on the bench of destr--err--healing today: this crappy 486 motherboard which needs an upgrade for its 14.318MHz oscillator.
some backstory: this motherboard claims to handle 3.3V processors (like the DX4-100) but it has no 3.3V regulator!
here is the PLL chip. there is a nearby crystal (14.318MHz) which it uses to generate all the clock frequencies used by the board.
i even found a datasheet for the clock chip! time to figure out the motherboard schematic, at least for this section.
well there's the problem. both capacitors are 10pF (measured out of the circuit).
the oscillator input pin has more parasitic capacitance than the oscillator output pin. this means the crystal sees an unbalanced load capacitance between both pins, so it pushes out the frequency!
i had a whole speech prepared about how nobody reads the datasheet--but in this case, you *can't* read the datasheet! is that 18pF? 16pF? 10pF? who knows!
I've replaced one of the caps with an adjustable trimmer cap. I've also added test points. let's see how well this works.
a bit off
I can't pull it very far, it's probably just a crappy crystal. this is as close as it gets
at least we're getting color now. the next rev of the Gremlin will feature a higher-quality onboard 14.318MHz oscillator. 😂
just gonna swap it out entirely.
with a socket, just in case
i just wasted a ton of time trying to tune the oscillator that just...won't...tune. now i'm wondering if my frequency counter has drifted.
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