I will try that. I have a vague feeling that I already tried, but I am not sure.
I am considering that sticking a comparator chip between 9 and 11, if all else fails. But I would rather understand what is wrong, rather than pile up mistakes.
Sadly, I definitely did that. It checks out at 5.01 kOhm with my multimeter. It is a takeout from a '1969 circuit board.
i
Yes... What I am definitely missing is, how could that happen (low amplitude on pin 11).
My hypothesis, stated vaguely, is this.
Pin 11 is a pull down pin. When necessary, it becomes grounded to V-, through a part of the XR2206 IC that is called "VCO" (see fig. 14). That draws current from Vcc and, due to large 5010 ohm resistance, creates voltage across the resistor. This voltage IS the signal. This is just stating the obvious.
What I suspect is that in the circuit that connects pin 11 with V-, there is some big resistor that is such high resistance that most voltage is concentrated across it, and not across pin 9.
If we accept this premise, where would we logically look?
The only other big resistor, IIRC, is the system connected to pin 3.
This line of reasoning is interesting to me, but I am not sure if it is in fact the correct one. I can try to make this chip to pulse at much lesser frequency (say. 0.1 Hz). Once I do that, I can trace with an ohmmeter and find out what other pin does pin 11 get grounded by VCO.
i
That sounds nice. I will save your post for the future. I think that what I am doing now is a frustrating, but unavoidable part of learning electronics and I need this learning experience of creating a simple timing circuit. Hence, I am reluctant to abandon my efforts with XR2206, too soon.
I also found a simple multivibrator with suty cycle using a comparator, of which I have many units.
iForgot to ask. How do you read figure 14. Are both pins 10 and 12 grounded? If so, what is the cap connected to. Or is pin 10 connected to ground with an electrolytic cap and pin 12 is grounded? (that's how I would interpret it)
i
Thanks. I will check stuff tonight, but I believe that I did it the way you read figure 14.
i
Measure the input resistance of the scope input with your ohm meter. If your scope has a 50 ohm input you may be right.
8-)
Before I start... I just had a thought. What if it is the oscilloscope that is messing it up? That when I connect the scope to the Vcc and pin 11, there is so little resistance that pin 11 -> Vcc voltage drops a great deal?
i
Yes, see my separate post, HOORAY! XR2206 problem SOLVED!!! That was, indeed, it. Changing it from 50 Ohm to 1 megohm did the trick. It was a Tek 2445. I bought it for resale, but may actually keep it and sell the old trusty Tek 475 instead.
i
UC3846
Cheers Terry
Be a bit careful - if you connect the scope to the power supply while it is in 50 Ohm mode, you can fairly easily let the smoke out of the scope and it might be very hard to fix. If you use a x10 probe (which confusingly actually divides the signal by 10) then you will be prevented from damaging the scope in this way.
Chris
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