SCRAM ASSEMBLY ============== WHAT IS INVOLVED If you are interested in the SCRAM 500 and would like to get one going, you need to know just what is involved. If you are going to build it yourself, you have to know how to solder! Count up the component pins on the main PCB - you will find there are more than 500! You will need a fine tipped soldering iron and fine multi-core solder. No big deal really, but don't attack the PCB with some great hulking 100 Watt Plumber's Friend. The RAM array is quite dense and care must be taken not to make solder bridges, but everything else is fairly routine. If you don't know how to solder, get someone good to give you some pointers and then practice a bit on some Radio Shack breadboard until you have the hang of it. You can buy a lot of resistors and breadboard for $5 and that will give you many hours of practice. When you feel confident, then proceed at will. An experienced technician could probably build a SCRAM 500 in 2 - 3 hours. If something goes wrong, a SCRAM board is not that hard to fix - it is fairly modular in design - the fault can usually be located visually. If you have access to a working SCRAM, chip swapping will resolve the problem in no time. With a full working set of schematics (provided on this disk) and PCB layout (also provided) the SCRAM 500 is not a mystery. The SCRAM 500 is easy to get going (excluding assembly errors). The production version of the SCRAM 500 currently enjoys a 0% return rate - simple, reliable technology. If the preceding paragraphs leave you perplexed and worried, take heart. One reason for releasing the SCRAM 500 design onto Public Domain was to encourage broad support for an Open Architecture SCSI/RAM peripheral in the Amiga community. Even if you have ten thumbs and two left feet, chances are that you know someone who is technical or you have access to a User Group or Computer Club that has a technical member. The SCRAM 500 can be a cheap way to upgrade your Amiga 500 - especially if you can provide some of your own parts and labour. The Naked SCRAM If you are the type of Amiga owner who likes wires, LEDs and PCBs hanging out of your machine then you will be happy to know that the SCRAM 500 will work fine bare. In fact if you run your Amiga bare or in a Rack Mount box or some other custom enclosure, then you won't need the Extender card stuff either. Just plug the SCRAM 500 card right onto the A500 expansion port. Note that you might have to install the 10K pullups on the Data Bus somewhere else (like in the empty locations on the A500 Main board). A naked SCRAM may cause some RFI - so be warned. MAIN PCB ASSEMBLY Start by installing all the sockets - GAL sockets, EPROM socket, PLCC socket. If you like, install sockets for all the glue chips - LS257, LS373, LS32, HCT393, F86, F521. The only CMOS device is the HCT393 which should be handled with care - so socketing this device (U26) may make life easier. TTL is pretty hard to blow up, so soldering it directly to the board is fairly safe. Next, solder in the RAM sockets. This is a tricky bit, and if you succeed here you should be OK with the rest of the assembly. If you run into problems, at least you won't have done any major damage and there is time to get some help. Only use a low powered soldering iron, and be carefull! Don't do any dry joints and watch for solder bridges. Fix each socket (or strip) in place by soldering a pin at either end. When all the sockets are in place, go down each column soldering every pin. Double check for unsoldered pins - in my experience this is the most common cause of handsoldered boards not working. Now solder in the TTL components - assuming you didn't socket them. Try not to solder any chips in upside down or back to front ( don't laugh I've seen it.) Solder in Resistors, R-SIPs, Electros, Bypass caps, and Jumper blocks. Avoid the temptation to solder in the LEDs now. They need to poke their noses over the front of the board by a critical dimension, so wait until you have the front panel screwed in place. This is the time to clean the PCB with FLUX CLEANER if you want to. This is important if using Acid core solder - ask someone who knows about this if unsure. NOTE - If using flux cleaner, make sure you don't let any get into the sockets on the component side of the card - this can lead to unreliable connections and extreme depression. Finally, you solder in place all the chunky bits. The DB25, toggle switch, and the 86 way edge connector. The first two are straightforward - if you like you could tack the DB25 to the PCB with Super Glue, but it is probably unnecessary. The 86 way connector is also easy, but TAKE NOTE! this connector mounts on the SOLDER side of the board. You will be doing your soldering on the COMPONENT side of the board. Don't make a mistake here or there will be much wailing and gnashing of teeth. Also note that you need to use this connector in the assembly of the extender card, so leave this operation till