Retro.Workshop

With the recent AmigaOS 3.2 release, and the prospect of further updates, it may be interesting to burn own EPROMs for the Amiga. Luckily there is no need to buy expensive equipment any more, as EPROM burners and erasers became affordable. This article explains how I burn Amiga EPROMs at home.

Shopping List

First of all, we will need fitting EPROMs of course. These types are compatible to most Amiga models:

• AMD AM27C400
• Macronix MX27C4100 [sic!]
• STMicroelectronics M27C400

It is important to pick parts with an access time of 200 ns or faster. Models like the Amiga 4000 can even be jumpered to an access time of 160 ns. EPROMs with 120 ns (or less) are easy to find, and definitely on the safe side. Note that Macronix truncates the trailing zero, so a MX27C4100-12 actually has 120 ns access time.

For burning the EPROMs, I use an XGecu TL866II Plus programmer and a 27C400 programming adapter.

Unlike modern flash memory, EPROMs cannot be erased electrically, but are erased by exposing the chip behind the quartz glass window to a strong UV light source. For this reason, an EPROM eraser is also recommended. Note that there are OTP-ROMs without that window, they cannot be erased at all.

None of these EPROM types are still in production. You might still find sources who sell NOS parts, but usually all chips on the market are refurbished. I am a bit careful with chips that look too new, or are claimed to be made by AMD. It is likely that these are just Macronix chips that were painted black and then laser-engraved with a fake AMD label. A cotton bud and a bit of nail polish remover will quickly reveal the scam.

These fake parts are perfectly fine. They are just sold at a higher price because of the alleged noble AMD origin, so you might as well order the Macronix chips directly and save money.

Preparation

Before burning a ROM dump, all even and odd bytes need to be swapped. For Amiga models with two ROM sockets, these dumps also need to be split into separate images for the upper and lower word. To make things even more complicated, a 256KB ROM dump needs to be duplicated to fill the entire memory space of a 512KB EPROM.

Luckily, the AmigaOS 3.2 CD already provides *.bin files that are ready for burning. For plain ROM dumps (those that can be used in emulators), my tool pynaroma can be used for byte-swapping, splitting, and duplicating.

Burning ROM

For the TL866 programmer, I prefer to use the minipro controller software. It is open source and runs on Linux, MacOS and many other Unix derivates, while the manufacturer's original software requires Windows.

The programming adapter is inserted into the programmer, and EPROM inserted into the ZIF socket of the adapter, with the notch pointing to the top, and the chip aligned with the bottom of the socket. Do not put the EPROM into the programmer without that adapter, otherwise it will be destroyed during operation.

The adapter simulates the pinout of an AM27C4096 EPROM, so --device 'AM27C4096@DIP40' must be selected. The --skip-id option must be given as well, otherwise minipro would abort the process because it detects a different type of EPROM.

For writing, the --no_id_error option needs to be used instead. By default, the AM27C4096@DIP40 profile uses a programming voltage of 13V, and a writing voltage of 6.5V. On my chips VPP=12.5V is printed, so I reduced the programming voltage with the --vpp 12.5 option. It might also be necessary to reduce the writing voltage using the --vdd option. Check the datasheet if in doubt.

The first step is to check if the EPROM is empty.

minipro --device 'AM27C4096@DIP40' --skip_id --blank_check

If it's not, it must be erased first. 15 minutes of UV light exposure in the EPROM eraser should be sufficient. If the EPROM still isn't empty after that, just repeat the process.

After that, the ROM image (e.g. amigaos.bin) can be burned to the EPROM:

minipro --device 'AM27C4096@DIP40' --no_id_error --vpp 12.5 --write amigaos.bin

minipro will automatically verify the content after burning, but you can also verify it manually:

minipro --device 'AM27C4096@DIP40' --skip_id --verify amigaos.bin

And for the sake of completeness, this is how to read the content of a burned EPROM to a ROM image file:

minipro --device 'AM27C4096@DIP40' --skip_id --read amigaos-read.bin

After burning, the window should be covered to protect the chip from stray UV light. A simple paper sticker is sufficient.

