back2roots/padua

home *** CD-ROM | disk | FTP | other *** search

/ back2roots/padua / padua.7z / padua / text / Hardware.Tricks < prev next >

Wrap

Internet Message Format | 2014-05-19 | 30KB

From bit.listserv.i-amiga Thu Mar 26 15:15:59 1992 Path: cs.tu-berlin.de!math.fu-berlin.de!fauern!ira.uka.de!yale.edu!jvnc.net!darwin.sura.net!wupost!sdd.hp.com!uakari.primate.wisc.edu!usenet.coe.montana.edu!news.u.washington.edu!milton.u.washington.edu!england From: england@milton.u.washington.edu (Gregory Miller) Newsgroups: bit.listserv.i-amiga Subject: Amiga hardware hacks Message-ID: <1992Mar24.205020.8620@u.washington.edu> Date: 24 Mar 92 20:50:20 GMT Sender: news@u.washington.edu (USENET News System) Organization: University of Washington, Seattle Lines: 496 Here's a text file describing various hardware hacks and fixes you can do to your Amiga. (YES!! It includes complete instructions on how to enable a fatter Agnus!) (on revision 5 AND revsion 6a boards!!!!) Standard disclaimer applies of course!!! == == == From - lae@mullauna.cs.mu.oz.au (Lewin EDWARDS) == == == -= A DIY Guide to Computer Repair & Modification (PC and Amiga 500) =- [Note : To people who saved my earlier post re: installation of the 1Mb Agnus, discard it and get a copy of this instead; the 1Mb Agnus installation procedure is included here] -=<* About This Document - Disclaimer *>=- This document is intended to provide electronics hobbyists with some useful notes the author has gathered whilst working on various systems. It is not intended to train or encourage novices to attempt service or modifications to expensive appliances. It is especially not intended to encourage unqualified personnel to tamper with dangerous appliances such as monitors, power supplies and other devices which contain vacuum tubes, live mains or high-tension rails, or other hazardous areas. Any such work carried out is done at the reader's own risk and the author accepts no reponsibility for damage to property or person which occurs as a result of attempting any of the procedures described herein. Having said that, I can vouch for the workability of the modifications listed here. I have personally performed them all myself on my own machines (this is, after all, a chronicle of my own experiences, chopped up under suitable headings for your convenience). With regards to the service information, whilst I have described problems and the solution(s) I found, keep in mind that this is rather like a doctor prescribing medication over the telephone or from an encyclopaedia. What I mean by this statement is that different problems may well cause the same symptoms; to take a trivial example, if you turn your monitor on and nothing happens, perhaps the fuse is blown, but on the other hand, perhaps it is not plugged in, or perhaps there is a more sinister fault in the power supply section. For this reason, I suggest you use the service information as a very vague guide ONLY. Remember; it is very rare to find two machines which are exactly the same inside, and so what I have found to be a suitable solution to a given fault may be totally inappropriate to your situation. -=<* The Commodore Amiga 500 *>=- i. Serial Killer ---------------- Problems with Amiga serial ports generally fall into three categories; problems with the flow control lines, problems with the actual data transmission lines, and problems with the line level (caused by faulty line driver chips). The first two can be relatively costly to repair, due to Commodore's monopoly on the CIA and Paula chips, so I advise exploring the line driver section of the circuit before rushing out to buy new Paulas or CIAs. Faults in the line driver chips (MC1488 and MC1489, U38 and U39 respectively in the schematic on page F-7 of the book 'Introduction to the Commodore Amiga 500') are typified by either total loss of transmission, total loss of reception, or both. If you look at the flow control lines, (CTS and RTS), you usually see that these lines are behaving normally (exactly why these lines are rarely affected is a mystery to me); if not, then you probably have a faulty CIA. Since the 1488 and 1489 only cost around A$1.00 each, depending on your source, it is well worth replacing these chips before you investigate Paula. Unfortunately, they are not socketed so you will have to desolder them (see "Desoldering Small IC's" below). When you have the chips out, put in a couple of sockets to facilitate later repairs. If, after replacing the 148x's, you still experience problems, it is time to look further. If you have suitable software, you can check whether the problem lies in the flow control lines (in which case you have a damaged even CIA, U8) or in the data lines (in which case you have a