last. Now you can mount the back panel, the front panel and finally the LEDs. The LEDs look better if you put something to diffuse them on the back of the front panel. Matt drafting film works, so does that paper based translucent sticky-tape stuff from the chemist. Observe correct polarity for the LEDs - the long lead is positive and this is marked on the silk screen overlay. The SCRAM PCB should now be complete. Double check everything and visually inspect the board for:- o Unsoldered pins o Solder bridges (tiny slivers of solder between adjacent pins) o Dry joints (dull, misshapen, blobby solder joints) EXTENDER CARD Solder the 86 way connector to the non-gold fingers of the extender card. Note that there is a .1" tongue on this edge which locates the extender card in the back of the edge connector. You will have to bend the pins down and up a bit so they touch the PCB. This is a little tedious, but just use a pair of pointy-nose pliers and do them one at a time. If somone can think of a good way to do this painlessly, I'd like to hear about it. It is a good idea to assemble the extender card before fixing the 86 way connector to the main board. Use the two connectors as a jig to aid in assembly of the extender card. Plug the gold edge of the card into the spare 86 Way and slot the other end in between the pins of the one to be soldered. You will now have a mechanically solid unit to solder. Tin the pins and pads first, then push the pins down to the pad and reheat the joint. If you are lucky enough to have a willing accomplice, get them to hold the pins down with a screwdriver while you solder the pin to pad properly. You may need a slightly hotter soldering iron here, as the screwdriver will act as a heatsink. Solder the four end pins first to fix the connector in place. This gives you a chance to make sure the connector is straight. After soldering all the other pins (as above) check a few with the multi-meter and that's it. The last thing to do is to solder the resistor paks to the extender card. Note that if you intend to use the RFI shield to cover the extender, you will need to need to lay these resistor paks over on an angle (away from the gold) to get clearance. CASE The main PCB has two threaded spacers (12mm) which are screwed to the 86 way edge connector. Put a split washer under each to stop them turning. The SCRAM 500 should now just slide into the extruded case from the back with the threaded spacers lining up with the two holes alongside the machined slot. Use the two 14mm threaded hex posts to fix the RFI Shield to the case and also to fix the internal edge connector to the case. The extender card should now fit snugly into the RFI Shield, mating with the edge connector and sitting up against the threaded posts. MAKE SURE the extender card is right way up - resistors to the front and ground plane up. Fix the extender in place with two M3 screws and Presto - Finished! Note The current PCB does not make any connection to the mounting points of the case, front panel, and back panel. To completely seal RFI from the SCRAM, you may have to connect these with some wire to one of the spacers that go to the case and RFI shield. Also, you may have to check that the RFI fingers associated with the 500 expansion connector contact the RFI shield. PASS-THRU If you decide to try pass-thru you will need an extra 86 way connector and a little extender card. The 86 way is soldered piggy-back onto the back of the normal connector - on the component side of the board. Don't solder pins 11 and 12 but instead put a jumper from pin 11 on the SCRAM to pin 12 on the pass-thru connector. This will daisy-chain the AutoConfig signal to the subsequent boards. You might trim the legs of the pass-thru connector before soldering it to keep the height down. The pass-thru extender card can be plugged into the pass-thru connector after the SCRAM is installed in the case. This gives you an edge connector just like the Amiga 500, so you should be able to connect any expansion device to it - hopefully. PRELIMINARY TESTING Before applying power, do some preliminary testing of the board. A multimeter is all you need. Do the following tests (preferably with the extender card plugged in). o use the continuity tester to check all GND pins and +5 pins o measure resistance between GND and +5 - should be about 20-40 ohms o check a few point to point connections for 0 ohms Don't plug in RAMs,EPROM, Custom Chips or SCSI chip until the board has been powered up with the smoke test. If you pass this test, install the custom chips, set the RAM jumpers to 0M (both shorted) and if you get the AutoConfig LED (green) on and a working Amiga, then you are looking good - now you can test RAM. Be careful plugging ZIP RAMs in the sockets. Check that all pins are seated and none are splayed outside the socket. Double check that pin 1 is to the top of the board. Populate from Bank 0 -> 3. Make sure the 4M jumper is set correctly. There, easy! NJJ