Hi Guys, hopefully someone can shed some light on this problem I'm having. Here goes:

I have built a 10 step sequencer for a synthesiser. The sequencer is from the MFOS website and has been tested so it should work fine, however, this is not the case for me. Everything seems to work on the sequencer except for the internal clock, (integrator, U3-A, followed by a comparator, U3-B). I am 99% sure this is where the problem is. I have also built the circuit again on a breadboard to troubleshoot. The only problem is I'm getting exactly the same results, the integrator is not outputting the expected ramped wave. Ive hooked up the integrator output to the scope and all I'm getting is a straight line. I have also checked the outputs of both the integrator and comparator with a DVM and both outputs are saturated to the positive supply, although the comparator is never going to work without the integrator anyway, so my focus has been on the integrator for this reason. Here is the schematic and the outputs that should be present:

As I say everything else seems to be working fine its just not automatically sequencing but I can step through the sequence manually with the step button.

Any help with this would be greatly appreciated as I don't know what else to try now.

 

Remove the fet Q11 source or drain terminals, and see what voltage range is on pin2 and Cr2 wiper it should be connected to +12V!! , check the supplies to the Ic3, and it should produce a sawtooth wave, that varies in frequency with the clock rate preset.

 

Your dran and Source pins seem to be flip-flopped on Q11. The clock rate potential too meter is connected to (-12 v) and ground. So your drain is currently connected to the more negative side of the circuit - I could be wrong on what happens to the op amp output but look at it.

Edit: arg, I took too long to explain - @Dodgydave beat me.

 

Remove the fet Q11 source or drain terminals, and see what voltage range is on pin2 and Cr2 wiper it should be connected to +12V!! , check the supplies to the Ic3, and it should produce a sawtooth wave, that varies in frequency with the clock rate preset.

Thanks for the help. Right, ok I tried that. I disconnected the drain from the circuit and the voltage on pin 2 is 10.02V and the cr2 point fully CCW is -5.65 and -00.6 fully CW. Both the supply rails on the chip is as expected. I should have mentioned I'm using a 15V supply atm. The output is still not producing anything and is still fully saturated to the positive rail.

 

Your dran and Source pins seem to be flip-flopped on Q11. The clock rate potential too meter is connected to (-12 v) and ground. So your drain is currently connected to the more negative side of the circuit - I could be wrong on what happens to the op amp output but look at it.

Edit: arg, I took too long to explain - @Dodgydave beat me.

Thanks for that. Do you mean I should try swapping the drain and source around?

 

Swap the Cr control voltage from -12v to the +12v, looks to me like the circuit is drawn wrong.

Can you show the circuit of the power supply?

 

Swap the Cr control voltage from -12v to the +12v, looks to me like the circuit is drawn wrong.

Can you show the circuit of the power supply?

Do you mean connect R22, 200K from -12 to +12? I tried that and nothing has changed on the output. The power supply i'm using I bought from ebay. It's just an AC to dual voltage DC supply board, i'ts worked with all other op amps designs I have built.

 

Ok, if pin2 is at +10V, pin3 is at half supply zero, then the output pin1 should be at -12V!!

Put a voltmeter across pins 2,3 and see what voltage difference is.

 

Ok, if pin2 is at +10V, pin3 is at half supply zero, then the output pin1 should be at -12V!!

Put a voltmeter across pins 2,3 and see what voltage difference is.

Ok I tried that. All are correct except Pin 1 is at the positive supply instead of the negative. So I'm getting +15v on pin 1. The volatge difference acroos pin 2 and 3 is 5.53V or -5.53V depending on which way round the test leads are.

 

If pin 3 is more positive than pin2, the output will be positive, if pin 2 is more positive then the output goes Negative..

 

If pin 3 is more positive than pin2, the output will be positive, if pin 2 is more positive then the output goes Negative..

Hmmm weird. I'm getting a positive voltage on pin 1 and 2. Pin 3 is at ground, Pin 2 is 9.34V and the output is saturated at positive supply. No matter what I do with the CV pot, pin 1 does not change. I don't know why this part of the circuit won't work. I can't get the op amp to produce anything on the breadboard or in the circuit i've built.

 

I would take the op amp out of circuit, and build the single integrator on its own in breadboard, or make a simple oscillator,

http://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.html


then you will know if the chip is blown.

 

I would take the op amp out of circuit, and build the single integrator on its own in breadboard, or make a simple oscillator,

http://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.html


then you will know if the chip is blown.

Yeah, thats why I built it on the breadboard because it wasn't working on the circuit board, but I'm getting the same results, that's the weird thing.
Ill try another chip now. As a side note I have noticed that by turning the pot I get a voltage change before the 1M resistor but after the 1M resistor the voltage stays at 9.36 V when I turn the pot, and it acts the same on the built circuit.

 

I just tried another chip and it seems to be working now. I must have had 5 bad chips in a row. Thanks for all the help I really appreciate it

 

Hi Guys, I have another question about this circuit. After managing to get it to work on the breadboard I switched the chip on the circuit board and i'ts not working. I probed the inputs and outputs of the integrator circuit on the board and i'm getting really weird results. First off, all voltages seem to be correct but when I probe it with the scope, I seem to be getting some sort of noise. Pin 1 and 2 look pretty much identical, which is a sort of imperfect sine wave but with a lot of noise. I also probed pin 3 which goes to ground and that is displaying the same fuzzy sine but alot lower in amplitude. What is going on here? Is this noise on the ground point that's causing the op amp to amplify it, even though the op amp is set up as an integrator? I really can't figure out whats going on here. Surely, if this noise on the ground point was causing problems within the circuit then I would be having problems with the whole circuit instead of just the integrator/comparator section.

As always any help would be awesome and greatly appreciated.

Thanks

 

I also probed pin 3 which goes to ground and that is displaying the same fuzzy sine but alot lower in amplitude.

That could be mains hum pickup on the scope lead.