damaged Paula). If you do not have, or are unable to interpret the results of, this software, you can make use of an old Amiga-tech wrinkle; exchange the CIA chips and see if your fault clears up. If it does, then ten to one the problem is in the CIA which started out as U8; if the problem persists, then it is most likely a Paula fault. ii. Floppy Hassles ------------------ In my life I have opened at least 40 Amiga 500's and have NEVER seen two which have the same make and model of internal floppy. It seems that Commodore change their suppliers (and also the colors of the LEDs on the keyboard) on a weekly basis. Let's see.. three possible LED colors (red, green, yellow), three LEDs on the keyboard, four floppy manufacturers (Sankyo, Chinon, Panasonic, Sony), two different styles of head connector (one film connector per head or one wide film connector servicing both heads), helical screw or belt-drive head carriage mechanisms.. that makes 144 possible configurations already. For this reason it is not possible to give specific "look-to-the-left- of-the-large-chip" directions, and I can only state a couple of possible faults and their possible causes : * Floppy refuses to acknowledge disk insert Probably caused by a defective odd CIA (U7). Try switching the CIAs over. Also check that the switches at the front of the drive travel freely up and down. Verify that the pads to which the switch assembly is connected have not flaked off the board (this can be caused by rough disk insertion). * Floppy will not read disks formatted in other drives The head alignment is faulty. There is a method of adjusting it without special tools and measurement devices, but it requires considerable patience and one of the X-Copy series of programs. The method is as follows : - Format a disk in a working drive. - Open your machine (or external drive). Remove the cover from the drive unit (it is usually held on by one screw on either side and another on top at the back). Locate the track zero sensor and loosen the screw which holds it in position. Load X-Copy and select the Toolkit option CHECKDISK. - You must now align the lower head. This can be accomplished by moving the track zero sensor a little, then checking the disk, and repeating this until you find a position in which the drive will read the lower side of the disk. When this is done, tighten the screw which holds the track zero sensor, to stop it shifting. If you have any Lok-Tite, a drop of that on the screw is advisable. - You must now align the upper head. Loosen the two screws which hold it to the head carriage assembly (Do NOT remove these screws, unless you relish the idea of going over the floor with a metal detector looking for small springs). Adjust the upper head by eye until it appears to be exactly over the lower head. Now repeat the above trial-and-error process, tweaking the upper head around until you find a workable position. Tighten the screws, replace the cover and reassemble. If the above procedure sounds rigmarolish and uncertain, it is ! But I have successfully performed it on a number of Amiga and PC 3.5" drives. And I have never had a failure with it. iii. Caps Lock Chaos -------------------- Many Amiga owners will know, to their detriment, that occasionally the Caps Lock light will start blinking and the keyboard will lock up. This is caused by the microprocessor in the keyboard detecting a keyboard fault. I first noticed this fault when our school Amiga room (two rev. 5a A500's) acquired an autofire joystick, a Quickshot IIb I think. When switched to autofire mode, this joystick would periodically jam the keyboard. If you have recently acquired an autofire joystick, you should investigate whether turning it off during boot fixes the problem. Otherwise, you can try the old "Atari Twist", ie pick up your machine in both hands and twist it. This is basically equivalent to reseating the chips on the motherboard; if you have a spare moment, I suggest you actually open the machine up and manually reseat them. iv. Bulimia of the Agnus ------------------------ iv-1. What is the 1Mb Agnus ? ----------------------------- The 1Mb Agnus (variously known as the Obese Agnus or Fatter Agnus), Commodore part 8372a or 318069-02, is the relatively new Agnus which is capable of addressing 1Mb of chip-RAM. This allows you to have twice the amount of sampled music, screens, and other custom-chip data in memory simultaneously. It also has the desirable effect of letting you run things like Deluxe Paint in 16-color 640x512 (or 640x400, NTSC users) mode, without having to disconnect external drives, close down WorkBench screens and the like. For A-Max users, installing the new Agnus and configuring it to 1Mb chip-RAM mode will also