Category - Amiga

Fitting an External, Boot – selectable Gotek Drive to an A500

6th March 2018

I wanted to avoid having to have one of these hanging off the back of my A500.

I wouldn’t really call this an upgrade as such, more taking advantage of the latest developments in the Amiga scene. In case you’ve never heard of it, the ‘Gotek’ drive is basically a 3.5″ Floppy Drive emulator. Instead of putting floppy disks in it you plug in a USB flash drive that can contain up to a thousands Amiga Disk Files or ‘ADF’s’ (floppy disk images).

There are quite a few versions of these floating around on eBay, internal and external. I decided pretty early on that I wanted mine external so I could keep my A500’s DF0 as a real floppy drive so I opted for an external drive. I wanted to have the install look as authentic and neat as possible so I opted to install my Gotek inside an old external floppy disk drive that I picked up off eBay for spares/repairs. If you want to do the same just be careful to pick one that houses a regular sized floppy drive and not a reduced height, slim one as the Gotek won’t fit. You can see what I mean from the photo below which shows my finished Gotek drive underneath a couple of Roctec slim drives – it should be pretty obvious that the slim drives don’t have the necessary height to accommodate the Gotek drive.

The other choices available now include the option to have an OLED screen that can display the name of the ADF file you select and the track info and also a built in speaker that can simulate the noises of the real thing. It’s more of a buzzer than anything else but it’s pretty effective and much better than sitting in silence waiting for Cannon Fodder to load!

Finished Gotek Drive under my two Roctec slim external floppy drives

Close-up of Gotek Drive with OLED Screen

Boot Selector Installed in Even CIA Socket

Mounting of Selector Switch

DF0 / DF1 selector switch

Amiga 500 Super Denise Upgrade

3rd March 2018

Steven

Amiga, How to's, Retro

19 Comments

The A500 machines shipped with a graphics chip called ‘Denise’ that is responsible for handling sprites and also all the different screen modes and resolutions you can use with Workbench and other apps. The later Amiga 500+ model sported an updated chip called ‘Super Denise’ which offered quite a few more screen modes and resolution options. The Amiga 500 Super Denise upgrade is a very straightforward modification which simply involves getting hold of the newer chip and replacing the old one with it. (It is an exact pin-for-pin replacement). For more detailed info on the Denise chip look here.

Getting hold of a new chip

The ‘Super Denise’ has the chip number 8373-R4 whereas the old regular ‘Denise’ is 8362. You can usually pick up Super Denise chips on eBay – here. I paid around £20 for mine which I though was fair… be patient and wait for one at a fair price – don’t get ripped off.

Removal & Replacement of the chips

You can remove the the old chip with just a small flat bladed screwdriver if you are careful. Just lever up each end slowly, alternating from one end to the other, making sure it lifts evenly from each corner so that the pins don’t bend. Ideally though, use an ‘IC Extractor’ – it makes the job a lot easier and is a useful tool to have in your retro computer toolkit anyway. Again these are readily and cheaply available on eBay – here. The new chip should just plug straight into the socket. Make sure the marked end of the chip matches up with the notched end of the IC socket. If it goes in the wrong way around it won’t work and may even damage the chip and/or the motherboard. If the pins don’t quite line up you can bend them gently into place using a pair of pliers.

Amiga 500 Super Denise Upgrade – In Pictures

Below you can see before and after screens showing SysInfo details, and available screenmodes along with the two different Denise chips fitted. The end result is a success with more screenmodes (eg Super-Hi Res modes) available straight away, even with my standard 1084S monitor. However in order to benefit from the rest of the modes the Super Denise chip offers (such as Productivity) I need to get hold of a proper Multi-Sync monitor that can handle the different refresh rates they need.

Sysinfo Screen with Standard Denise Fitted

Screen Modes Available with Standard Denise Chip

Standard 8362R8 Denise Chip Fitted

Replacement 8373R4 Super Denise Chip Fitted

Sysinfo Screen with Super Denise Fitted

Screen Modes available after fitting Super Denise Chip

Repairing a Roctec Floppy Drive for an Amiga

2nd March 2018

Steven

Amiga, Gaming, How to's, Retro

12 Comments

This is a guide to repairing a Roctec floppy drive for an Amiga 500 computer. I thought I’d write this up as much for my own benefit as other peoples so I can refer back to it in another 20 years! LOL.

Roctec Amiga external floppy drive

If you ever pop a disk in your external Roctec drive and hear a strange whirring noise and can’t read any of your disks then the chances are you are suffering from a perished or broken drive belt.

Perished Drive Belt

These drives use a rubber belt to connect the drive motor to the spindle hub. The rubber belt only has a finite life and given most of these drives are getting on for 30 years old now it’s hardly surprising that they expire.

No tension in old perished belt

The good news is that it’s pretty easy to replace them and I’ll give an overview of what you need to do here.

Getting Started on the Repair

The first thing you need to do is undo the 6 screws on the underside of the drive case using a small philips screwdriver.

Remove these 6 phillips screws.

This particular drive case is made from a solid metal rectangular tube so you need to slide the floppy drive out. The aim here is to slide everything out of the BACK of the case. The first step is to gently slide the floppy drive forward out of the case far enough to detach the plastic fascia from the front of it. Unless you do this it won’t slide right into the case and out the back. There are 2 small tabs on each side of the fascia which you can gently bend with a small flat-bladed screwdriver and it will pop right off. Don’t force anything or it will break – it should come off easily.

