Dam problem with my vco (No Sawtooth) :-(

[maclean]

Hi Everyone,

As was suggested I done some kits before getting my xoxbox, however have now made a start on it and have been doing fine up until a problem with my VCO.

Basically the last test measurement I am able to successfully take is the step below on the instructions (obviously all previous measurments where successfull):

"Verify the op-amp is working. Test the DC voltage as shown on pin #7 of the chip.
It should be the same as the control voltage in the previous step, 3V "

http://www.ladyada.net/make/x0xb0x/fab/vco.html

The next steps ask you to attach an oscilliscope (which I don't have) however it then says:

"If you don't have a scope, you should at least measure the DC bias, which is about 8.5VDC"

This step measuring R105 with my multimeter set to dc produces 0v when I am expecting something close to 8.5v.


I have spent a ridiclious amount of time checking each compent and for the life of me sware that I have the components of the VCO correct, therefore I then tried resoldering asmany conections as my sanity could handle and essentially I am in the exact same situation - no further forward (or back thankfully)

I am seriously starting to wonder if its a pcb fault or a failed component, however I'm hoping someone with experience could help me narrow down where the faut must lie given the step that im having trouble with.

I also tried using some headphones into r62 to hear the saw wave as per instructions but obviously that is not producing a result at all either.

Thanks if you can help at all!

[maclean]

I managed to find out the cause of my frustrations in the end!!!

I know its a pain reading up on forums without solutions so encase anyone stumbles upon this in future the reason I was having problems is:


There is a step in my paper manual but isn't on the online version which suggests putting in R105 with the longer leg south so it can be measured easily.. I ovcourse put the resistor in with the long leg north which ment I was trying to measure the wrong side of the resistor - which I can only presume that at the stage im at is quite significant that you do the measurement from the south leg of R105.

Upon going behind and putting my headphones to the south end of R105 I can hear a perfect saw wave which changes pitch when I change my control voltage! and get an 8.something voltage reading on my multimeter, exactly what I was after!!

Happy days, I can continue my build with confidence that all is well!

[MMM]

Congrats for finding this out yourself; I probably would have "poked in the dark" trying to help here.

There is a step in my paper manual but isn't on the online version which suggests putting in R105 with the longer leg south so it can be measured easily.. I ovcourse put the resistor in with the long leg north which ment I was trying to measure the wrong side of the resistor - which I can only presume that at the stage im at is quite significant that you do the measurement from the south leg of R105.

It is quite significant indeed - if you look at the schematic you'll see that the other end of R105 is connected to ground. So when you try to measure the "wrong" end here you would essentially try to measure from ground to ground - and the results are predictable.
So we should always keep in mind that it is a good idea to look at the schematic to see WHERE exactly we are measuring WHAT kind of signal.

Thanks for letting us know about your progress; this probably will be helpful for other readers. And good luck with the rest of your build!

[maclean]

Thanks mmm, I spent so long checking every resistor, polarity and resoldered just about every joint that I was convinced something was broken.. I did have a look at the schematic in an attempt to trace it back and voila first step was it!

I must admit I find the schematics hard to follow but I guess practise makes perfect!