Author Topic: LM13700 vco not oscillating  (Read 1027 times)

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Ryan O’Malley

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LM13700 vco not oscillating
« on: May 27, 2019, 01:31:20 pm »
Hi everyone, I’m working on a vco that I built using a schematic from page the Texas Instruments datasheet for the lm13700. I’m using the triangle/square circuit found on page 19, and I can’t get it to oscillate at all. I I have everything on my breadboard connected exactly as in the schematic, except that I’m using 3 10k resistors in series for the input bias current on pin 16 instead of 1 30k. I have a 200pf ceramic disc capacitor, and the control voltage into pin 1 is 4 Vdc. I have my channel 1 oscilloscope probe hooked up to the square wave output , and my channel 2 probe hooked up to the triangle wave output. When I turn power on to the circuit, the trace stays flat on channel 1, and channel 3 gets pulled downwards. I will include a picture of the schematic. I’m pretty stumped here, and any ideas of suggestions would be greatly appreciated. Thanks, Ryan
 

Online JPortici

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Re: LM13700 vco not oscillating
« Reply #1 on: May 27, 2019, 02:48:25 pm »
the circuit should work without issues.
Two ideas:
- check the power supply, should work between +/-5 and +/-15. Some nodes are at GND and others are at -V!!
- Maybe the darlington buffers are damaged. Besides being merely passable in specs, they are really fragile. You can try using an opamp voltage follower instead
 

Ryan O’Malley

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Re: LM13700 vco not oscillating
« Reply #2 on: May 27, 2019, 11:37:32 pm »
Thanks! I have a few lm358’s laying around so I’ll try using one of those for the voltage followers. I certainly want a circuit that’s relatively robust since I plan on building an analog synthesizer using this vco design.
 


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