Drive Fascia removed

Now you need to prise the back part of the drive out of the case using something thin enough to slide into the tiny gap between the case and the backplate, I found the blade of my pocket knife to be perfect for this but be careful not to injure yourself or damage the plastic/paint on your drive! Once you’ve got it moving slide it out slowly (a slight side to side wiggle can help here). It will be attached to the floppy drive via a ribbon cable so keep going until you’ve got the floppy drive out too.

Roctec floppy drive detached from controller board

The floppy drive has a thin metal cover that protects the mechanical innards and it is held in place by some little tabs on the edges and a solitary screw at the back right (when viewed from the front).

Roctec Floppy Drive Rubber Belt Path

Replacing the Belt

Once the top is off you can see the drive mechanism, motors, heads and so on. Carefully remove what’s left of the old belt with a pair of tweezers. Be careful not to touch the drive heads with anything metallic or you may damage them. The photo above shows the path the rubber belt follows, indicated by the yellow line.

The next step is to remove the drive motor which is attached to a metal bracket that runs across the back of the drive. It is held in position by one screw in the far left corner and another on the right a few cm down from the corner. Don’t touch the two screws with the large flat heads either side of the motor. Be careful you don’t pull the bracket too much as there are two tiny wires connecting the motor to the circuit board here so be careful to support the bracket whilst you are handling the drive to replace the belt.

How to detach the motor and bracket

Drive motor and bracket removed

Once the bracket and motor are out of the way you can carefully thread a replacement belt around the large drive wheel, small tension wheel and keep a loop ready at the top right to go on the motor spindle. Follow the yellow path in the earlier photo and take your time as it can be a little tricky to get the belt into place. A small flat bladed screwdriver and a pair of tweezers are essential for this. Don’t forget the belt needs to thread behind the small silver pully wheel as this is what tensions the belt.

Close-up of the small tension wheel you need to thread the belt behind

Once you’ve got it threaded around the wheels correctly you need to get the final loop onto the motor spindle. It helps if you grab the loop with some needle nosed pliers here and pull it taught with one hand whilst guiding the motor spindle into the loop with the other. Once this has been achieved (it may take a few goes as it’s quite fiddly) place the bracket back in place and rotate the large drive wheel slowly and check the belt stays on, is running where it should and isn’t fouling any components.

Look carefully through the tiny gaps in the top of the drive motor you should be able to see the motor armature slowly rotating as you move the belt. If it is then job done, time to put it all back together! If it isn’t then don’t fret, just double check the belt is following the correct path, isn’t twisted anywhere and hasn’t slipped off any of the wheels.

Congratulations, your Roctec drive should now be fit for active duty for another several years now!

Upgrading my Amiga A500 to 1MB Chip RAM (1MB Chip RAM Mod)

16th February 2018

Steven

Amiga, How to's

43 Comments

Introduction

My A500 motherboard is a Revision 6A which means that it is quite a straightforward modification to get a 512K trapdoor expansion to be seen as additional Chip RAM. The 6A comes with the Enhanced Chip Set (ECS) variant of the Agnus chip called ‘Fat Agnus’ which is able to support up to 1MB total Chip RAM. This 1MB Chip RAM Mod will combine the 512k onboard RAM with the 512k Trapdoor RAM to give 1MB total Chip RAM, just like you get in the later A500 Plus models.

Chip RAM is needed for stuff like graphics as it is the only memory that the custom Amiga chips are able to access and 512k goes nowhere! Opening Workbench windows, increasing the screen resolution, adding a wallpaper all gobble up precious Chip RAM. The popularity of WHDLoad as a means to run old Amiga floppy games off a hard drive is also a problem for 512k machines as most of the games require 1MB of Chip RAM to function.

Checking the Amiga board revision

You can check what revision your Amiga is by opening it up and removing the floppy disk drive. The revision number will be etched into a small area underneath along with other interesting information such as the serial number and when it was made. It looks like my board was designed in 1988 (30 years old this year!) but that it might have been assembled in 1989/90 going by the serial number sticker?

Revision 6A motherboard confirmed

Here’s a screenshot from SysInfo confirming the presence of an ECS Agnus chip – needed for this mod.

Under the knife!

The first thing to do is cut the small trace between the lower two pads (2 & 3). I used a small craft knife but any similarly sharp knife should do the job. Be very careful not to slip with the knife as there are several other tracks on the motherboard that you do not want to damage! To check that you have successfully cut the track you can use a multimeter – there should be no continuity between the two pads if the track has been cut properly.

JP2 – Track cut between the lower 2 of the 3 solder pads with a sharp craft knife.

Bridging the gap

After successfully cutting the track between pads 2 and 3 the next task is to solder the top two pads (1 and 2) together. I found the easiest way to do this was just to keep adding blobs of solder to one of the pads until there was enough to drag across to the other pad and bridge the gap.

JP2 with pads 1 and 2 soldered together.

The shortcut

Traditionally the next part of this modification required cutting a track on JP7A but just by chance I stumbled across an advert for a 512K Trapdoor expansion on eBay that bypasses this requirement completely. This particular RAM expansion has a series of jumpers on it that you can use to enable/disable a number of features including using it to expand Chip RAM. It also features a clock with battery backup so it’s a very complete little package. Anyway, in order to have the 512k of RAM added to the pool of Chip memory you need to remove the blue jumper – this basically has the same effect as cutting the track on JP7A.

512K RAM Expansion with blue jumper disabled to enable it to be seen as Chip RAM.

Testing

The final step (after putting everything back together of course) is to turn on the Amiga and check that it now has a full 1 megabyte of Chip RAM. Easiest way to check for sure is to load up good old SysInfo and go to the Memory Information screen and you should see 1.0MB Chip RAM displayed. Job done!

SysInfo screen confirming 1MB of Chip RAM 🙂