Giranda wrote: ↑Sun Feb 26, 2023 10:29 am
Firstly, thanks very much for sharing your board design! I hope you don't mind but I had a few of your issue 5 pcbs made up
Not at all! I haven't made any of these myself, so it'll be nice to see the result.
I'm not sure if it's okay to use standard 20pF crystals for the 4.4 and 14.2 MHz ones?
I used a TTL output oscillator for the SAM so I've not actually tested any crystal. The SAM datasheet recommends 5pF, but I'd just try what you have...
As for the chroma crystal - I used a 20pF one from Farnell and it works fine.
Also, do you know what I can replace the two germanium diodes with that are part of the reset circuit with, they seem to be unobtanium. I'm guessing there's something particular about that type of diode (forward voltage drop?) which means I can't just swap it out for a 1n4148?
Voltage drop was my guess too. I went for a "BAT43" purely because Vf was specified at 15mA and seemed low enough. All I can say is that resetting the machine seems to do the right thing
I couldn't find any BC212L with the same pinout but I did find BC212As in stock, so I am hoping they'll work okay once I've re-arranged the pins to match the PCB.
I couldn't determine anything special about the PNP in the video circuit. I assume maybe it's used to exaggerate the sync pulses (relatively low frequency)? Anyway, I used a BC177 and it seems fine.
For the NPNs there I went for BSX20.
And yes, seems like everything has different pinouts. Careful now
I did find an EDAC edge connector in stock but one not with the board lock bolt tabs. I have other machines like the C64 and VIC-20 which have right-angled cart connectors, and that are also are not secured to the PCB with bolts. These don't seem to fail at all, so I am hoping the one I sourced will be okay
Julian Brown donated a couple of these without mounting holes - seems pretty stable anyway!
In case it helps, I think I've sourced everything else to exactly match your design, so if there's anything you, or anybody is having problems sourcing, please let me know and I'll post where I got mine from.
Julian also kindly donated a reset switch, as the exact footprint doesn't seem to appear on RS or Farnell, but it's possible I haven't looked hard enough - might be good to know that one!
..ciaran