VC-LFO2 Troubleshooting

billak · Joined: Jul 30, 2015 Posts: 3 Location: Melbourne

Hi guys,

I've just completed my build of the VC-LFO version two and unfortunately the magic smoke is escaping from R1 and R2.

I've re-checked all of my traces to look for shorts and found none, checked all the component orientations and found them all correct and checked that all of my resistor values are correct. I can't seem to find anything wrong with the build but the resistors are still smoking.

I'm using a TLC555 for the timer if that makes any difference. I also substituted BC547 for the BS170, though I don't understand how they can be a direct replacement in the circuit as they seem to be of two different types?

My only clue is that U3 and U4 seem to be getting very very hot but I've checked around them and found no shorts on the board.

Any ideas where to check next? I've attached a couple of pictures though is is a little difficult to see the traces. Any help is much appreciated.

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LFLab · Posted: Mon Mar 21, 2016 2:51 am Post subject:

I can't see a reason why R1 and R2 should be smoking, at 15v they will cause a current of 0.15mA to flow which will not cause any smoke. And that's on their own, they have several other high value resistors attached to them.

That is, if you are using the same component designators als Yves is, on his site.

Check component values!

billak · Joined: Jul 30, 2015 Posts: 3 Location: Melbourne

My mistake, it is R37 and R38 the two 10ohm resistors that are smoking. I just assumed they were R1 and R2 without checking because they are the power supply resistors.

I've checked all component values and everything is as specified by Yves. I still think there is a short somewhere that I haven't noticed, I can't think what else it could be. Maybe my transistors have an incorrect pin orientation?

marfoski · Joined: Jan 26, 2011 Posts: 36 Location: Italy

LFLab · Posted: Mon Mar 21, 2016 12:18 pm Post subject:

The mosfet is not directly between the rails, so i don't think that's causing the short.
If not caused by the etching, the only other thing i can think of is the polarity of the electrolytics (found out that right angled boxed headers have their polarity reversed this way).

Edit:

You are using tlc274 for the quad opamp near the middle of the board. That's a single supply opamp, does it work in this circuit? Datasheet mentions a cap from output to negative supply.

And, you are using a single opamp in the other one? The 741 in yves' schematic.

billak · Joined: Jul 30, 2015 Posts: 3 Location: Melbourne

Hah...yep, there it is you've identified my problem LFLab. I thought I had the TL074 in there and it was instead the TL274. Mixed up my parts. It's working perfectly now with the correct op-amp in there.

Thanks for your replies guys.

LFLab · Posted: Mon Mar 21, 2016 11:42 pm Post subject:

Excellent! Glad to be of help.