Please help me understand how the LM358 Oscillates in a Small Clone

[theundeadelvis]

I've had a thread going about a Small Clone I've been trying to fix, and I've gotten really close. I've eliminated the buzz, it works bypassed, and when it's on I get clean guitar output with no effect (doesn't matter which way the depth switch is positioned).

My voltages are not oscillating on pins 1,5, and 7 on the LM358, and I'm trying to understand how this should work. When I touch the components between pin 1 of the LM386 and pins 1,2, and 3 of the CD4047 I can get a bit of chorus (almost works like a theremin). I've learned more working on this pedal, than I have with any new build. Anything anyone can teach me about how this oscillation works is greatly appreciated. Thanks guys!

[DougH]

http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf

[theundeadelvis]

Thanks Doug! I'll give it read. It's a pretty intimidating article (for me)!

[PRR]

Looking at THIS Small Clone.....

That's not a SINE oscillator? (What the TI paper discusses.)

At least it looks to me like a triangle. IC2b is a comparator with hysteresis ("Schmitt Trigger"). Its output slams high or low, maybe +1V and +8V. When high, current in the RATE pot charges the 2.2u Tant cap around IC2a. The output of IC2a slowly slants down. When it reaches the other threshold of the comparator, IC2b slams low, RATE pot current runs the other way, 2.2u cap charges the other way, and IC2a output rises slowly.

http://www.national.com/an/AN/AN-20.pdf#page=11
FIGURE 27. Triangular-Wave Generator
"The generator embodies an integrator as a ramp generator and a threshold detector with hysterisis  as a reset circuit. The integrator has been described in a previous section and requires no further explanation. The threshold detector is similar to a Schmitt Trigger in that it is a latch circuit with a large dead zone. This function is implemented by using positive feedback....."

Stuff to go wrong:

Wrong part values (6.8K and 68K look the same!).

Right part in wrong place (even if you looked 13 times).

Bad cap (I'm not sure an Aluminum electrolytic will work here? Try a 2uFd NON-polar (loudspeaker crossover cap) or a 0.5uFd poly (it will run too fast, but we'll deal with that)).

> not oscillating on pins 1,5, and 7

What are the DC voltages?

[theundeadelvis]

Pin 1 is 8.31V
Pin 5 is 7.76V
Pin 7 is 8.25V

These voltages seem too high (aside from not oscillating)?

I went ahead and replaced the 2.2uF Tantalum cap (with a new 2.2uF Tantalum).

Thanks so much for your help!

[PRR]

First, set the Depth pot to mid-point; I have a suspicion this oscillator won't go right when Depth is full-up and that 10uFd cap shunts pin 1.

Also turn Rate to minimum resistance, just for clarity.

OK, let's stop trying to short-cut.

Voltages on ALL pins. (Including both supply pins.)

FWIW: _IF_ the battery is 9.0V, pin 3 should be 2.23V, pin 6 should be 4.09V.

This is in-fact an LM358? (The 2V bias at pin 3 would be kinda low for some older opamps.)

[theundeadelvis]

PRR - It is an LM358N, and this is a Vintage Small Clone, so instead of the Depth pot I have a switch. I'll post the voltages for the switch in both positions. Also, fwiw, my battery is at 8.5V.

Switch On
1 - 8.31V
2 - 7.87V
3 - 7.94V
4 - 8.52V
5 - 7.75V
6 - 7.85V
7 - 8.22V
8 - .2mV

Switch Off
1 - 8.34V
2 - 7.87V
3 - 7.94V
4 - 8.51V
5 - 8.10V
6 - 7.99V
7 - 8.26V
8 - .2mV

Thanks so much for the help! This guy has been keeping me awake at night.

[Govmnt_Lacky]

From the tonepad circuit, the LM358 should be reading ground at pin 4. Not pin 8. Do you have your chip in backwards? Are you sure you are reading the chip correctly? Pin 8 should be about 8.4V as it only goes through the 47ohm resistor before the IC.

[theundeadelvis]

Good call Govmnt_Lacky. Funny thing is, I soldered a socket in for this IC, and when I put the IC in I purposely put it in the socket the wrong way because I had convinced myself I soldered the socket in wrong.

So that fixed it! Unfortunately, I don't not fixed the other problem, but at this point I'm just happy to have it going! Thanks so much guys! 

Now I need to set the trimmer.

[Govmnt_Lacky]

Once you set the trimmer correctly, you should get your chorus effect. The trimmer in my Small Clone has a fairly large "gap" in which to get your chorus effect. Hopefully, your will be the same way.

What is the other problem that you are having that is "not fixed?"

[theundeadelvis]

Thannks Govmnt_Lacky! The original problem IS fixed, I just don't know at what point I fixed b/c I had the LM358 in wrong. It originally had a horrible buzz when it was on, and didn't work bypassed, but now it works like a charm!

[Govmnt_Lacky]

Good deal. Glad to help!

[Thomeeque]

 Hi!

For some reason I am simulating this LFO at the moment by LTSpice



SmallCloneLFO.asc

T.

[theundeadelvis]

Cool! I need to download and play around with Spice sometime.