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Atari 65XE RAM upgrade to 128K

17th Sep 2017

I've recently been looking for something to do with my 65XE and came across some information which indicated that the 65XE can be upgraded in several different ways. One of the easier ones seemed to be RAM - so I've started with that.
There are two options: Fit an additional 64K RAM for a total of 128K or purchase a module from Lotharek which replaces the CPU with a PCB which has a replacement CPU and RAM. I'd prefer (for now) not removing the existing CPU so I've gone for the 128K option.

As the processor in the 65XE is a C014806C-29 (6502C - a derivative of the MOSTek 6502) it can only address 64K of RAM/ROM at a time. This means that the 65XE must have to page the extra RAM which is why an EMMU chip is required to handle this.

Tools & materials

Pliers
Screw Driver
Soldering Iron
CMOSTTLDesigner
Philips/Cross head screw driver
Weller WSD 81i solder station
Basic set of snub nose plaiers
Philips/cross head screw driver
Weller WSD 81i 80 watt solder station. A cheaper fixed temperature iron can be used of course
Solder
EMMU chip
RAM chips
Solder
C025953-20 EMMU chip
Hitachi HM4864P-3 chip
Multicomp 60/40 / Lead free / 0.5mm / Rosin, 250G, O53C990358
1x EMMU chip
8x Hitachi HM4864P-3 RAM chips
Solder Sucker
Flux Pen
Solder braid
Solder Sucker
Flux Pen
Solder Braid
Solder Sucker - or desolder station if you're rich
Flux of some kind. In this case a NoClean flux pen
Good quality solder braid. Do not go cheap on this otherwise you'll regret it
Capacitors
Solder Sucker
8x 100nF (0.1uF) ceramic capacitors


Taking the computer apart
Taking the 65XE apart is quite easy. Four screws need to be removed from the underside, the computer turned over so the keyboard is facing up and then the top plastic part of the case pulled off.

Atari 65XE RAM upgrade, external case, rear Atari 65XE RAM upgrade, external case, top Atari 65XE RAM upgrade, external case top removed
(click an image to enlarge)

Next, the keyboard needs to be lifted up a short distance at the front so that the ribbon cable connecting it to the 65XE can be unplugged. Like all edge connector/ribbon cables, you need to be gentle doing this, rocking the cable from side to side and pulling up until it comes free.

Atari 65XE RAM upgrade, removing EMI shielding Atari 65XE RAM upgrade, removing EMI shielding
(click an image to enlarge)

Several metal "clips" are twisted so as to prevent the top metal EMI shielding from coming off. I used a set of pliers to straighten these so that the shielding could be removed. There are six of these clips : Two at the front, one on each side, and two on the back of the shielding. Also, remove the screws near the clips.

Atari 65XE RAM upgrade, removing EMI shielding Atari 65XE RAM upgrade, removing EMI shielding Atari 65XE RAM upgrade, removing EMI shielding Atari 65XE RAM upgrade, removing EMI shielding
(click an image to enlarge)

The mainboard PCB can now be seen. The existing 64K RAM is located on the left hand side of the board in the form of 8 ICs (chips). Mine are marked NEC D4164C-15. I couldn't find a datasheet for these ICs, but I can surmise that they at 64kbit (65,536 bit) x1 DRAM with a maximum access speed of 150ns.
Getting hold of just one of these was relatively expensive, let alone eight of them. The good news is that a alternatives can be purchased as long as they fit the above criteria of 65,636 bit x1, are DRAM, and have an acess speed of 120ns or less. In my case I've purchased eight Hitachi HM4864P-3 ICs, although I subsequently found out that the these RAM ICs have an access speed of 200ns. Well, I can test and see if it works and if not then try to see if I can get then HM4864-2 version.
To the right of the existing RAM ICs can be found eight IC positions - just solder-filled holes where ICs can be soldered - marked U26 to U33.

Atari 65XE RAM upgrade, showing RAM bank positions on PCB and EMMU position
(click image to enlarge)

RAM - Removing solder from the holes
This is the most onerous part of the project: Removing the solder which currently fills the leg holes of where the RAM ICs need to go. Ideally this can be done with a deoslder station - something which I don't have. As such, I have to use a solder sucker/pump and some good quality solder wick. This is all about perserverance and not burning the PCB. Experience has shown me that setting my temperature controlled soldering iron to 350 degrees C, holding the flat of the tip to the solder hole for 3-5 seconds and then immediately using the solder sucker to suck the molten sodler away work *most* of the time. With a few of them this simply doesn't work and so I use a combination of melting more solder into the hole, which causes the sodler further in to melt, and then using solder wick to absorb it all.
Once the IC positions have been desoldered, go ahead and do the same for the capacitor positions C113 to C120 - each one positioned to the left of one of the new RAM positions U26 to U33 which have just been desoldered.

