Hi guys I am working on an apple ii replica this time and i ran into a strange problem with the 14.31818MHz crystal oscillator circuit. I am using this crystal that i bought from Mouser: https://www.mouser.co.uk/ProductDetail/ABRACON/ABL-14.31818MHz-B1U?qs=DZ%2FvJMgWWIuY3tOrhGQEVQ%3D%3D.
The rest of my board is almost complete, but then i found out that there was no voltage rails present at all when the crystal oscillator was installed. Then the SN74LS00 next to it at location A2 started to get really hot to the touch. I don't understand what's wrong here. I have all the correct resistor values installed and the 2n3906 transistors are facing the right way.
This is a modern crystal designed for a parallel resonant circuit, so you need to add a small trimmer cap in series in order for it to resonate at the correct frequency.
Take a look at this thread and look for the word "parallel": https://www.applefritter.com/content/problems-replica-apple-ii-rev-0
While the series/parallel resonance point is relevant, there appear to be bigger problems with the board given the observed symptoms.
Hi, thanks for your response. How come on a real apple ii there is no trimmer capacitor in series with the crystal? Isn't that what the color trim variable capacitor is for?
The short answer is that it is meaningless to speak of a crystal as having a frequency by itself. The crystal only oscillates when it is driven by an active circuit, and there are multiple topologies of such circuits. When using a series circuit, the same crystal resonates at a higher frequency than it does with a parallel circuit—so the frequency that the manufacturer stamps on the crystal case is correct for either series or for parallel, but in the other topology it is incorrect.
The Apple II uses a series circuit for the oscillator and the crystals that work with it are marked for series operation. When a crystal is marked for parallel operation, the same frequency on the can is a different crystal inside.
It's actually the other way around. When using a series circuit, the same crystal resonates at a lower frequency than it does with a parallel circuit. This is why when you put a crystal what would resonate at 14.31818MHz in a parallel circuit inside the Apple II's series circuit, you get a lower frequency than 14.31818MHz, which you must then raise by adding a trim capacitor in series.
The kind in the picture is not the kind of crystal that Apple used. The packages are completely different.
You might want this one instead:
https://www.mouser.com/ProductDetail/IQD/LFXTAL034763B
Lol all of a sudden when i plugged it in again the 74ls00 wasnt getting hot to the touch and all the voltage rails were present. Strange
Bad link.
Were pins poking into that breadboard?
Is there no short circuit between the crystal connection point and GND?