One year or so after I got my Amiga 500, I extended it with a GVP Impact Series II A500-HD+ SCSI host adapter and a 45MB harddisk. It retired together with the Amiga 500, and was stored in the basement for decades. But while the Amiga survived the years in a surprisingly good state, the GVP had suffered from the moisture. The case was yellowed, but also got mold stains, and the metal frame got some flash rust.

All in all, it seemed to be in a bad shape that was difficult to restore. But on the other hand, it would be sad to write off this nice piece of hardware, while all the other Amiga stuff got a general overhaul.

I sent the case to the CBM Museum Wuppertal for cleaning and whitening. It was a bit embarassing to send it in that bad condition, but they said it can be cleaned and will then look as good as new. Let's see if they can do miracles.

Flash Rust

The base frame had a lot of dirt and flash rust from the storage, especially in the areas that are frequently touched. I used some Nigrin car metal polish paste to clean it, but probably every kitchen metal polish would have done the job as well. It was a bit of work, but after that it almost looked as new.

Self-Powering

There have been two things that were always annoying me on this controller. One was the tiny fan that was supposed to cool the harddisk, but produced a lot of noise. The other one was that a separate external PSU was needed to power the harddisk.

I always wished that the controller would just source itself from the Amiga, but it was clear to me that the mechanical harddisk was drawing too much power for that. The original Fujitsu drive consumed about 10W! With a SCSI2SD adapter, the power consumption is considerably lower, so a self-powering is feasible. The SCSI2SD V5.2 adapter draws only about 10mA, or 0.05W.

The controller can easily be modified to get its power from the Amiga. There is a blog post by davem2 explaining it. All one needs to do is to bridge the CN5 and CN6 pads with some solder.

Since the SCSI2SD adapter also does not need active cooling, I could finally keep the loud case fan disconnected for good.

After the modification, make sure never to connect the GVP PSU to the controller again. It would power the Amiga via the card connector, which is very likely to cause damage to your hardware. Also, do not use mechanical harddisks after the modification. If you want to keep using the original PSU instead, you should let a technician check it first.

Firmware Upgrade

By a lucky chance, I found the latest firmware v4.15 in Ralph Babel's Amiga archive. By another lucky chance, I also found a 27128 EPROM in my spare part box that was once stripped from an old mainboard.

The original firmware would have worked fine as well, but if there is an opportunity for a free update, why not take it?

If you should use a 27256 EPROM instead, make sure to burn the image twice, as only the upper half of the memory will be accessed by the hardware.

Final Works

As there are no electrolytic capacitors on this board, there is no need for recapping. I still did a minor modification: I replaced the LEDs with blue (power) and red (disk) ones, as it became a kind of signature color for all my computers.

After that, I gave the board a thorough bath in IPA, and cleaned the edge card connector with a contact cleaner.

I also found two 1MB/70ns 30-pin SIMM modules for a few Euros on the Bay, so I could double the available Fast RAM to stunning 4MB in total. (Remember to change the jumpers accordingly, as there is no automatic detection.)

Reassembly

Meanwhile the CBM Museum Wuppertal had returned the cleaned and whitened cover. They did an excellent job! The case looks almost as good as new. The mold and dirt stains are gone, and the whitening brought back the original "Amiga beige" color.

As the SCSI2SD adapter is delivered without any kind of mounting frame, I had to 3D-print one myself. Unfortunately it collided with the case fan, so eventually I just removed it completely.

And that's it. The GVP harddisk controller is reunited with its Amiga 500 again. I am sure they have missed each other. 😉

The ugly duckling finally became a beautiful swan again!

In the first part, I took the Amiga apart and repaired the main board. In this second part, I will take care of a new PSU, and then I will put the system back together.

New PSU

There are also capacitors in the PSU that dry out over the years and need to be replaced. But as I'm not a trained technician, I keep my hands off all kind of hardware as soon as mains power is involved.