RAM - Soldering in IC sockets and capacitors
I could just solder in the new HM4864-3 RAMs directly into the (now solder-free) positions on the mainboard PCB, but to do so have several disadvantages:
- If any of the RAM ICs is (or becomes) faulty then you have to desolder it from the board
- ICs of any kind can be quite sensitive to temperature and if the soldering iron is left on one of the legs for too long them damage can occur to the RAM IC.
Of course, the down side of using IC sockets is that they are not super cheap - especially if, like me, you use the turned pin variety rather than the older style ones. I got eight 16 pin turned pin IC sockets from Farnell at the cost of £1.25 + VAT (£1.50) each. Total £10+VAT (£12). The make/part number is Multicomp/2227MC-16-03-09-F1. Soldering them in is very easy as you'd expect. To keep them straight while being soldered you simply use some masking tape, solder one pin at a corner in then one in the opposite corner. Check the socket is still straight and if not then desolder one pin and shift it, then resolder it. Once down, solder the rest of the pins.
I do recommend coating the leg hole sin the board in flux just before solderijng the socket. This cleans the contacts and makes things much easier and gives better solder joints.

Atari 65XE RAM upgrade, showing IC sockets soldered to PCB Atari 65XE RAM upgrade, showing IC sockets & capacitors soldered to PCB
(click an image to enlarge)

EMMU
The EMMU IC (marked C025953-20) is necessary as it makes paging between the two banks of 64K possible. I got mine from atarifreakz on ebay for the sum of £2.99 + £1.50 postage.
The EMMU needs to be fitted to the IC position marked as U34 which is to the right of U27. Curently, you will find three zero ohm value resistors fitted:
- One goes from pin 2 to pin 17
- One goes from pin 6 to pin 15
- One goes from pin 16 to a pad just above, however this is simply joined to pin 1 of U34.
These three resistors should be desoldered and removed (and the other holes desoldered while you're there). A 20 pin IC socket needs to be soldered in its place. As I don't have a spare 20 pin one I'm improvising by fitting a 16 pin one and then the back 4 pins of a spare 8 pin one to make up the extra 4 pins. Although this may look a bit odd, it does sort of fit consdering that the EMMU IC is actually on 16 pins and only occupies pins 1 to 8 and 13 to 20 - i.e. the last 4 pins to the right are not used. I could just not bother soildering anything in these holes and just soldering in the 16 pin socket, but I'm adding the extra 4 pins just in case they are ever needed in the future.

Atari 65XE RAM upgrade, showing resistors to be removed and EMMU position on PCB Atari 65XE RAM upgrade, showing  EMMU socket on PCB Atari 65XE RAM upgrade, showing resistors to be removed and EMMU position on PCB
(click an image to enlarge)

Fitting the RAM and EMMU ICs
Fitting the RAM and EMMU ICs is the easiest part: Just remember to fit them the correct way around with the notch in the ICs facing to the left - just like all the other ICs on the mainboard. Also, remember to wear an ESD grounding strap or at least ground yourself prior to handling the RAM and EMMU ICs.

Atari 65XE RAM upgrade, showing resistors to be removed and EMMU position on PCB
(click image to enlarge)

Testing
A good test is simply to turn the 65XE on. If you get a red screen (or nothing at all) then quickly turn the machine off and go back and check that the ICs are in the right way around (facing left), that they are well seated (maybe reseat them even if the look good) with no legs misaligned/sticking out and also (using a magnifier of some type) check all of your soldering. If in doubt about a solder joint simply use the solder wick to remove the solder entirely (remembering to first remove the RAM IC), apply some flux, resolder the joint and then refit the RAM IC. Never just try reheating the joint if you can help it.
Now, if you get to the BASIC prompt then indications all is well. Note that you cannot use the inbuilt memory diagnostics as this only tests the first 64K. The are some RAM testing applications available for download and you can always download a 128K rated game and see what happens.

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