Examining my stock SE/30 motherboard and the BOMARC schematic, I see this...

Looking at Bolle’s Github Gerber files (via JLCPCB website viewer), I see on the top side of the board, he has designed it to accept either the old cylinder-shaped crystal (shown above) or the kind more commonly used today:

And on the bottom of Bolle’s PCB, we see pads for the required two capacitors:

So in terms of what should work on an SE/30 Reloaded Motherboard, if one wishes to use the aforementioned ATS169-E on Mouser, which clearly shows a Load Capacitance of 20pF (which is a very common value, by the way), we need to calculate what each of the two external capacitors should be (C33 & C34).

I believe the board is using a Pierce Oscillator circuit , which is typically used with MCUs.

We unfortunately must make some assumptions before proceeding. There is something called parasitic (stray) capacitance inherent to any circuit, which pertains to how the circuit is designed (trace length, width, proximity to other traces, etc.) We also have input and output capacitances of the microcontroller (Cin & Cout). Unfortunately, Cin & Cout are almost never given in datasheets (even if we had one), and the parasitic capacitance is also an unknown.

Assuming stray capacitance (inherent to the board design) of 3pF is reasonable (but it could be more because thru-holes are used).

Assuming Cin & Cout to be 5pF each, is also reasonable.

We can combine those two assumptions into Cstray = 8pF.

The total Load Capacitance (CL, as defined in the XTAL datasheet, which in our case is 20pF) is Cstray plus the external capacitor value you must choose, which we can call Cext in the following formula:

CL = Cext/2 + Cstray 20pF = Cext/2 + 8pF Cext = C33 = C34 = 24pF

If the assumptions are correct (and only testing would show if they are), you would need 2pcs of 24pF C0G with a voltage rating anywhere from 16V to 50V (whatever physical size will fit the pads on the bottom of the motherboard).

If your C33 & C34 are currently the stock 33pF values, we can see why using the ATS169-E (CL=20pF) is not producing the oscillation expected by the circuit. The frequency is being shifted too far off due to C33 & C34 being non-optimal values. But because we’ve made some assumptions, it’s best to have a range of capacitor values to solder-in and test. Once the correct value is found for C33 & C34, that helps everyone because you will know what XTAL p/n and what C33, C34 values to use.

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