Surprisingly, a standard SFX form factor PSU perfectly fits into the power supply opening at the back side of the Amiga case. Even the two screw holes of the case match perfectly, almost as if the SFX form factor was just invented for that purpose. There is sufficient space below the PSU for the fan to transport the warm air from inside the case to the outside. There is also enough space for the longer SFX-L form factor. Most of them use a silent 120mm fan. All it needs is a frame where the power supply can sit on.

Not all SFX PSUs are suitable for the Amiga though. The reason is that in modern PCs, the main load is on the 12V line, and there is also a 3.3V line. The Amiga however has its main load on the 5V line, the load on the 12V line is neglectible, and the 3.3V line is not needed at all. Most modern PSUs are regulated on the 12V line. If there is too little load on it, the other voltages may become instable. In best case, the Amiga will then just crash. In worst case, its hardware will be damaged. There are experts who generally advise against using PC PSUs as a replacement power supply, so let me give some warnings before I go on with the article.

WARNING: I recommend to keep the original Amiga PSU, and have it overhauled by an expert. Using a different PSU may cause permanent damage to your Amiga, possibly years after the modification. Self-made power adapters can damage your Amiga if incorrectly wired, and may even cause a fire if underdimensioned or short-circuited. You reproduce the following modification at your own risk. If in doubt, keep using your original Amiga PSU.

The original Amiga 4000 power supply has a maximum load of 145W. Even the smallest SFX PSU is able to deliver far more than that, so basically you can choose any PSU which provides at least 90W (18A) on the 5V line. Still the actual choice is very small. Firstly, the PSU should generate the 5V by a separate DC/DC converter, so the voltage is stable even if there is almost no load on the 12V line. Secondly, the PSU fan needs to be active all the time, as it is the only fan in the Amiga that transports the warm air out of the case. Many modern PSUs have a hybrid fan control though, and operate in a passive cooling mode on low load conditions.

For my Amiga, I use:

• A be quiet! SFX-L Power 500W PSU. According to the manufacturer the 5V is generated by a DC/DC converter, and all supplied voltages have a required minimum load of 0A. Also the fan is permanently operating, but still almost inaudible. Some of the connectors of the modular design collide with the power switch, but that's not a problem unless you plan to use more than three drives.
• A 3D printed SFX adapter frame.
• A Canal PSD-1 power switch. They are not manufactured any more, but can still be found at online marketplaces. If you cannot get one, you can also take the one from your original PSU. (Do not open the PSU case, but ask an expert to remove it for you.)
• An ATX to Amiga power adapter. I made one myself using an "ATX to Acer 12-pin" power adapter, a TE Connectivity Mate-N-Lok 6 circuit plug that is still available today, and a crimp tool. Some Amiga shops also sell readily assembled adapters.

To build the power adapter, the wires need to be connected between the Amiga power plug and the corresponding pin of the ATX power connector. The PS_ON# and one of the COM pins are connected to the power switch. All wires should be sufficiently dimensioned.

When everything is assembled, make sure (make double sure, make even triple sure) that the wiring is correct, but do not plug the connector into the mainboard yet. Now press the power button and check the voltages. PWR_OK should have 5V, but it may flutter or even be 0V because the PSU has no load right now. The other voltages must be correct and within a tolerance of 5%. Keep the test short, as it may stress the PSU.

For the first live test, I removed the CPU module, the SIMMs, and all Zorro boards. The Amiga won't boot like that, of course. But if something should go horribly wrong, the damage would be limited to the mainboard only (which is still bad enough).

Then I turned on the power, for the first time in maybe 20 years. The power LED lighted up. There was no smoke, no smell of burnt electronics, all the chips stayed cool. I checked the power lines, and all voltages were within the expected range. This was looking really good! I turned the power off again.

Now I was confident enough to remount the CPU module and the SIMMs. I also inserted a scandoubler, and connected a monitor to it. Then I turned the system on again. And a few seconds later, I got a picture.

I was probably never that happy to see the Amiga boot screen! 🥲

I had invested some money and many weekends into the refurbishment project, with unknown outcome. All the patience finally paid off.

