NEC PC-9821Ap/U2 (PC-9800, 1993)


When it comes to PC-9800 computers, the "A-Mate" (9821A) line can be both amazing and horrifying. They're great for anyone wanting to play DOS games, they're horrible for anyone who hates repair work. This is the story of a very cheap 9821Ap which I found tested, booting, but never maintained.


This is the PC-9821Ap/U2. It's got an i486DX2/66, 3.6MB of RAM, integrated FM (PC-9801-86) audio, two 3.5" FDDs, and gosh knows what else.

The two 3.5" FDDs are what really makes it a /U2. The other "U" configurations, /U7 or /U9, would only have a single 3.5" drive. The /M2 configuration would have two 5.25" drives, /M7 would only have one.

By pressing the two tabs on the sides and pulling out, the front bezel pops off. This exposes the ports, "File Bay" for expansion devices, floppy drives, speaker, lights, and hard drive sled. Wait, hard drive sled?

Pull straight out on that wire handle (if you have one) to expose your hard disk sled. This is a PC-9821A-E05, a sled which gives you IDE support! Pretty neat.

If you want to open the drive sled, there are some screws on the handle side and the edge connector side.

A NEC D3765 (177MB IDE) disk is inside my drive sled. I won't be keeping it in, I don't trust old spinning disks very much, but I will be attempting to image the disk for experimentation later on.

This is a good time to bring up a limitation of these A-Mate systems: The NEC PIO interface in these earlier IDE 9821s can only address disks up to 543MB in size. Anything bigger will make it throw a fit.

This is also a good time to bring up a solution to these limitations: Sampson on nfggames has patch files available for disk support up to 120GB.

If you're wondering what the bare sled looks like, it ain't much. Just that little board, an IDE cable, and a power cable. Nice and simple.

Here's the backside of the computer. We have, from right to left, another expansion board connector (with a SCSI card in it!), power stuff, the centronics port, line in/out for the 86, the CN50 external FDD connector, our serial port, and some sort of board attached to the analog RGB port?

You can also see more confirmation that this is a /U2 and, neat thing, was made in August of '83. Good stuff.

Here's that SCSI card. It's a PC-9821A-E10, by NEC, which is basically the same thing as a PC-9801-92

It's not a horrible board, they removed the things like the "NEC check" of the PC-9801-55 (the first three letters of your disk manufacturer identifier had to say "NEC" or it would reject it, rude), but it's not the greatest SCSI card out there.

Someone must've gotten quite hungry as a bite of one of the SIPs on the right is missing.

Here's the board which was connected to the display out connector. It's a PC-9801-E02 High-Resolution board which connects through the 32-bit NESA Local Bus, not the usual 16-bit C-Bus, and lets you push 1150x750 (I think?) video similar to the PC-H98 series.

I don't honestly know anything which uses this apart from CAD software but it is pretty neat.


Disassembly

Tearing down a 9821Ap isn't too hard.

You'll first need to remove the upper cover of the system. Remove the two screws on the sides of the system. Also remove the two screws securing the upper cover to the backplate along the top edge.

Now, looking down at the system, we can see a few "areas".

Firstly, we'll need to remove the backplate. Simply remove the rest of the screws on the back then pull straight off. See the first image in this section for which ones I'm talking about.

Any C-Bus cards need to be removed. Empty slots with slot covers don't need to be touched, you can leave the slot covers on, as these don't mechanically engage with the card cage itself.

We'll remove the power supply next. See those three screws on the metal tabs around the main rectangle itself? Take them out. Once done, pull straight up on the power supply. It should lift out with little to no force.

After the power supply, we need to open the CPU cage. Start by removing the two screws with a black plastic housing. Lift the top of the cage off.

To remove the CPU card, lift the plastic ejector tabs upwards and it'll pop right out.

To remove the RAM card, grip the metal top plate and lift straight up.

To remove the CPU cage itself, there are four screws you need to take out. Two are in-frame, on the left. Two are just out of frame to the right. I'll update this with a better picture during reassembly.

Make sure you unplug the front LEDs and speaker from the motherboard before lifting the CPU cage up and out of the system!

We'll unscrew the two remanining screws holding the C-Bus cage in next.

These at an awkward angle for some screwdrivers. If you have something short enough to fit vertically in the cage, or with a long enough shaft to travel through both sets of cutouts, then great! If not, I've found that unscrewing these at a slight angle is usually fine.

The C-Bus backplane is held in using a card edge connector. It may take a little force but, if all four screws are out, it will lift out without issue.

You'll see the audio daughterboard under the C-Bus cage. Simply grab it and lift straight up to remove it.

Unscrew these two screws on the right side of the remaining floppy/hard disk mounting bracket/cage assembly.

Also unscrew these two screws underneath the hard disk sled.

The same as it was for the C-Bus cage, you may need a notably short or long screwdriver to get these out. I just use a fairly standard-length thing at a slight angle and it seemingly works fine.

You can now pull the whole Floppy/Hard Disk assembly out.

Congratulations, you've disassembled your 9821Ap!

If you want to remove the motherboard from the baseplate, simply remove any remaining screws you see.

If you're wondering how many screws you should have, here's an image.


So it turns out the person who recapped this before didn't really do any cleaning. I went in with a Q-Tip and oh look at that, this cap popped right off of the board! My Q-Tip has a bunch of green on it.

Remember, always clean your board before you go putting new components down on it. I guess I'll be having to properly revive this board now.

(See how the other cap in-frame is lifted off the board? Heh.)


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