Welp, I plugged my power cable into the “link” jack on the back of the 1222 VCO. Dumb mistake, wasn’t paying attention. I was wondering why I had no sound, and then it got really hot, and the TL074 started to smoke and popped. I replaced it, but something is still broken. Unsure where to check next. Has anyone else made this dumb mistake? Any help is appreciated!
1 Like
You are not the first person here to make this mistake, and you won’t be the last. You may need a new 3340 chip. @analogoutput wisely made the link cable on his 3340 oscillator have a different connector for exactly this reason.
4 Likes
Is there any reason not to defer installing the link connector until you have a need for it? The VCO will operate just great without it.
3 Likes
I skipped 'em. But I also skipped the screen and the arduino.
4 Likes
It’s a common issue.
Mine does not have the socket.
4 Likes
PUT AN EMPTY CONNECTOR ON THE LINK HEADER. Or a piece of tape over it. Or something.
Within a couple of days of building my Hero VCO I must have started to try to put a power cable on the link header two or three times. No chance of succeeding, because the link header’s a different size, fortunately. But I’d probably fry a 1222 too if I had one and left the link header open.
Not installing it in the first place is a viable option too.
As for fixing the broken one, follow the usual troubleshooting procedure. (Look in the FAQ category.) It might be the 3340 but no point in buying a new one if the old one is okay, and they do seem to survive power to the link header fairly often. And who knows, maybe if it’s not OK it’s because something else is damaged and you’d damage a new one if you put it in too. So rule out other possibilities first.
6 Likes
yep did that to the only 1222 VCO of 4 that I hadn’t tapped over , the TL074 is what cooked for me . yeah it should be noted in the build docs to leave that connector off the board unless needed .
5 Likes
None, except it isn’t stated in the build instructions that it is totally useless…
5 Likes
When I re-designed my own, I still had a link connector but it was for the refence voltage.
2 Likes
Update!
Replaced all of the ICs and it’s working again! (mostly)
The ramp and triangle outputs are fine, but I’m not getting anything out of the pulse. I’m not totally sure where to start. Any ideas?
3 Likes
Nothing on pin 4? (Check with scope or audio probe.) Then:
- 3340 chip is bad or
- pin 4 is shorted to ground or some other voltage or
- voltage on pin 5 is too low or too high so pulse width is 0% or 100% or
- something else but I can’t think of any
6 Likes
Will do! Thank you, I really appreciate the help!
3 Likes
Update!
I’ve tried two different 3340s and got the same results with both.
Everything works perfectly, but now the pulse wave is there, but the volume is extremely low. Ramp and triangle are regular volume. Any ideas? 
1 Like
Audio probe or scope on pin 4? The amplitude there should be 0 to 10.5V or so, so should be quite loud. If it’s loud there but quiet on the output then there’s probably a bad connection at or on the way in or out of the op amp buffer.
If the pulldown resistor (47k or 51k I think) connection is bad that might give low volume at pin 4.
Only other thing I can think of is it seems quiet because it’s a very narrow pulse width, but changing the PW knob setting would fix that.
4 Likes
I really appreciate your help!
Unfortunately, I don’t have an oscilloscope or audio probe, so I can’t test the 3340 as suggested. I’ve tried two band new ones and get the same results.
I’ve replaced the 47k resistor, still nothing.
The PWM works like it should, but the volume is still as quiet as the triangle wave and there is a slight hum.
I tried a different power supply and also replaced the pulse wave jack out of desperation. I also replaced both PWM pots. I’m going crazy trying to figure this out!
1 Like
$10 via PayPal to the first person that helps me solve this 
1 Like
An audio probe is basically a piece of wire with a capacitor.
I GOT IT FIXED!
I made an audio probe and when I probed pin 3 of the TL072, the pulse wave was at the correct level. When I probed pin 2, the level was drastically lower. I knew this was a new TL072, but I swapped it out with a different one anyways, and it worked!
Richard, thank you so much for taking time to help me out, I really appreciate it! I’m glad it was a dumb mistake, but more importantly, I’m glad I learned a new troubleshooting skill!
4 Likes
So you need my PayPal address? 
4 Likes
DM it to me!
20 character limit!
2 Likes