Geofex Lera not working properly

[freshmex18]

Built RG Keen's Lera from Geofex to add to a PedalPCB clone of the Walrus Julia. 

http://www.geofex.com/article_folders/lera/lera1.gif

http://www.geofex.com/article_folders/lera/lera.htm

The On/Off switch works and the Lera does affect the sound of the Julia by adding a warble.  The speed is quite fast.  Neither the ramp up/down switch, the instant/delayed switch, nor any of the three trimpots have any effect on the sound. 





I added the optional 10uF capacitor to the output of the 78L05 but no difference to the sound.

The build doc says "Once you get it wired up, check it out with a voltmeter on the 100uF capacitor to see that it does indeed ramp up and down, and on the + and - inputs to the opamp - they should go up and down in concert with the capacitor voltage." I checked and there is no ramp up or down on the 100uF cap.  Unfortunately, the doc doesn't say what to do if there is no ramp up/down detected.

I did use an LM358 as I could not find an LM158.  I also tried a RC4558 and no difference.

Can anyone see what I did wrong?  Or is the Lera simply incompatible with the Walrus Julia?

[ElectricDruid]

It's not exactly compatible.

The PedalPCB Caesar Chorus uses a pot to ground as a voltage divider, not as a variable resistor, which is what the Lera is expoecting.

Putting the Lera across the pot in this situation changes the value of the pot, but not the wiper position, so it doesn't work like it's supposed to.

I guess the LFO uses this method of speed control because it reduces the depth as the speed goes up, which is what you want for chorus. The alternative method (variable resistor for R6/1M) gives the same output depth across the speed range, which gets a bit warbly for chorus.

You could try it though - put the Lera's LDR across R6 instead of the pot and see how you get on.

[freshmex18]

I tried your suggestion of connecting to R6 instead and it did not work.  More of the same but with a different speed of warble affecting the signal but still no switches or trimpots doing anything.  I suppose that means that the LERA just isn't compatible with the Julia.

I shall start looking for another chorus with a speed pot that acts as a variable resistor as you mentioned.

Thank you for the help

[ElectricDruid]

I shall start looking for another chorus with a speed pot that acts as a variable resistor as you mentioned.

Hang on a minute! While the incompatability explains some of the problems you're having, it doesn't explain why nothing happens when you put it across R6. This part explains that:

The build doc says "Once you get it wired up, check it out with a voltmeter on the 100uF capacitor to see that it does indeed ramp up and down, and on the + and - inputs to the opamp - they should go up and down in concert with the capacitor voltage." I checked and there is no ramp up or down on the 100uF cap.  Unfortunately, the doc doesn't say what to do if there is no ramp up/down detected.

Basically, your Lera isn't working correctly just yet. First we need to fix it, and then we'll worry about whether the LFO response is ok!
Sorry, I didn't pick up on this first time around.

Firstly, we need to get two speeds. Leave the Instant/Delayed switch open (so "Instant") and switch the other switch. The voltage at pin one of the op-amp should go up and down. Presumably it doesn't or you'd have more working than you report. But check this. Then check that the output of the 5V regulator is 5V (I always put those things in back-to-front, and they don't work that way around!). Then check (with the power off) that you've got the expected resistance between the down speed connection of the Up/Down switch and ground. It'll vary with that Down Speed Trim/Pot, but you should see between 24.7K and 4.7K.

HTH

[freshmex18]

Voltage at output of regulator is 5.03V

Voltage at pin 1 is 6.70V with no fluctuation up or down

Resistance between Up/Down switch connection for the 4.7k and ground is 0

[ElectricDruid]

Voltage at output of regulator is 5.03V

Ok, that's all good then.

Voltage at pin 1 is 6.70V with no fluctuation up or down

That's not right. 6.7V is possible, but we ought to see a change when the Up/Down switch is switched.

Resistance between Up/Down switch connection for the 4.7k and ground is 0

That's not right either.
The resistance from that contact on the Up/Down switch and Ground should be the 4K7 resistor plus the pot, whatever that's set to (so something between 0 and 20K). But there shouldn't be any way it can be zero. The minimum is 4K7, since that's the resistor value. Check this wiring carefully.

[freshmex18]

Ive gone through the wiring three times and nothing is in the wrong place and every connection is solid.

There was a solder bridge between IC pins 3 and 4 but that's all I found wrong.

I thought maybe I put the NSL 32 in backwards so I reversed that but no effect on the chorus pedal so that's not it either.

I'm at a loss

[Mark Hammer]

Some LFOs will use the configuration that the Caesar/Julia does, with a voltage divider (R4 setting the minimum speed) and a series resistor, functionally equivalent to R6.  But others will skip the voltage divider, and just vary the series resistance going to the integrator stage (IC1-2).  The LERA should work in that second format.  I will suggest sticking a 100k trimmer and 1M5 resistor in series between pin 1 and pin 6, and removing the pot and resistor to ground, as an experiment to see if this form yield usable sweep rate and depth.