Reassembly

The old mechanical hard disks were loud, slow, and produced a lot of heat. Since I want my Amiga to be as silent (and modern) as possible, I decided to use a SCSI2SD hard drive emulator instead. Alternatively, an IDE to Compact Flash adapter can be connected to the internal IDE header.

Besides that, I only left a floppy disk drive in the drive cage, but maybe I'm going to replace it with a Gotek floppy drive emulator later. The minimalistic equipment and the modular design of the PSU gives a clean and tidy look on this side of the case. It is also good for the ventilation.

On the other side, the freshly recapped MaestroPro and Toccata sound boards have already moved back to their Zorro slots.

Let's have a look at the outerior. The CBM Museum Wuppertal did an excellent whitening job again. The keyboard looks as good as new, and the outline of the old sticker on the front is almost invisible now. The paint shop did a good work as well, the cover now looks almost brand new.

I've tried to use my original Workbench installation from the 1990s, which I kept on using in an UAE emulator after that, but it was too messed up and crashed the real Amiga while booting. Finally I gave up and installed a fresh Workbench 3.2 from scratch.

What is still missing is a ZZ9000. It will serve mainly as a HDMI graphics card and Ethernet card. After that, my good old Amiga 4000 is ready for some serious Amiga programming again. 😉

Kudos

A refurbishment project like this is not possible without the help of a number of people. First of all, I want to thank the members of the A1K.org Amiga Board, especially halbvier for organizing the rare parts that I needed for the RTC repair. I want to thank the people of the CBM Museum Wuppertal for carefully whitening the plastic parts, and for the nice talk we had. I also want to thank Jan Beta because his YouTube channel inspired me to refurbish my two Amigas.

This project is dedicated to my brother Robert, who had taught me how to solder and how to repair old hardware. I miss you.

A few years after I got my Amiga 500, I bought an Amiga 4000 from the pay I got doing my civilian service. At its peak, it was housed in an RBM Big Tower case, and had all kind of expansion cards and an 68060 accelerator board. It was my pride and joy. Later, after Commodore went bankrupt, I switched to Linux systems. My Amiga 4000 was put on a diet and moved back into a desktop case.

The restored Amiga 500 is meant to be used for playing old games and watching demos. In contrast to that, the Amiga 4000 is supposed to become a workstation again. It should be connectable to modern monitors, and should be as silent as possible.

This is the state of the case before restauration.

The computer has suffered a lot in the past decades. The powder coating of the metal cover got a lot of deep scratches from CRT monitors that were placed on it. The plastic front got a deeply yellowed tint. To make things worse, there was a sticker on the front that kept the plastic from yellowing, but now the outline is permanently "burned" into the front instead. The keyboard was in a similar state, although not that badly yellowed.

The CBM Museum Wuppertal already did an excellent job with whitening my Amiga 500, so I decided to also let them whiten the front panel and the keyboard. The experts at the museum warned me that the logo might stay visible after whitening though, but then I could still resort into dying the front black, or even 3D print a new front.

Meanwhile a paint shop is taking care for repainting the metal cover. I decided to keep the original color, RAL 7044 (silk grey).

Let's have a look at the inside now.

Leaked Battery

A true Amiga killer is the NiCd battery that serves as a power backup for the RTC. Sooner or later it leaks and spills battery lye onto the PCB. The lye corrodes the components and traces in its vicinity. If untreated, the board can become irreparably damaged over time. When I first heard about the problem eight years ago, I immediately cut out the battery, but sadly it had already leaked.

For the restauration, I needed to repair this part of the PCB. I generously removed all affected components. Then I neutralized the lye with vinegar essence, rinsed the area with water, then cleaned and dried it with IPA. After that, I used a fiberglass pen to remove the solder mask down to the bare copper, to have an unobstructed look at the damage.

I lost two pads that seemed to be too damaged by the lye. They just came off while I was cleaning the board. Luckily one of them was not connected, and the other one could be replaced with a short piece of wire.

After that I rang all the traces. Three of them were open and also needed to be fixed with a piece of wire. I then used solder to tin the bare copper traces and protect them from corrosion. Finally, I soldered in fresh components.

