I need help with breadboard LFO

[CorvinIan]

Hello fellow DIYers

I startet building synths a couple of months ago. Started with breadboarding, but didn't really get something working out of it, so I switched to PCBs. No I'm at the breadboard again and it's just like when I started. Nothing is working... I'm kind of frustrated so I decided to look here for help.

I'm trying to rebuild these schematics:
http://www.davidhaillant.com/wp/wp-cont ... 151120.pdf

I just don't get the LFO to work. If I connect the output to an amp it just makes high pitched noises. I went through the whole schematics two times but I can't figure out what I've done wrong.
The 1n cap is in there because I tried different caps to see if something changes. But it's also not working with the 1u cap as in the schematics.
It would be nice if someone could help me.

[Pav]

Hi, have you confirmed with a digital multimeter that power is reaching pins 4 and 11. I have problems with Breadboard connections all the time and the meter probes on the pins of the chip are best. That's first test I would do.

I'm not sure if your breadboard links up all the sets of five holes, so I'm not sure the green wire to pin11 is connected to -12v.

[CorvinIan]

Hi, have you confirmed with a digital multimeter that power is reaching pins 4 and 11. I have problems with Breadboard connections all the time and the meter probes on the pins of the chip are best. That's first test I would do.

Yeah, I've already checked that. Both pins get the right voltage.

[Pav]

Ok, that's good. Would you have a 2nd tl084 to rule out bad ic?

[Pav]

It is not easy to see on picture if the trimmer to left of ic has its pins correctly wired ...when compared to the other trimmer, it looks like it should be rows 8,9,10 .

[guest]

looks like the ground connection is missing from the comparator section.

[joem]

What do you mean by "If I connect the output to an amp it just makes high pitched noises."? Depending on what you mean by that, it might be working correctly. Using a 1n cap for the frequency cap should make it go faster (and therefore higher pitched if you're listening to it) than if you were using either of the caps spec'd in the schematics. The smaller value the cap, the shorter the time it takes the cap to charge, and therefore the faster the frequency.

More general debugging tips:

For debugging purposes, if you don't have an oscilloscope to monitor the output on, I suggest hooking up the LED driver part of the original schematic. For LFOs I find the LED to be a good way to figure out what it's doing, especially as I adjust the frequency pot or try different caps.

Breadboarding can be finicky, with connections breaking if a cable is jiggled and such, but also common pin headers with square posts can tend to deform the insides of the breadboards, making connections even less reliable. So try jiggling every connection while monitoring the output and see if one of them might be loose.

You could simplify things a little bit by eliminating R9 -- that just controls the amplitude. Less parts means less possible loose connections or mis-wired things.

[joem]

looks like the ground connection is missing from the comparator section.

Oh nice catch!