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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i printed a new panel insert that has two BNC sockets on it (the original probes would have had captive cables)
the tricky part is that this oscilloscope has a 333K ohm input impedance instead of 1M like umm every other scope on the planet. fortunately someone designed a little conversion circuit...
in honor of Sim Wong Hoo, here's a quick historical review of the Sound Blaster! 🧵
but first we have to go back to their first sound card, the Creative Music System from 1987. (image credit: Bratgoul on Wikipedia)
this card has some special "CMS-301" chips. if you peel back the sticker, they have had their top marks removed. they're actually SAA1099 synthesizer chips made by Philips. Creative wrote music composition software for musicians on the PC.