increase A-Max compatibility. If you have more than 1Mb of RAM, then it will make A-Max virtually 100% identical to a Mac Plus, memory-map-wise. NOTE WELL : This modification requires 1Mb of RAM to be visible ! Commodore, not wishing to make two chips where one will do, is now shipping the 8372a in most (nearly all Rev. 6a) A500's, without telling anybody. In these machines, the extra chip-RAM capability has been disabled so that if you connect an A501, it will still think it is "slow" [see below] RAM. iv-2. An Aside on Memory Speeds ------------------------------- The Amiga has *three* types of RAM. Chip-RAM, Fast-RAM and "Slow"-RAM. Contrary to what many people assert, RAM at $C00000 (the default location of the A501 expander, and the second 512K of RAM in A2000's) is NOT fast-RAM, even though the system reports it as such. Because this RAM is refreshed and accessed via Agnus, it is affected by buss contention in just the same way as chip-RAM. It is simply a dead area in RAM which is not accessed by the custom chips and which is not faster than chip-RAM. Moving this RAM to $80000 (which is what you do to enable 1Mb chip-RAM) will have no effect on machine speed at all. iv-3. Compatibility ------------------- To date (20-Oct-91, 18:11EST) I have personally tested at least 400 pieces of software with the new Agnus. I have found three programs which do not work : * Double Dragon II (Completely screwed up) * VideoFX 3D (Again, completely useless) * Impossible Mission II (This game requires memory at $C00000. It does not mind the Agnus, but if you want to run it you must drop back to 512K chip/512K slow mode. Lousy AllocAbs() programmer !) In addition, Dragon Ninja thinks you are running on a 512K machine and will not play in-game music. Again, this can be rectified by switching into 512K chip/512K slow mode. Some other games also think they are running on a 512K machine, notably TechnoCop and R-Type, but this is of no real consequence. iv-4. Identifying the Version of your A500 ------------------------------------------ There are several versions of the A500 out there. I have only tried fitting the new Agnus to the following : a. The Revision 5 board shipped until early 1989 b. The new Revision 6a board supplied from May (?) 1989 onwards The way to identify your machine type is to check the expansion port which lurks under a plastic cover on the left-hand side of the computer. The Rev. 5 machine has a very fat strip (double width) at the back of the connector; the Rev. 6a board has two normal-width strips instead. iv-5. Installing the 8372a to Revision 5 A500's ----------------------------------------------- 1. Remove the 8371 from its socket. If you are an NTSC user who does not wish to use the PAL mode, go to step 3. 2. Pin 41 of the new Agnus controls whether the machine starts up in PAL or NTSC modes. When it is tied low (to ground), the machine boots in NTSC mode. When the line is tied high or left floating (ie unconnected), it boots into PAL mode. On the Rev. 5 board, pin 41 is connected to ground by tracks on BOTH sides of the board, making it virtually impossible to cut the track without desoldering the socket. This means if you just plug in the new Agnus, you will be locked in NTSC mode !! There are a couple of ways to go about circumventing this problem; I will detail here the one I undertook on my own Rev. 5 board. I will assume you want to install some sort of switch to toggle PAL/NTSC mode. If you only want to run in PAL mode, you can simply bend out pin 41 of the Agnus socket (don't bend the pin on the chip) to prevent it from making contact with Agnus. If you do this, proceed to step 3. Otherwise, get yourself an SPDT or SPST toggle switch and two lengths of telephone (or other thin, stiff single-core wire). Solder wires to the switch : +---+ To a convenient ground--+-o | BACK VIEW OF SWITCH +--------+-o | When held as shown, UP=PAL, DOWN=NTSC. | | o | | +---+ Uninsulated --> | | <-- This side is insulated \-/ <-- Fold the wire thru 180 degrees 3. Insert the new Agnus. If installing the switch, you should now poke the loose end of wire (see above) from the switch into pin 41 of the socket so that the bare copper side touches the pin on the chip and the insulated side touches the pin on the socket. In order to fit the wire down the hole you may find it necessary to bend out or remove entirely [Not recommended !] the pin on the socket. Secure the wire with a tiny drop of cyanoacrylate adhesive. 4. If you wish to install a memory mode switch, go to step 5. Otherwise, locate JP2 (between 68000 and ROM). This consists of three pads. The center and lower pads are joined by a thin track. Cut this track and place a drop of solder between the center and upper pads. Now locate the _EXRAM line coming from the A501 port (it comes from pin #32 and runs on the upper side of the motherboard parallel to the A501 connector). Cut this line. You have now completed the installation procedure. 