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Troubleshooting VCF
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Please be positive and constructive with your questions and comments.

Troubleshooting VCF

by jbdiver on Mon Jul 02, 2007 4:41 pm

I finished the build and now I'm doing some troubleshooting. The VCO looks (and sounds) good. But the signal doesn't seem to make it through the VCF.

I can see the square/saw signal make it to the east side of R62. But then it disappears on the other side of R62. I just see a flat 8.02VDC. Hmm. I'll admit my electronics expertise is just above basic. I've checked all the parts and performed circuit traces looking for shorts. I'm not finding anything. I included some voltages for vr3, vr4, and vr5 pins below (with just power turned on). Any advice where to check next is appreciated.

VR3
1-4.7
2-5.4
3-11.8

VR4
1-0
2-0
3-3.18
4-3.19
5-3.18
6-0

VR5
1-2.7
2-2.4
3-0.02
jbdiver
 
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More troubleshooting

by jbdiver on Thu Jul 05, 2007 12:18 am

Okay, here are some more details that might help me troubleshoot. I took VDC readings at different sections of the VCF. I took these readings with the power on and with R62 removed. My readings are in red. I'm hoping someone can help me identify if these readings look okay.

Image[/img]
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Scope pictures

by jbdiver on Thu Jul 05, 2007 12:21 am

Here is the scope picture from the east side of R62:

Image

And here is the scope picture from the other side of R62. What happened to the saw?

Image[/img]
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by guest on Thu Jul 05, 2007 3:44 pm

what scale is the scope on
in the second picture

the signal should be about 50mv
on the other side of r62

what is the 1.418v reading
near c29 referencing
that should be 3.53v
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by jbdiver on Thu Jul 05, 2007 9:48 pm

I updated the scope pictures so that the scale would be the same. The scope was set to 0.5V/div and I used a x10 probe so 5V/div. I read 8.25V on my meter for the east side of R62 and 8.09V on the other side.

I also updated the diagram. My note for c29 was incorrect. It does read 3.53V.

I really appreciate the guidance. Thanks
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by guest on Fri Jul 06, 2007 3:03 pm

at 5v/div the signal wont be noticeable
the signal is divided down via r62 and r70
if you cant find any signal after r62
check to see if c17 is in backwards or bad or shorting
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by guest on Fri Jul 06, 2007 3:05 pm

also
there is a second 3.53v near r116
what is this referencing
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by jbdiver on Mon Jul 09, 2007 4:45 pm

The second 3.53v near r116 should be on the other side of that resistor. It was a little unclear from the diagram.

Well the good news is that I think my VCF was working well all along. I just didn't realize that the signal was so small. I set my scope to AC mode and then set the voltage scale down to the 1mV level and this is what I found after R62:

Image

My recommendation would be to update the VCF testing instructions to note the scale of this signal. It would help newbies like me. ;)

I think one or more of my jumper cables broke during all of the troubleshooting. I resoldered all of the jumper cables and now #608 is working perfectly. Thanks for the help!

Image
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Please be positive and constructive with your questions and comments.