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MOTM 300 Ultra VCO - no sine wave
MUFF WIGGLER Forum Index -> Music Tech DIY  
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...

pin5 should be at -14.4V (assuming -15.0V rail). pin6 should be 0V with a signal on it. if pin5 isnt at that level, then the CA3080 is busted.

Pin 5 is a diode parallel with a base-emitter to V-, so about 0.7 volts higher than what is connected to V-.
The 3080 will blow if the current thru this pin to V- exeeds 2 mA

Here the datasheet
whats the vlaue of R52?
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