5. Complicated. Locate JP2 and cut the track which joins the center and lower pads. Locate the _EXRAM line from the A501 port (see step 4) and cut the track. Solder wires to all three pads of JP2 and one wire to either side of the gap you cut in the _EXRAM track. Wire a DPDT switch as follows : +-----+ BACK VIEW OF SWITCH Upper pad of JP2 --+-o o | When held as shown, UP=512K CHIP, DOWN=1M CHIP Center pad of JP2 -+-o o-+- To one side of the gap in the _EXRAM track Lower pad of JP2 --+-o o-+- To the other side of the gap +-----+ You have now completed the installation procedure. iv-6. Installing the 8372a to Revision 6a A500's ------------------------------------------------ 1. Remove the 8371 from its socket. Insert the new 8372a. NTSC users who do not wish to use the PAL mode should go to step 3. When installing the new Agnus, note that on the 6a board, the Agnus socket has been rotated 90 degrees anti-clockwise from the orientation on the revision 5 board. For this reason, plugging in the Agnus correctly is no longer an intuitive operation ! 2. JP4 controls PAL/NTSC video mode. If you are in North America or Hong Kong or anywhere else the National Television Standards Committee TV format is used, this should be bridged by a track or a blob of solder. If you are in Australia, Europe or anywhere else the Phase-Alternated Linescan TV format is used, this jumper should be cut open. If you wish to install a switch, then simply connect it across the two pads of the jumper. 3. Cut the track joining the center and upper pads of JP2, and the track joining the center and upper pads of JP7a. (JP2 is between the CPU and the ROM and JP7a is just to the left of the A501 connector). If you wish to install a memory switch, proceed to step 4 otherwise solder the center and lower pads of JP2 together. You have now completed the installation procedure. 4. Connect a DPDT switch as follows : +-----+ BACK VIEW OF SWITCH Upper pad of JP2 --+-o o | When held as shown, UP=512K CHIP, DOWN=1M CHIP Center pad of JP2 -+-o o-+-- Center pad of JP7a Lower pad of JP2 --+-o o-+-- Upper pad of JP7a +-----+ You have now completed the installation procedure. v. Is My Jumper On The Line ? ------------------------------ Number Rev5 ? Rev6a ? Function ------------------------------------------------------------------------------- JP1 yes yes Connects reset timer (NE555) to main reset JP2 yes yes Maps A501 at $C00000 or $80000 JP3 yes yes Swaps _CAS of RAM banks (of use for test purposes) JP4 no yes 8371 Agnus : TEST line [solder closed !!] 8372a Agnus : PAL/NTSC video mode select JP5 no yes 28MHz base clock to Agnus JP6 no yes 7MHz clock on pin 7 of expansion connector JP7a no yes Controls _EXRAM to Gary JP7b no yes CIA tick from VSYNC or pin 32 of A501 (default VSYNC) JP8 no yes LPEN from FIRE0 or FIRE1 JP9 no no ??? A non-existent jumper. Future Rev. 7 ??? JP10a no yes Connects AUDIN to pin 18 of serial port JP10b no yes Connects AUDOUT to pin 11 of RS232 port JP11 no yes Connects CSYNC to BCSYNC or video hybrid ------------------------------------------------------------------------------- NB: These are A500 jumpers only. A2000, 2500, 3000 have different jumper assignments. The 1000 *may* have similar settings but I cannot guarantee this. vi. Hol(e)y, Hol(e)y, Hol(e)y ----------------------------- Owners of Rev. 6a A500's will notice that they only have 4 RAM chips as opposed to the 16 in the Rev. 5 and earlier machines. They will also note that in between the RAM chips are empty sets of holes which look like something ought to go into them. Firstly, fear not - the reason you have a quarter the number of RAM chips as older machines is that the 6a uses 44256 4-bit x 256K chips which are four times as large as the 41256 1-bit x 256K chips used in the 5 and earlier revisions. All the modern Amiga memory hardware (except possibly the A501) uses these larger chips because they are slightly cheaper on a dollar-per- megabyte basis. For example, the older Series I A590 hard drives use the 41256, and they will accept up to 2Mb. The new Series II A590's use the 44256 (and consequently have enough space to fit 8Mb of RAM). But, back to those extra holes. They are indeed designed to accept something; viz, another set of four 44256's to take your machine up to 1Mb without having to buy an A501 or other expander (this is particularly useful if you have a memory expansion which uses the A501 slot, and you want to have 1Mb chip-RAM). If you want to install your own RAM there, you should do as follows : Purchase four 20-pin IC sockets (preferably machined-pin). Make sure you have a solder-sniffer. Now look at those holes and you will see that they are clogged by solder (this is due to the way the boards are soldered; the process covers all exposed copper areas with solder). You must clean this solder out by heating the board from one side and sniffing the solder out on the other (or blowing the solder through; if you choose this method, make sure to remove all the little splashes of solder from the board and your lap). When you have cleaned out all the holes, solder in the sockets. Now plug in four 44256 chips and you have 1Mb of memory for slightly less than the cost of an A501 (in Australia, the difference is roughly A$20). With respect to the speed of RAM chips required, I use 150ns chips with no problems at all; however, I suspect that Commodore would recommend 100ns devices. The price difference is minimal so you may as well buy 100ns chips so that if you ever buy a series II A590, you can shift the chips over and have 100%-guaranteed-not-to-overheat fast-RAM. vii. 1Mb of Motherboard Memory on the Rev. 5 -------------------------------------------- It is possible to install 1Mb of RAM onto the revision 5 motherboard. However, this modification is flaky at best and I cannot recommend it (having tried it in a moment of sheer boredom). Also note that I could *not* get this extra memory to be mapped at $C00000 (the computer would only recognise it if I had JP2 set to map the A501 at $80000); which means that it is only of potential interest to owners of the 1MB Agnus. (This failure to work as $C0 RAM may well have been due to some stupid omission on my part). I will deliberately be vague because this is not a trivial modification. If you are unable to work out what to do with confidence, you do not have enough knowledge to perform the mod. The procedure can be summarised as follows: Take sixteen 41256 RAM chips (for speed information, see the end of subsection vi. above). Bend the refresh address strobe pins (_RAS, pin 4) on all the chips up and over so that it is well clear of the other pins and easily accessible from the top of the chip. Bend in all the other pins a little. Now, piggyback the new chips on top of the existing RAM, making sure to install them in the same orientation as the original chips. It is not possible to reach all the pins to solder them, so you will have to content yourself with only soldering the corner pins. Solder wires from the bent-up _RAS pins to pin #38 of the A501 port (or any convenient take-off point of this signal). You should now have 1Mb of RAM. (If you want to try using it as $C00000 RAM, you should ground the _EXRAM pin, pin #32, of the A501 port. I had no success with this). This mod sounds all very well, but in the current climate it may be slightly cheaper to buy a secondhand A501 than to buy 41256 chips. In addition, due to the precarious hold the piggybacked chips have on the originals (because there isn't enough room to solder all of the pins down), the mod is sensitive to shock; if you tap the table, you can sometimes guru the machine. For this reason, this is a "possible but ill-advised" modification. viii. Double Trouble -------------------- The hack to accelerate your Amiga to 14MHz has been published in countless articles, but I may as well include a brief description of it here. The hack involves removing the original 68000 from your Amiga and substituting a version rated at 16MHz (confusingly called the 68000P-12; the Motorola chips have "16MHz" stamped on them, but the SGS-Thompson clones do not). The clock pin is bent out and soldered to the output of a divide-by-2 counter whose input is connected to the base 28MHz clock. THIS MODIFICATION SHOULD BE INSTALLED WITH A SWITCH !! If not, you WILL (not may) find that some games will refuse to load because they use timing-dependent loaders. Aside from the problem mentioned above, the only compatibility down-side to this modification is that some floppy drives will not handle the increased step rate. If you find that your drives give read/write errors at the high speed, you can either replace them or run a program called "DFDelay" to slow down the step rate. (If df0: will not work, you have a problem. Try swapping df1: for df0: - to do which you will need a Commodore df1:. The mechanisms used in the old A1010 drives are EXTREMELY reliable and much quieter than the new rubbishy things; if you have an A1010, you can be fairly certain that it will support the new speed). To perform the mod, remove your old 68000 and embed it in a perspex block for future reference. Now rig half of a 74S74 dual D bistable as a divide-by-2 counter. Connect the input of this to a convenient take-off point of the 28MHz clock line on the motherboard (I use pin #34 of Agnus, but anywhere will do). Now bend out pin 15 of the new CPU and solder a wire to this pin. Take a piece of stiff telephone wire, strip approx. 