VCO Controller being naughty. (SOLVED).

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eggpie
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VCO Controller being naughty. (SOLVED).

Post by eggpie » Fri Feb 14, 2020 6:06 pm

Hi folks.
Have recently built a vco controller and it is misbehaving.
Here are the symptoms;
Built in LFO runs from about 10Hz to 6kHz instead of very slow to about 50Hz
LFO sine looks good -5 to +5v.
LFO square looks good but 0 to +5v.
Can't trim scale of CV1, pitch seems to glide upwards on its own, weirdly.
Oct +1 switch raises pitch but -1 lowers the pitch to inaudible LFO rate .
I am getting a good 5v ref at U5. Power is fine as are module and panel grounds.

Using LF412CP for U1 and U7, LT1013 for U2,3 and 4, TL072 for U8.
I've double checked all resistor values, especially around U1.
Values of pots and trimmers check out.
No solder bridges or shorts
VCA section seems to work ok.

Any ideas greatly appreciated of course.
Last edited by eggpie on Sun Feb 16, 2020 11:08 am, edited 1 time in total.

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Synthbuilder
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Re: VCO Controller being naughty.

Post by Synthbuilder » Sat Feb 15, 2020 6:14 am

eggpie wrote:
Fri Feb 14, 2020 6:06 pm
Built in LFO runs from about 10Hz to 6kHz instead of very slow to about 50Hz
R18 could be the wrong value. It should be 100K.
Can't trim scale of CV1, pitch seems to glide upwards on its own, weirdly.
That sounds like a poor solder joint somewhere. Possibly pin 3 on U2, or maybe the pins on R10.
Oct +1 switch raises pitch but -1 lowers the pitch to inaudible LFO rate .
Check you are getting +5V at pin 7 of U2. If not check again the resistors R21 and R25 and their solder pads. Maybe even the soldering of the trimmer -OCT.

Tony

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eggpie
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Re: VCO Controller being naughty.

Post by eggpie » Sat Feb 15, 2020 8:53 am

Hi Tony. Thanks for the reply. Just been doing some more tests.
I'm getting some bizarre behaviour!
R18 reads 100k but I will pull it to check it out of circuit.
Pin 3 of U2 is fine, as are leads of R10, soldering looks fine.
Getting 5v at pin 7 of U2 and -5v at pin 7 of U3.
Here's the weird bit. With the VCO controller powered up, if I play lowest C on the keyboard, then a high note, the pitch glides up (slowly).
Some high notes don't trigger at all.
If I then switch the octave+1 on the controller, the pitch glides again. Switching in -1 oct gives just LFO style clicks at about 1 per 3or 4 seconds.
This happens via the buss or via pitchCV patched direct from the mididac.
When I remove the VCO Controller, the weirdness goes away.
I have slide turned all the way off on the mididac.

Would a dodgy trimpot give this sort of behaviour?
I'm at a bit a loss with this one!
Any more ideas most welcome and many thanks. Tom.

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Re: VCO Controller being naughty.

Post by Synthbuilder » Sat Feb 15, 2020 10:58 am

Does the second CV channel work, or does it do the same as CV1?

Does everything work with the VCO Controller module powered up but not used?

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Re: VCO Controller being naughty.

Post by eggpie » Sat Feb 15, 2020 4:11 pm

Hi Tony.
Thanks for your help with this.
I have made some progress and I think I have isolated the problem.
I don't think there are any problems with the module itself. The issue seems to be with the socket 8 pcb.
I noticed I had continuity between pins 5,6 and 7 on the 8 way header of the socket 8 pcb.
Pins 5 and 7 are both sine wave out but pin 6 is connected letting pitch cv bleed into the LFO, this was the high frequency I was getting.
Now, when I connect an external LFO via CV IN, the module behaves normally, both octave switches do their thing (+1 and -1).
I thought I had a solder bridge under the header but can't find any.

Have I made a connection on the socket 8 board that I wasn't supposed to?
The socket 8 board is fully wired with both 8 and 6 way headers and L1 not fitted.

This, along with the bus connections is getting extremely confusing, I'm not sure if the pitch cv is coming from the module, or the bus.
It seems like there is a normalised connection that is not supposed to be there.
Hopefully, I have overlooked something simple.

Thanks. Tom.

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Re: VCO Controller being naughty.

Post by Synthbuilder » Sun Feb 16, 2020 4:36 am

Pins 5, 6 and 7 should be connected together when nothing is plugged into the CV IN socket. All three pins should be carrying a sine wave from the LFO. When you plug something into CV IN then the LFO sine wave is disconnected from pin 6, and whatever you've plugged in to the CV IN should now be seen at pin 6. Pin 6 is the LFO/CV signal and its effect on the two KCV outputs is controlled by the Vibrato Depth control. Anything being plugged into the CV IN socket, or the normalised internal LFO sine wave, should be able to be switched off by the Vibrato Depth control being set to its minimum position. So if there's anything naughty going on at pin 6, it should be able to be switched off with the Vibrato Depth control.

Pitch CV should be present on pin 6 of the six way header, and also pin 5 if you are using the Bus connector and not have anything plugged into the Key CV socket.

It may be worth checking again the wiring of both the 6 and 8 way board interconnects. That is, pin 1 goes to pin 1, pin 2 goes to pin 2, etc.

Tony

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Re: VCO Controller being naughty.

Post by eggpie » Sun Feb 16, 2020 11:02 am

Hi Tony.
I finally found the problem.
Nothing to do with the socket board.
Nothing to do with anything except an elementary and idiotic error on my part.
C1 was 1nF instead of 100nF.
What is even more embarrassing is that it took two days to find it!

However, the module works perfectly now and has calibrated nicely.
Thanks for your help and suggestions with this Tony.

Regards, Tom.

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Re: VCO Controller being naughty.

Post by Synthbuilder » Mon Feb 17, 2020 4:38 am

eggpie wrote:
Sun Feb 16, 2020 11:02 am
C1 was 1nF instead of 100nF.
:tu:

It doesn't explain the gliding or scaling problem you had initially with the KCV1 output though. C1 only really affects the speed of the LFO. You probably sorted those things out by the various other bits of work you did on the module.

But welcome to the sometimes frustrating world of fault finding. I've spent countless hours trying find faults on vintage synths only to find that the actual fault was one simply component failure which took a minute to replace.

Tony

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