Replacing A5000 CMOS chip

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mark71
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Replacing A5000 CMOS chip

Post by mark71 » Tue Apr 07, 2020 9:33 pm

Hi All

After cleaning up the A5000 mother board from the battery leak damage, in which the CMOS Chip fell off in the process.... I have a question about replacing it and seeing if it will boot to the desktop. I did try firing up the old girl without the chip and got the Red screen then a blank screen, so something works..

After searching the net and the forums looking at Retro-kits website https://www.retro-kit.co.uk/page.cfm/co ... -an-A5000/

I plan to replace the CMOS chip with board similar to the one shown on the retro-kit website my question is about the replacement capacitor c153 shows 15p which to me means a 15pf cap, would a 10pf cap work?, and can it be ceramic rather than a tant?. This sort of thing is not my expertise, can build a board and solder but the design side, only touched on as a mechanical design engineer.

Thanks

Mark
RTC-sml.png

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myelin
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Re: Replacing A5000 CMOS chip

Post by myelin » Tue Apr 07, 2020 11:50 pm

That's the load capacitor for the crystal... 10pF might work; I believe you can be off a little bit and still OK, although it could fail to oscillate if you're too far away from what it needs to be in resonance, so you'll have to scope it up and see how it looks.

Ceramic should also be fine; I would have expected ceramic for such low capacitance already TBH.
SW/EE from New Zealand, now in Mountain View, CA, making BBC/Electron hardware projects for fun.
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IanS
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Re: Replacing A5000 CMOS chip

Post by IanS » Wed Apr 08, 2020 11:10 am

You could buy the podule based replacement - http://www.cjemicros.co.uk/micros/indiv ... CJE-RTCPOD If you don't want to build it yourself.

mark71
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Re: Replacing A5000 CMOS chip

Post by mark71 » Wed Apr 08, 2020 9:42 pm

IanS wrote:
Wed Apr 08, 2020 11:10 am
You could buy the podule based replacement - http://www.cjemicros.co.uk/micros/indiv ... CJE-RTCPOD If you don't want to build it yourself.
It's an option i've considered, but as have not much to loose, and enjoy a challenge in solving problems and a bit more time on my hands with the current situation.....

mark71
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Re: Replacing A5000 CMOS chip

Post by mark71 » Thu Jun 18, 2020 7:45 pm

Hi All

Working on a veroboard RTC CMOS replacement, based off of the website on retro-kit.co.uk, I have a question as want to replace the diode from the +5v D15 on the Circuit diagrams any recommendations for a non SMC equivalent?, IN4148?

Mark

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IanS
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Re: Replacing A5000 CMOS chip

Post by IanS » Thu Jun 18, 2020 9:09 pm

mark71 wrote:
Thu Jun 18, 2020 7:45 pm
Working on a veroboard RTC CMOS replacement, based off of the website on retro-kit.co.uk, I have a question as want to replace the diode from the +5v D15 on the Circuit diagrams any recommendations for a non SMC equivalent?, IN4148?
On other machines it's listed as a iN4005, any 1N400x should be fine.

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Re: Replacing A5000 CMOS chip

Post by philpem » Fri Jun 19, 2020 12:25 pm

For the current needed, I'd say a 1N400x was a bit over-spec. A 1N4148 is probably more than sufficient, or you could substitute a Schottky small-signal diode or rectifier if you wanted to squeeze a bit more life out of the battery.

I routinely use 1N4148s when I'm modding Acorn machines to take CR2032s.

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