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MOTM 300 Ultra VCO - no sine wave
MUFF WIGGLER Forum Index -> 5U Format Modules  
Author MOTM 300 Ultra VCO - no sine wave
I've been trying to troubleshoot the sine generator on one of my MOTM 300s, which has never managed to create a sine wave. As I understand it the Sine wave is created by feeding the Triangle into the CA3080 then saturating it into a sine shape.

The module is creating the triangle all well and good at 10v p-p.
I can see the triangle wave at pin 2 of CA3080 (although it's a lot lower signal level at around 0.18v p-p)
pin 3 is connected to ground
pin 4 is -15v
pin 5 seems strange; i'm reading -15v here although it's connected through R52 to +15v..?
pin 6 has no output (although this should be current rather than voltage output so not sure if I can measure that on a scope)
pin 7 reads 15v

so the anomaly seems to be pin 5; I traced it back to R52 and here I read +15v one side of the resistor and -15v on the U8 side of it. Is this weird? Is it a dead CA3080?

I haven't got another one on hand to quickly swap out to test...

Pin 5 is the bias current input and should have a voltage of about 1.2V above the negative supply, so if you see -15V and not around -13.8V something seems wrong here. If this is the case, you can try to measure the resistance between pin 5 and pin 4 to see if there is a short. Again, if there is a short measure both with the chip and without if it is in a socket to determine if the problem is with the board or the IC.

Pin 6 is a current output so it may go to the negative input of an opamp in which case you will not see a signal anyway, but you should see it at the output of that amplifier if you can find it.

0,18Vpp at pin 2 does seem reasonable. You just want to round off the peak of the triangle wave, not completely saturate the input stage of the 3080 and get a square wave at the output.
Hey Synthiq thanks for those pointers - you're right there is a short (very low resistance of 3ohms) between pins 4 and 5 on the CA3080 so it must be a funky chip.

I measured resistance between pins 4 & 5 on my working osc and there is no circuit so that backs up everything.

Be careful buying replacements, I found that the 3080 chips from ebay did not work like the CA3080 of old. My Digisound design uses 3080 chips for sine converters and it took me a while to realize this.

If you can find an RCA branded CA3080 then it will probably be the chip you want.
Yeah I've had my fair share of dodgy CA3082s from eBay. I just ordered some from Synthcube so hopefully they're legit.
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