Amiga 500 Mainboard

#Restauration

ZX Spectrum "Portugal"

And yet another Speccy that I could buy for a good price. The seller said it was "untested", but I allege that he knew very well it was broken. It's fine for me as I mainly buy those things for the repair fun. ๐Ÿ˜

The computer was in a sad condition when I got it. What's remarkable is that the machine was "assembled in Portugal". It's the first time I see this, and to be honest, it was one of the reasons why I wanted to have it. According to the very few information I found on the internet, those machines were intended for the Portugese and South American market, but some of them also made it to the UK and other European countries.

The faceplate was heavily bent, and a connector of the keyboard membrane was broken off. It seems that the previous owner tried to replace the membrane, but wasn't able to remove the faceplate.

The new Speccy is in a poor condition. One of the membrane connectors was broken off and missing.  It was assembled in Portugal.

That's the first hint that the machine wasn't "untested", but underwent a botched repair attempt.

I got the second hint when I tried to power up the machine, but found that it was completely dead, with all the voltages missing. The 5V is generated by an 7805 voltage regulator. It could just have died of old age. But considering the other hint, I rather guess that the previous owner has tried to power this machine with a standard 9V power supply. It has a reversed polarity, which kills the 7805 instantly, and usually damages the lower RAM chips and other components.

Let's have a look inside. There's an Issue 6A board inside, which is the final revision of the board. But besides that, there were no surprises. Anyway it's the first Issue 6A board I own, so I'm happy to have it.

An Issue 6A board, probably built around end of 1984.

The 7805 regulator is definitely broken, but I would have replaced it with a Traco Power DC/DC converter anyway. After I replaced it, the 5V line was back. To my surprise, the 12V and -5V lines were also back, so at least there was no further damage to the power supply.

I did my usual composite mod. Then I connected the computer to my monitor and powered it up to find out what else is broken. To my surprise the start screen appeared, and the Diag ROM also found that all RAM chips are working.

The Speccy just booted up. The Diag ROM found no further defects.

Okay, so much for the "repair fun" I was hoping to get. On the other hand, this board has a second custom chip, the ZX8401, also known as ZXMUX chip. If it would have been damaged, repair would have been a lot more difficult. Not impossible though, since the ZXMUX can be simulated by a few standard SMD chips.

Now that the Speccy was repaired, I continued with replacing the electrolytic capacitors. I also found and fixed a lot of cold joints at the lower RAM chips. The refurbishment of the board was completed after that.

The board after repairing and recapping. A lot of cold solder joints.

Let's have a look at the case. The membrane connector was broken, but luckily there are new membranes available at retro shops. The previous owner tried to remove the faceplate, which is most often glued to the case. Most often, but not here. On this computer, the faceplate was just held in place by four brackets. All that would have needed to be done was to open these brackets and then easily pull of the faceplate.

The faceplate is held by four brackets that can be easily seen on the inside. All that needs to be done is to open them. The faceplate itself is not glued to the case.

Sadly, thanks to the botched repair attempt, the original faceplate was bent too much to be recoverable. It also had some visible scratches. I wished I could have salvaged it, but I decided to replace it with a new one instead. This time I took a metallic red faceplate, which looks as hot as a sunset in Portugal. ๐Ÿ˜‰

My new ZX Spectrum "Assembled in Portugal".

And there it is, another ZX Spectrum for my collection.

ZX Spectrum "Beauty"

When I started to refurbish old computers in 2021, I couldn't imagine that it was so much fun. ๐Ÿ˜ The other day I bought another ZX Spectrum. According to the seller, it had some strange artefacts on the screen and also stability issues, so it was sold as defective. When I tried it at home, it was even worse. I just got a black screen on a white border.

Screenshot made by the seller. Here the screen was just black with a white border.

Inside the case I found an Issue 2 board. The previous owner has added a composite output on a separate connector. As the age of TVs with tuners is definitely over, there is no need to keep the modulator output. I will do my own composite mod instead, and remove this ugly cable that was hanging out.

The manufacturing dates of the components tell an interesting story. This computer has probably been manufactured around the end of 1982. However, all chips that are related to the upper 32KB RAM are socketed, and some were made in 1983. I guess it was originally built as 16K model, and has been extended to the full 48K a year later. As the only chip on this computer, the ULA was made in 1984, so maybe it had been replaced around then.

It's an Issue 2 board with a composite mod on a separate connector. I removed the ULA for testing.

My main suspicion was that the ULA was broken, so I put it into one of my working Spectrums, and was happy to find it in working order. The problem must be somewhere else.

The usual first step is to check the voltages. And bingo, the 12V line had around 7V, and the -5V line was flat. This sounded very familiar, and a look at the coil confirmed my suspicion. The coil had a purple color, and a short between the primary and secondary winding. I guess the coil was already pre-damaged when the Spectrum was sold, causing the artifacts because of poor voltages on the lower RAM chips. When I powered up the computer at home, I eventually killed it.

