definitely controls the intensity of the sweep and works really well after powering up and turning it all the way down. Once it's on, it's great. And it's a verified design that many have built successfully so it very well could be user error. I've taken it off the breadboard and replaced everything a few times now but get the same result every time. It's probably something I'm overlooking.

[duck_arse]

  BTW, I assume what you call Vb in your schematic is the exact same voltage as pin 3 on U1 (i.e. direct connection to it). If not then it probably should be.

I'd say the opposite. what is your "Vb" connected to? it should be to a stable, bypassed bias point, whereas  the pin 3 bias can't be bypassed as it wants to hop and jump w/ the switching. but I'll stand to bow to your superior knowledge, opamps aren't my best.

[DrAlx]

  BTW, I assume what you call Vb in your schematic is the exact same voltage as pin 3 on U1 (i.e. direct connection to it). If not then it probably should be.

I'd say the opposite. what is your "Vb" connected to? it should be to a stable, bypassed bias point, whereas  the pin 3 bias can't be bypassed as it wants to hop and jump w/ the switching. but I'll stand to bow to your superior knowledge, opamps aren't my best.

Yes you're right.  Pin 3 is not stiff at all.  I was just wondering what Vb was connected to.

[moosapotamus]

bree - Have you breadboarded the original/unmodified LFO from Rick Holt's Causality 4 mkII phaser, or did you just start with your modified schematic? If so, did you get the same behavior with the original version that you are describing now with your modified version? If you haven't, starting with a known, confirmed design (especially on a breadboard) might help rule out or point to some possible causes.

[bree9217]

  BTW, I assume what you call Vb in your schematic is the exact same voltage as pin 3 on U1 (i.e. direct connection to it). If not then it probably should be.

I'd say the opposite. what is your "Vb" connected to? it should be to a stable, bypassed bias point, whereas  the pin 3 bias can't be bypassed as it wants to hop and jump w/ the switching. but I'll stand to bow to your superior knowledge, opamps aren't my best.

Yes you're right.  Pin 3 is not stiff at all.  I was just wondering what Vb was connected to.

For some reason I can't post images right now but Vb is connected to a standard bias voltage divider consisting of two 220k resistors and a 10uF electro. This is an OTA based phaser with the OTA's and remaining opamps being connected here as well when needed.

bree - Have you breadboarded the original/unmodified LFO from Rick Holt's Causality 4 mkII phaser, or did you just start with your modified schematic? If so, did you get the same behavior with the original version that you are describing now with your modified version? If you haven't, starting with a known, confirmed design (especially on a breadboard) might help rule out or point to some possible causes.

I started with it unmodified as actually the MKI version, everything worked fine. Then I breadboarded the unmodified MKII. I didn't notice the problem until after I modified it so I went back to the original MKII and the problem persists. This is why I'm starting to believe I'm overlooking something or perhaps set it up incorrectly

[moosapotamus]

So, for your modified version, you replaced the Shape pot with an on-off-on toggle switch. Did you change or even just move anything else on the breadboard when you went from MKII to your modified version? Seems really odd that simply replacing a pot with a switch would prevent it from oscillating on power-up. Sorry. Wish I could be more helpful.

[bree9217]

So, for your modified version, you replaced the Shape pot with an on-off-on toggle switch. Did you change or even just move anything else on the breadboard when you went from MKII to your modified version? Seems really odd that simply replacing a pot with a switch would prevent it from oscillating on power-up. Sorry. Wish I could be more helpful.

That was the main modification. I may have tweaked a resistor value or two but even after going back to the original, it still happens. I'm considering a different LFO all together