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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
AK/DK
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...

Cheers!
Synthiq
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.
AK/DK
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.

Cheers!
craigyb
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.
AK/DK
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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