5mm of insulation from one end, and stick this end down pin 15 of the 68000 socket. Insert the new CPU in the socket and verify that pin 15 is well clear of the wire in the socket. Now wire a SPDT switch as follows : BACK VIEW +---+ When held as shown UP=7.14MHz, To output of divide-by-2 counter -+-o | DOWN=14.28MHz To pin 15 of 68000 -+-o | To wire in pin 15 of 68000 socket -+-o | +---+ The 74S74 should be mounted somewhere convenient; I break off all the unused pins, bend the remaining pins out, and stick the chip on top of one of the data path IC's; this has the advantage of allowing me to take power directly off the existing components rather than stringing yet another pair of wires through the machine. If you only have chip and/or slow memory (see subsubsection iv-2.) then your machine will run at approximately 150% normal speed (on CPU-intensive operations; DMA rates will be unchanged). If you have true fast-RAM then code running from fast-RAM will execute at close to 200% of normal speed. Try placing a large perpective brush with anti-aliasing HIGH in DeluxePaint III (or IV !) and you will see the speed increase. ix. The Numbers Game -------------------- This is how PLCC chips are numbered (using Agnus as an example) : 1 1 8 8 8 8 8 7 7 7 7 7 1 0 9 8 7 6 5 4 3 2 1 4 3 2 1 0 9 8 7 6 5 BEVELED CORNER | | | | | | | | | | | | | | | | | | | | | /-------------------------------------------| 12 -| o |- 74 13 -| |- 73 14 -| |- 72 15 -| |- 71 16 -| |- 70 17 -| |- 69 18 -| |- 68 19 -| |- 67 20 -| 8372A |- 66 21 -| |- 65 22 -| |- 64 23 -| TOP VIEW |- 63 24 -| |- 62 25 -| |- 61 26 -| |- 60 27 -| |- 59 28 -| |- 58 29 -| |- 57 30 -| |- 56 31 -| |- 55 32 -| |- 54 |____________________________________________| | | | | | | | | | | | | | | | | | | | | | 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 x. A Change Of Face ------------------- Installing the ECS Denise is simplicity itself. Simply remove the old Denise and plug the new one straight in. Note that to use the new video modes, you require the 1Mb Agnus (not necessarily in 1Mb chip-RAM mode, though). By installing the new Denise, you also eliminate the annoying video glitches which occur in NTSC mode when using the new Agnus with the old Denise. xi. Snap, Crackle and Pop ------------------------- [For non-Australians, the above is the slogan a local cereal company uses to advertise a tasteless soggy mess called Rice Bubbles]. Symptom : Your monitor produces snapping or popping noises, and sometimes goes unaccountably blank after being switched on for a while. I have no idea why this problem seems to affect Commodore monitors so badly. It is caused by arcing of the EHT rail (around 14kV, from memory) to lower-voltage areas of the set. Initial treatment is to turn off the monitor, leave it for a couple of days (to allow capacitors to discharge), and then to remove every last speck of dust from the inside. This usually works. If it does not, however, sterner measures are called for. Open your monitor and (here comes the dangerous part) leave it running, open, for a few hours. If you actively work against Murphy's Law by watching it from behind something or inside something, where the monitor can't see you, you MAY eventually be lucky enough to actually catch it in the act of arcing. Mark the place with a spirit pen and spray on some anti-gaussing compound. (This stuff has a very high dielectric constant and hence helps to prevent arcs). xii. Yo Ho Ho And A Packet Of Disks ----------------------------------- I sometimes describe A-Max as the only useful productivity tool for the Amiga; Mac owners will agree with me when I say that Macintosh applications are incomparably better than their Amiga counterparts. However, due to suppression of the source of 128K ROMs by a certain computer mega-corporation named after a piece of dead plant, the future of this marvellous product looks slightly less certain than Melbourne hosting the 1996 Olympic Games (grin). There exists a software patch which will allow you to [illegally] run the emulator using a ROM image file on disk; however, if you wish to use a Mac drive with the A-Max cartridge, you must have ROMs or EPROMs in the ROM sockets. Problem ? No. Simply insert two blank 27512 64K x 8 EPROMs in the ROM sockets. NB: The above procedure is wholly illegal and the author is not explicitly condoning nor advocating the practise. I personally own original Apple 128K ROMs (purchased before the great ROM drought). The above information is supplied for interest's sake only.