The lye may run to the bottom side through the vias, so it should be checked as well, and repaired if necessary. Luckily I could see no damage on my board.

Since the old rechargeable battery almost killed my Amiga, I decided to use a 3V button cell for backing up the RTC. To prevent the cell from being charged when the power is turned on, I replaced R179 with a standard diode.

Since I removed the crystal and capacitors, the RTC needs a recalibration. I connected pin 17 of the RTC chip (U178) to a scope, and then used a plastic screwdriver to adjust VC190 until the measured frequency was exactly 32768 Hz.

Recapping

Electrolytic capacitors may leak, similar to the battery, and it is recommended to replace all of them with modern premium caps. Some people replace them with ceramic capacitors. They cannot leak, but may have other disadvantages. There are good arguments on both sides, so it's a decision that everyone has to make themself. I decided to order the premium capacitors from my Amiga 4000D Bill of Material list. They are made by Panasonic and have an expected lifetime of up to 10,000 hours, which is maybe the tenfold of the original caps. They are also certified for temperatures of up to 105°C, which further extends lifetime.

There are different ways to remove the old electrolytic capacitors. The recommended way is to use two soldering irons, or a hot air rework station. I decided to use a different method that I read about, and twist them off with pincers. It worked surprisingly well, and it took only a few minutes to remove all the caps. To avoid causing damage to the pads, care must be taken that the caps are only slowly twisted, but not pulled from the board.

On the Amiga 4000 the leakage usually starts in the "audio corner", where many SMD caps are concentrated on a small space. I was lucky because all the caps were still intact there. On my board, C317 has leaked though. It was visible by the corroded solder joints around it. When they were heated, there was also a typical telltale smell of microwaved fish. Again, I generously removed the components around the damaged part, cleaned the PCB, and soldered in new parts.

There are also a few axial caps on the daughterboard. They usually don't tend to leak as easy as SMD caps, but since we're on it, they should be replaced as well.

When I repared the battery damage, I also had to remove two of the SIMM sockets. The original sockets have plastic brackets that easily snap off, so I decided to finish what I started, and replaced all five sockets with new ones having metal brackets.

In a final step, I washed the board with IPA. It was then repaired, cleaned, and ready to be put back into the computer case.

In the next part I will take care of the power supply, and I will put the system back together.

From my first days of home computing, I still have two ZX Spectrum 48K. The first one is my own one, which I restored in the previous part. This second Speccy was a donation from a friend. It was broken and written off as irreparable, so he wanted to throw it away, but I asked him to give it to me instead.

Let's find out what we have here…

Memorable Surprise

This Speccy also has an Issue Two board, but it seems to be a bit older because it has an older ULA 5C112E-3, while my own one has an ULA 6C001E-6.

What surprised me was the tiny daughterboard that is used for IC26.

I first thought it was some kind of post-production fix for a PCB error, but it turned out to have a much simpler explanation. For the upper 32K RAM, Sinclair used eight 32KBit DRAM chips of various manufacturers. Those chips were actually 64KBit chips, but one half of the memory turned out to be defective after production, so they were sold with half of the capacity for cheaper.

To run a Spectrum, all eight of the chips need to have the defect in the same half. A wire bridge on the board then configured whether the "upper" or "lower" half of the RAM was to be used. For the OKI M3732 chips that were used on this board, the internal memory cell addressing is a bit different though. Let's put it that way, on these chips either the "left" or "right" half was defective. The tiny daughterboard just takes care of the necessary modification on the address lines to run the OKI chips. Maybe they have just been the cheapest around when Sinclair produced this batch. Starting with the issue 3 boards there were jumpers for the OKI chips, and the daughterboard was not needed any more.

Damage Assessment

In order to see anything, I first did the "composite mod" that I also did on my other ZX Spectrum. It just needs a wire and a few minutes of work, so it is well invested time even if this ZX Spectrum actually turns out to be irreparable.

After that, I connected the Speccy to the TV, took a deep breath, and then turned on the power.

