Issues with newly built VCO

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

Hello, I recently build my 3 VCO, the LM394 type. I'm experiencing some issues with it and I'm not sure where to look since I lack the experience on this.

When I power up the unit I get a nice sawwave, I am being able to balance it around 0v. So all good there.

When I check the triangle, it is running hot, like, real hot. It ranges from +10 to -10, way to much if you ask me. Thats problem number 1.

On to problem number 2 and 3, and those are ones concerning me the most, I get no output or whatsoever on the sine and the square. Just a flatline on the scope.

Are there any measurments I can take so I can check some components? I've already replaced the 79l15 with a new one since the 1st one was broken.

Thanks in advance!

wackelpeter · Posted: Sat Dec 10, 2016 12:36 pm Post subject:

How much p/p voltage has your saw output? If it's not okay that could also produce your problems in your waveshaping afterwards...

The square seems to be generated as one section of the TL074 is used as a comparator. If the level isn't strong enough to reach the threshold the comparatoroutput wouldn't switch...
With that flat line on the square output you mean 0V or a steady positive or negative voltage?

As you wrote one of your regulators blew up... i would check the voltage too and there should be a reason while it blew maybe meanwhile you have fixed that or it's still there and is having impact somewhere in your circuit.
Do all the IC's get their voltage supply and is it within correct range?

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

As far as I could measure the saw ranges from +5v to -5v, like it should.

I will try to swap the TL074 to see if it is faulty. Who knows. I have to measure if the flatline has a voltage, I thought it was 0v, but I have to measure it to make sure.

The regulator didn't actually blow up, it was broken, like literally, 2 of the legs broke of, so I replaced it. I'm going to check the voltages anyway, see if I get the desired +14,2 and -14,2 volts.

Regarding the IC's getting their power supply, I have to check how to do this, is it as simple as checking the specific datasheet on voltages and measuring these? I'm sorry, I'm not really familiair with this part of electronics yet, but I want to learn it Wink

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

This has been resolved. D6 was broken.

I wasn't being able to find this myself, but luckily we had an old Neve technician walking around our shop. He was happy to help, fixed the issue in like 5 minutes.

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

Alright, this is almost resolved. Yesterday I've tried calibrating and tuning the VCO, but the tracking is really bad! So I think I have a fake LM394 or something.

Yves, I saw on your website that new LM394's are available at EricaSynth. From what I understand these are not pin to pin compatible with the one used in your design. So for clarification, I have to cut pin 1 and 8 from the new chip and place it backwards a bit? How do I connect the Tempco resistor then? Can I just use pin 2 and 7 for it as well? It seems to be connected on the PCB.

Thanks in advance!

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

The ERICA model requires a simple shift as you said you can cut pin 1 and 8 and install the IC such that pin 2 of the IC goes into hole 1 of the socket and pin 7 goes into hole 8. You can solder the tempco normally in its dedicated pad.

10T5 · Joined: May 29, 2018 Posts: 2 Location: France

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

I repeat : You can solder the tempco normally in its dedicated pads on the PCB

10T5 · Joined: May 29, 2018 Posts: 2 Location: France

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

Yes that's correct but here you might have leave long legs (with tubing) to the tempco resistor such that you can bend it over the IC when installed on its socket.