Well, it's not the first time I had to deal with a broken coil. I unsoldered it, rewound it, and replaced the semiconductors that usually get grilled as a result. Then I powered the system again, and found that all voltages were back to normal. Success!

A shorted coil. The purple color is looking very familiar. It was the same on another Spectrum. The repaired coil.

I put the ULA back into its socket, so I could check what else is broken. And (to my displeasure, to be honest) the computer just came up and was working again.

This is looking good! The computer is working again!

What a spoilsport! I was hoping to have some more repair fun with that machine. ๐Ÿ˜‰

Okay, what next? I started with replacing the electrolytic capacitors with fresh ones. Then I found something strange: A wire link was missing that was supposed to be there.

There is supposed to be a wire link here.

That link is important. The upper 32K RAM chips are actually 64K RAM chips, where one half of the memory turned out to be faulty after production, so they were sold with half the size for cheaper. The link configures which half of the memory is to be used. There is no pull-up resistor, so keeping it open is not a valid option. It might cause the upper RAM to randomly flip between the working and faulty memory half. I doubt that this computer has ever been working stable after it was modified to 48K. This link has just been forgotten by whoever did the modification.

The RAM chips are TMS4532-20NL4. The trailing 4 indicates that the upper part of the memory is to be used, so I added a link between the center hole and the "+5V" hole. A trailing 3 would require a link between the center hole and "0V".

I soldered in the link and replaced all electrolytic caps with Vishay ones. I also replaced the 7805 voltage regulator with a Traco Power TSRย 1-2450. This modern DC/DC converter is a drop-in replacement that needs no heatsink, and is small enough to still fit into a classic ZX Spectrum case.

Wire link added and electrolytic capacitors replaced.

Issue 2 Spectrum boards have two variable resistors, VR1 and VR2, for color calibration. With the aid of an oscilloscope, calibration is a matter of a minute. I connected the scope to the composite video output (or to the video input of the modulator), and then adjusted both resistors until the signal was looking as smooth as possible. There is a blog article at Spectrum for Everyone that gives more details about the calibration.

Finally, I ran the ZX Spectrum Diagnostics tool. All tests passed, even those of the upper RAM.

All diagnostic checks passed.

Another repair job well done. ๐Ÿ˜„

So there is my 3rd ZX Spectrum. Above all, I like the exceptionally good condition of the case. It seems that the computer has barely been used in its 40 years. The keys and faceplate actually look pristine, and there are also only very few and small scratchmarks.

The case is in an excellent state, considering it's 40 years old.

Action Replay

I got this Action Replay MK-I module. According to the seller it was untested, and for that reason sold as defective. It was in aโ€ฆ let's say very used state. The case was dirty, to a point that it was almost revolting to touch it. A side of the case was cracked open, and a knob was missing. The module must have been dropped at some time.

The Action Replay, in the sad state that I got it.

I carefully opened the case. The top and bottom shells are just stuck together, there are no screws, so it was easy to pull them apart. Inside I found some kind of coating on the PCB, so perhaps a drink had been spilled on the module as well. I also found a lot of fine paper dust like from a cardboard, and a small dent at the corner of the PCB that was caused by the drop.

Even the inside is filthy.

The first thing I did was to give the entire module a proper cleaning in an ultrasonic cleaner, just with warm water and a drop of dishwasher detergent. And yes, I also washed the PCB that way, then dried it off and sprayed it with IPA to remove the last traces of water. That bath did wonders.

Before cleaning: Dirt everywhere, and also this strange matte coating on the PCB. After cleaning, the board looks almost as new. I removed a few parts before the bath.

I expected that the dirt also reached the inside of the mechanical parts, so I decided to replace them all. They were a bit hard to find as replacement parts, but still available. As the original knob was lost, I used a different potentiometer that came with a knob. Unfortunately the new one is white, while the original one has likely been black, so I couldn't fully restore the original outside look.

New electrolytic condensors and mechanical parts for the rejuvenating cure.

The case was cracked open at one side because two pins inside were broken off. I fixed the pins with superglue. After that I exposed the case to the sun for a day, which removed quite a bit of the yellowing. Then I could put everything together again. Compared to the original state, the Action Replay is now looking nice and clean.

The Action Replay, shiny and as good as new.

I gave it a test run in my Amiga 500, and it was working fine! Now I have an Action Replay for my Amiga collection. The only sad thing is that it cannot be upgraded to an MK-II or MK-III, as these modules are constructed differently.

Amiga 1200 Restauration

Ever since I got my Amiga 4000, I was pondering about if an Amiga 1200 would have been a better choice. I mean, the Amiga 4000 is nice because it has a lot of space for extensions. But on the other hand, it is rather bulky and heavy, so it isn't much fun to take it to a friend or a party, unlike the compact and light Amiga 1200.

But why not have both? ๐Ÿ˜‰ I had found an Amiga 1200 offer on the Bay that was too good to be ignored, so I bought it.

My new Amiga 1200, unboxed. The keys are yellowed, and the Amiga badge is just a cheap sticker.