Yes, this computer is definitely broken.

New Coil

The first thing that should be tested on a broken ZX Spectrum is if the voltages are correct. The 4116 RAM chips need three of them: +5V, +12V, and -5V. The +5V were there, but instead of +12V I only got +7V, while the -5V were completely missing.

With further checks I found the culprit: the coil was shortened. And there must have been a lot of heat involved, as the insulation plastics was completely melted and got a dark purple color. The left photo shows this coil, the right one shows a good coil for comparison.

That kind of damage usually happens when an expansion cartridge is removed while the ZX Spectrum is still powered, causing a short circuit on the power lines. This poor computer must have given one last smoke signal before its decease.

The coil was custom made for the Spectrum. One can still get remakes today, but they are quite expensive. So why not just wind a new one myself?

First I thoroughly removed all the old copper wire and the charred insulator plastics. I was hoping that I could just unwind the old coils and count the number of windings, but the insulation was melted to a single lump of plastic. The wire eventually tore, and I had to use a cutter to get the remains off the ferrite core. When I was done, it looked like the coil just exploded on my desk.

Luckily, the circuit diagram gives us all the information we need to know.

The original coil wire had a diameter of 28AWG (or 0.32mm), so we need insulated transformer copper wire of the same strength. For the inner coil we need about 30cm of wire, for the outer coil about 100cm.

First we start with the inner coil. Wind a bit of the wire firmly around the pin marked in red on the next photo, then do 13 turns around the ferrite core, then wind the wire firmly to the other pin. The windings on the ferrite won't need to be perfect, but should still be as tight as possible. I recommend to rewind the inner coil even if it appears to be intact, as the insulation might already be damaged.

After that, we do the same with the outer coil, having 39 turns. It is important that both coils are wound in the same direction. It doesn't matter whether both coils are wound clockwise or counter-clockwise, as long as you use the same direction for both coils. The original coils are wound counter-clockwise when looked from above.

Finally, use a lighter to remove the insulation on all four pins, then use flux and a bit of solder to fix the wire ends to the pins. Now check with a multimeter. Both the primary and secondary coil should have less than 1Ω, but there should be no resistance between both coils.

Now the coil can be soldered to the board again. The fifth pin serves as a key for the correct orientation.

A shortened coil always causes secondary damage, so I preemptively replaced the components that usually fail as well:

• TR4: It can (and should) be replaced with a ZTX651, which is stronger and more reliable. They can still be found at good electronic retailers. I was researching for a standard transistor as replacement, but even though there were some types, the ZTX651 was always the strongest recommendation.
• TR5: The original type is not available any more, but can be replaced with a ZTX751 or a standard BC557 (which must be mounted facing in the opposite direction).
• D16: This can be any standard 5V1 Zener diode.

After that, I connected it to power, and (to my surprise, to be honest) all the three voltages were back and correct.

What's Next?

The picture on the TV was still unchanged, but I had already expected that more components would be damaged.

I checked the temperatures of the ICs with my finger. If you try this at home, be very careful because a broken chip can get so hot it can easily burn your skin within a second.

The ULA got warm, but that's normal. The CPU also got a bit warm, which wouldn't be a surprise on modern computers, but the Z80A is supposed to stay cold. I unsoldered it, and replaced it with a 40 pin socket and a used SGS Z80A CPU that I once recovered from a broken ZX-81.

I powered it up again, and it just worked! 🎉

So there was just a burnt coil and a broken CPU. This repair was much easier than I had expected.

Finishing Works

Like on my other ZX Spectrum, I first replaced all the old electrolytic capacitors. I also used a fresh 7805 voltage regulator, and thermal paste for better cooling.

My first Spectrum got a transparent case and a chrome faceplate. For this ZX Spectrum I decided to keep the original look, so I just replaced the broken keyboard membrane. The old faceplace had some visible dents and scratches, so it was replaced as well. I then washed the original case in warm water with a bit of dish detergent, and then put it all back together.

And that is the story of the two sisters who got a nice makeover, and are now fit for the next 30 years. 🙂