I Made some RAM for my NEC PC-8201a — and it’ll work on a Tandy Model 100 too!

Original expansion RAM for the NEC PC-8201a and Tandy Model 100 is pretty hard to find these days -- so I made some.

Ori­gin­al expan­sion RAM for the NEC PC-8201a and Tandy Mod­el 100 is pretty hard to find these days. Lucky for us, there are mod­ern third-party replace­ment options avail­able, either premade and sold on eBay or as DIY kits.

While I’ve pur­chased premade, I’ve been more sat­is­fied by assem­bling the modules.

The pro­cess is pretty simple, just select and order the Prin­ted Cir­cuit Board (PCB) you want, order the asso­ci­ated com­pon­ents, and wait for them to be delivered.

The designs have BOMs (Bill of Mater­i­als) for major com­pon­ent sup­pli­ers, which makes order­ing easy.

I built the above design a couple of months ago.

And this oth­er design last week.

For this most recent RAM build, I ordered this sur­face mount lay­out, prin­ted at OSH­park. Com­pon­ents came from DigiKey.

I thought I was going to be cool and have some nifty black boards, but I did­n’t look at the PCB design and descrip­tion closely enough.

If you look closely, you’ll see that the sub­strate is indeed black, and the solder mask is indeed clear, but I’d neg­lected to notice that the excess cop­per lay­er was­n’t removed in the ori­gin­al board lay­out. So the black sub­strate was covered by the cop­per, gleam­ing nicely through the clear solder mask 🙂

I think the cop­per look is grow­ing on me 🙂

On to building them.

The first step is to clean up the boards. There are a few rough points where the boards were attached to the main pro­duc­tion boards. A quick file of the ‘nibs’ made the edges smooth. Then clean with iso­p­ro­pyl alcohol.

Sur­face mount sol­der­ing is pretty easy once you get the hang of it (great video on it here), and I had no prob­lem with the integ­rated cir­cuit chips.

The res­ist­ors capa­cit­ors were anoth­er story. This was the first time I’d tried res­ist­ors capa­cit­ors. The silly things are so tiny they just would­n’t stay put as I soldered them down.

Even­tu­ally I did man­age to get all four on, though I’d not call it my best work. As it turned out after test­ing, one end of one res­ist­or capa­cit­or had a cold solder joint and I had to reflow that end.

And finally the pins. The board design­ers used an innov­at­ive meth­od of sim­pli­fy­ing pin install­a­tion which is bet­ter detailed in this post. It worked quite well.

As did the RAM! After a clean­ing off the solder res­in paste and installing into my NEC PC-8201a, I was happy to see 16k more memory avail­able in the first RAM bank. At least on first blush.

After a few tests, it appeared that one RAM unit was­n’t quite right. Inspec­tion using the Mark One Eye­ball revealed a couple of solder joints that could use a bit more work, so I reflowed all solder joints just to be sure.

End res­ult — quite satisfying.







5 responses to “I Made some RAM for my NEC PC-8201a — and it’ll work on a Tandy Model 100 too!”

  1. CloudInMyHead Avatar

    I just ran across this post and I am amazed on well it turned out and how well it looks. The sur­face mount and the chips legs have me wor­ried but I hgave seen You­Tube videos where they explain it well and they are successful.
    Great work and thanks for shar­ing. I do not have my NEC PC-8201A yet as it is arriv­ing next week INOP. Hope­fully I can get it run­ning and indulge in this upgrade.

    1. brad Avatar

      I too was con­cerned about sol­der­ing sur­face mount com­pon­ents, but tak­ing it slow and watch­ing a lot of videos about it helped set me up for what to expect. Good luck with your NEC — it’s kinda fun/frustrating to track down issues but there’s a good com­munity out there focused on the device. Not sure if you found it yet, but this Dis­cord group is pretty respons­ive: https://discord.gg/vXYCAhSZVH

      Good luck with it!

  2. CloudInMyHead Avatar

    Get­ting 404 errors on the links to the PCB boards.
    Can you send me or update the link?

    1. brad Avatar

      Sorry about the delay get­ting back to you — this blog isn’t send­ing me noti­fic­a­tions when com­ments are left.. must fix that.

      Also, it looks like the author mod­i­fied or moved the design I linked to. Here’s a cur­rent one: https://oshpark.com/projects/jX6Yde5B/view_design

      1. brad Avatar

        If that does­n’t work, try this: https://github.com/bkw777/Model_T_RAM

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