[sdiy] MCU Xtals

[brianw]

I cannot remember when they started appearing, but some processors have the active electronics for an oscillator that can drive a crystal. These sometimes need series resistors and/or capacitance (to ground) to get the oscillation just right. In such a design, the circuit is technically analog, and you'll see something like a sine wave on an oscilloscope for the external signals. If the drivers inside the chip have burned out, then the crystal won't work. But any of the passives - resistor, capacitors, crystal - could also be at fault, so consider all the pieces involved. These types generally have two pins, because one is driving the crystal and one is sensing the output to turn it into the clock.

There are also many of these old systems with inverter gate(s) set for feedback with a crystal in the circuit, just as Jay described. I think the TR-808 is like this.

When I was younger - before the web - I would probe the PCB with a meter and draw the circuit by hand. Once you have a picture of the components and how they are wired together, you can diagnose problems more easily. It's a real pain, though, converting a PCB into a schematic that's readable, but sometimes you gotta do what you gotta do.

Before that, though, there's always the tactic of reseating all the chips to see if dislodging some vintage oxidization will clear up the functionality.

Brian


On Aug 23, 2022, at 10:30 AM, Jay Schwichtenberg via Synth-diy <synth-diy at synth-diy.org> wrote:
> Do you know who made the processor?
> 
> Some processors can take either a crystal or an external oscillator. TYPICALLY if the crystal is hooked up directly it is usually hooked to 2 processor pins. If there is an external oscillator then one pin with the other either open or grounded. USUALLY if an external oscillator is used inverting logic chips are used for the circuit. Might look to see if the crystal is hooked to an inverter chip.
> 
> Then there are parallel and series resonating crystals.
> 
> https://www.maximintegrated.com/en/design/technical-documents/tutorials/7/726.html
> 
> Jay S.
> 
> On 8/22/2022 9:22 PM, Michael E Caloroso via Synth-diy wrote:
>> I have a dead Lync LN1000 keytar I bought a few years ago, it has been on the back burner during elder care with my aging parents and job relocation.
>> 
>> I'm hardly a novice restoring embedded systems but this thing has me stumped.  There's no schematics.  The last time I was in this thing, I repaired the voltage regulator and the unit sometimes worked, then not at all.  I'm powering it from a bench supply not from batteries.  I traced the problem to the master clock on the mcu, nothing there.  Tried replacing the xtal with no luck.
>> 
>> Now I'm beginning to suspect that I might have installed the wrong xtal.  Where's a good technical resource on mcu crystals?
>> 
>> Also the mcu is one of those quad leadless packages and I can't for the life of me figure out how to pry the mcu out of its socket.  Need to probe the pins in the socket to confirm power rails and signals.  At the moment I don't have the mcu info in front of me.
>> 
>> MC