Dragon 64 motherboard replacement

A place to discuss everything Dragon related that doesn't fall into the other categories.
sixxie
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Dragon 64 motherboard replacement

Post by sixxie »

Throwing progress images/videos here for now:

http://www.6809.org.uk/tmp/d64-mb/

As you can see (or rather hear) in the last video, I've got reason to be happy about the progress :) Tomorrow I'll solder on some more sockets and see if I get any sort of video.
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robcfg
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Re: Dragon 64 motherboard replacement

Post by robcfg »

Nice!

What's that board that you have plugged into the SAM socket?
sixxie
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Re: Dragon 64 motherboard replacement

Post by sixxie »

robcfg wrote: Fri Jan 06, 2023 11:34 am What's that board that you have plugged into the SAM socket?
Just a little SRAM board - buggy, but good enough to give me some RAM and a clock source for testing :)
sixxie
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Re: Dragon 64 motherboard replacement

Post by sixxie »

And now it works! Hooray! (More pics and video clips in the url above)

I updated the KiCad files here to include all the changes I made before getting the PCBs made. And one more change for a slight missing bit of info on the original schematic.
jedie
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Re: Dragon 64 motherboard replacement

Post by jedie »

Absolutely impressive!

I made a german crosspost here: https://forum.classic-computing.de/foru ... d-nachbau/
... too many ideas and too little time ... Related stuff written in Python:
Dragon 32 emulator / PyDC - Python Dragon 32 converter: https://github.com/jedie/DragonPy
DWLOAD server / Dragon-Lib and other stuff: https://github.com/6809
Giranda
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Re: Dragon 64 motherboard replacement

Post by Giranda »

Hiya,

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, and I'm trying to build one from mostly new parts. Would it be okay to ask about a couple of the components needed to finish the board? Building new-old stuff is just my hobby, so I don't have the background in electronics needed to know how to select the correct ones.

I'm trying to work out what capacitance the crystals need to be to function properly. The WDC 6551 documentation says it needs a series crystal, so the 1.84MHz one is sorted I think, but I'm not sure if it's okay to use standard 20pF crystals for the 4.4 and 14.2 MHz ones? 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?

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 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 (I might live to regret that in a few months when I have to break out the desoldering gun and try to replace it!).

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.

Cheers in advance for your help, and have a great day! :)
sixxie
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Re: Dragon 64 motherboard replacement

Post by sixxie »

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
sixxie
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Re: Dragon 64 motherboard replacement

Post by sixxie »

Oh, and I must ask: what's the mounting hole positioning like on the "issue 5"? I moved them around a bit based on where things ended up on the previous revision. Printing it out on paper, it seemed ok, but would be nice to know if anything still needs a little tweaking.
Giranda
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Re: Dragon 64 motherboard replacement

Post by Giranda »

Thanks very much for the help, I'll get some BAT43 diodes ordered and try out the 20pF crystals I have. I don't know about the mounting hole positions, I'll try and test fit the board into my D32 case tomorrow evening and let you know how it goes.

I got the reset switch from Mouser - https://www.mouser.co.uk/ProductDetail/688-SPUN190900 , I think it's the right one, it seems to fit okay at least.

Cheers! :)
Giranda
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Re: Dragon 64 motherboard replacement

Post by Giranda »

Morning,

I had a chance to check the board in the case, the case itself is from my Dragon32, it has the extra space for a serial port like the D64s have with a blanking plate over it.

The main board mounting holes seem to line up well, it's just the reset switch and the lower mount for the cartridge shield that are a little off...
D64Issue5 small marked.jpg
D64Issue5 small marked.jpg (1.49 MiB) Viewed 15460 times
The reset switch seems like it needs moving ~1mm up the board towards the PCBs top edge (on my board I think I can just move the switch itself up a little as it's soldered into the centre of it's holes at the moment, and they're quite large).
D64Issue5 reset switch closeup.jpg
D64Issue5 reset switch closeup.jpg (63.56 KiB) Viewed 15459 times
The the lower cart shield mounting hole looks like it should move to move about 1.5-2.0mm up...
D64Issue5 lower cart shield closeup.jpg
D64Issue5 lower cart shield closeup.jpg (519.23 KiB) Viewed 15459 times
Additionally, both cart shield holes might want to move about 0.1-0.25mm to the right, so the bolts go in completely straight.
Last edited by Giranda on Mon Feb 27, 2023 11:56 am, edited 3 times in total.
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