If it does, then bridge the 1M5 and vary the resistance of the trimmer to identify a suitable "fastest" speed.  Unbridge the 1M5 and see if that yields a usable "slowest" speed.  You may find that either value is a little too high OR too low, so adjust accordingly.

One suitable values have been identified, THEN you can start monkeying around with photocell values to provide a usable set of speed ranges.  You may also find you have to tinker with the value of C3 as well (higher value = slower overall range).

[ElectricDruid]

Ive gone through the wiring three times and nothing is in the wrong place and every connection is solid.

There was a solder bridge between IC pins 3 and 4 but that's all I found wrong.

I thought maybe I put the NSL 32 in backwards so I reversed that but no effect on the chorus pedal so that's not it either.

I'm at a loss

We've all been there. Staring at something is generally only a good way to see what you *think* should be there.

Go for a long walk. Bake a cake. Play with your dog. Do whatever, but don't think about it. Come back in a couple of days and look again. You won't remember any more what you thought you knew about it, and you'll be much better able to see what's *actually* going on.

[freshmex18]

I tore the whole thing down and rebuilt it after taking a few days away from it.  Changed the NSL-32 for another one for good measure.

The Up/Down switch now works.  It makes pin 1 read 1.35V and 5.03V respectively

Resistance at the 4k7 connection on the switch reads from 4.65 through 25.26 at each end of the trimpot's range

With the Lera attached to R6, the Up switch makes the Julia wobble very very fast.  When in the Down position, there is no effect on the Julia; it sounds exactly the same as when I flip the On/Off switch

[R.G.]

Can you post a link or schematic for the Julia? I'll take a look.

[freshmex18]

For the man himself?!?!!  Of course

https://docs.pedalpcb.com/project/PedalPCB-Caesar.pdf

[R.G.]

From a quick glance, I think the problem may be related to the way this LFO works. The rate pot selects a variable fraction of the output of IC1 at pin 1, and applies this to the integrator of IC1 part 2. Pin 1 bangs from the most positive to most negative output voltages of IC1 part 1. For the TL07x family, the output can only go to about 1.5V less than its positive supply and 1.5V more than its most negative supply. So the rate pot applies a fraction of this to the integrator.
R6 (1M) is not a really good place to change the rate in this circuit. I would try to think of how to make the LDR give a variable fraction of the pin 1 voltage. The simplest way I can see is to connect the input end of R6 to the top of R4, and then put the LDR in parallel with the rate control pot.
That makes a voltage divider of the LDR and R4. Scaling the LDR's contribution to this might or might not work easily. With it in parallel with the existing rate pot (and its wiper being disconnected) the range of fast to slow might be OK. If it's not, you could add fixed resistors in series with the LDR to keep its resistance from going too low (and the speed going too high), or parallel with the LDR to keep its resistance from going too high (and the speed going too low).
That's where I would start. Let me know how it works and I'll try to help refine it as needed.

[freshmex18]

So if I am understanding this correctly, you are advising me to

1. Add a jumper between R4-1 and R6-2
2. Connect the Lera with Rate pot lugs 1 and 3
3. Disconnect Rate pot lug 2

[R.G.]

Yes.

[freshmex18]

Have done as suggested and it does work.  In the Up position, the sounds is a lovely warble that I can adjust with the speed trimpot.

In the down position, the effect is barely audible and I thought it wasn't doing anything until I listened really hard and was able to hear only the slightest of modulations.  So that needs to be modified.

You did say "add fixed resistors in series with the LDR to keep its resistance from going too low (and the speed going too high), or parallel with the LDR to keep its resistance from going too high (and the speed going too low)."

I assume this is the situation you meant with the down switch causing the speed to go too low.

Where exactly would I put a resistor to make it parallel with the LDR?  And any suggestions as to what values to try?

[R.G.]

I'm a little surprised that the remaining 1&3 pin connections on the speed pot aren't doing this right now, but then I'm guessing about the characteristics of the LDR. They vary a lot. Try another 100K fixed across the LDR. It's a raw guess, but maybe we'll get lucky.

[freshmex18]

I don't have any experience with octocouplers so forgive my question, but which of the four leads does "across the LDR" mean?   

[bluebunny]

The two leads that aren't the LED (which likely has one leg marked by a dot).

[freshmex18]

I was wrong.  No need to add a resistor.  The Lera was in fact generating warble in the down position.  My crappy desktop amp at low nighttime test volumes wasn't picking up on it but today on a full sized amp at daytime volumes there is a distinct, lovely slow warble.  I think the Lera troubleshooting might be complete.  Now to finish the full pedal build and see how it all functions together.