It's an Amiga 1200 from the days after Commodore went belly up, and when Escom took over and sold the last Amiga stocks. The good news is that the Escom cases were made of ABS with an anti-UV treatment, so they will never yellow. The bad news is that the keycaps were not treated, and are very visible yellowed by now.

I sent the keycaps to the experts at the CBM Museum Wuppertal for whitening. The case itself is almost in a mint condition, all it needed was a bath in warm dishwater.

Let's take the computer apart and have a look inside.

After opening the cover and removing the keyboard.

There's a 2.5" hard disk, which turned out to contain a Workbench, a few games, and also many bad sectors. I'm going to replace it with an SD Card solution anyway. The shielding has a bit of flash rust and was bent around the ROMs, probably from forceful prying out the ROM chips with a screwdriver. Besides that, the overall status is quite okay.

The mainboard is in a good condition. I've already changed the Kickstart ROMs.

Under the shielding, I found the mainboard in a good condition, especially without flash rust on the modulator. To my surprise, it is a Rev. 1D.1 board, which was actually the first broadly sold board revision. In one of the last Amigas that have ever been produced, I had rather expected to find a revision 2 board. Anyway I was lucky because the 1D.1 revision is said to be the most stable one, and it also has a good Lisa chip that was manfactured by HP. On both CIA chips I found traces of flux, so the board seems to have been repaired before.

It's a rather early Rev 1D.1 board. I didn't expect to find that in one of the last Amigas ever sold. It also seems that this board has been repaired before.

According to my contact at the CBM Museum Wuppertal, Escom sold everything they could find at the Commodore remainders. Allegedly they also produced "new" Amiga computers with refurbished mainboards. Maybe this is one of them?

I first inserted diag ROMs and checked the hardware, but found no problems, so I upgraded the system to AmigaOS 3.2. The next thing on my to-do list was to replace the electrolytic capacitors, as they tend to leak over all those years, causing damage to the PCB. I have already done that on my Amiga 4000 before, but on this model the space was a bit more limited. I even had to remove a freshly replaced SMD capacitor because another capacitor did not fit next to it any more.

Two capacitors are overlapping. It's a tight space. Eventually I replaced all electrolytic capacitors.

To enhance the stability of accelerator boards, it is recommended to remove the capacitors E123C and E125C on the bottom side of the PCB. The easiest way is to use two soldering irons like a pair of tweezers.

The bottom side, with E123C and E125C removed.

After a thorough wash with IPA, the board was then ready to move back into the case.

The board after recapping, ready to move in again.

I want to modernize the Amiga so it can be connected to a HDMI monitor. The RGB to HDMI solution of the Amiga 500 won't work on the AGA chipset though, so I decided to get an Indivision AGA MK3 from Individual Computers. It is plugged onto Lisa and one of the CIA chips, and offers an HDMI output even with sound. (Which is quite an accomplishment, as both chips are not connected to a sound line.)

Since I was on it, I also extended the memory with an ACA1211. Unfortunately it turned out that AmigaOS 3.2 is incompatible to the ACA1211, and the system won't boot in this combination. I had to return to the original AmigaOS 3.0 ROMs again. Eventually I traded the ACA1211 for an ACA1234, which is also an accelerator and works fine with the latest AmigaOS.

To make the wire mess complete, I replaced the floppy disk drive with a GOEX drive from Centurion Tech.

The Indivision AGA MK3 and the ACA1211 mounted. The GOEX floppy simulator replaces the original floppy disk drive.

Centurion also offers LED boards with customized colors. I have picked blue as power LED, green as floppy drive LED, and red as harddisk LED.

New LEDs in my favorite colors! ๐Ÿ˜

In the meantime, I got the whitened keycaps back. They were almost white again, but sadly there is still a slight, but visible yellow tint. Maybe I will buy a new set of key caps once they are available. The Amiga would then look as new.

The original Escom label is just a cheap sticker. It looks ugly to me, so I replaced it with a replica Commodore badge.

And then, for the first time after my purchase, I could close the Amiga 1200 case again.

This is my refurbished Amiga 1200/030.

Please welcome the newest addition to my Amiga collection!

The Ugly Duckling

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.

Dirty, yellowed, mold stains, rust at the bottom frameโ€ฆ This poor device has suffered from storage.

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.

A lot of flash rust, fingerprints, and dirt. After applying a metal polish, it was a lot harder to make a photo. ๐Ÿ˜„

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.

The CN5 and CN6 bridges enable powering from Amiga. The controller's PSU must not be connected after the modification though.

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?

A firmware update after 30 years!

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.

The PCB is nice and clean again!

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.

The cover was cleaned and whitened successfully.

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.

The SCSI2SD adapter on its mounting frame.

And that's it. The GVP harddisk controller is reunited with its Amiga 500 again. I am sure they have missed each other. ๐Ÿ˜‰

Don't they look happy?

The ugly duckling finally became a beautiful swan again!