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no sine wave on Oakley VCO [SOLVED!]
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author no sine wave on Oakley VCO [SOLVED!]
jonny_w
Built my second Oakley VCO (first one worked great!) and have discovered a problem - there is no sine wave output. Triangle, Sawtooth, and Pulse all work properly.

When I put U9 (TL074CN) on my scope, here's what I see:

Pin 4 = +14.8V
Pin 7 = -14.8V (no sine wave)
Pin 8 = normal pulse wave
Pin 14 = normal pulse wave

The fact that I'm seeing a pulse wave indicates to me that the TL074CN is operational.

Anybody have any ideas?
sonicwarrior
I don't have the schematics but according to the Builder's Guide U9 doesn't have anything to do with the sine wave out. It says that it's formed from the triangle wave and as that is OK, I would take a closer look at U8 and the surrounding circuit which forms the sine shaper.

Have you got the schematics?
jonny_w
sonicwarrior wrote:
I don't have the schematics but according to the Builder's Guide U9 doesn't have anything to do with the sine wave out. It says that it's formed from the triangle wave and as that is OK, I would take a closer look at U8 and the surrounding circuit which forms the sine shaper.

Have you got the schematics?


I do indeed, and finally solved the problem! It turned out to be a solder bridge underneath the U8 socket, between pins 5 & 6 - effectively tying the output to -15V (which would explain what I was seeing on the scope). I guess a bit too much solder got through, and created the bridge.

This was a great lesson for me, as now I know to look for hidden bridges underneath components (I've only ever visually inspected the bottom).

Question: when removing the old IC socket, I pulled up 3-4 pads on the surface side. The bottom still had the pads intact, and I was able to verify that the circuit is now working. Should I be worrying about the longevity of the joints, now that some of the top pads are gone?
Synthbuilder
jonny_w wrote:
Should I be worrying about the longevity of the joints, now that some of the top pads are gone?


Only worry if the missing pads were connected to copper tracks on that side of the board.

If you don't have a desoldering station this is the stuff to remove any IC, pot bracket, etc:

http://uk.farnell.com/chip-quik/smd1nl/kit-smd-removal-pb-free/dp/1850 218

Yes, it's a bit expensive but it'll last you years. I use it to remove 40-pin DIL ICs and all sorts of surface mount stuff.

Tony
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