I still own two ZX Spectrum 48K from my very early days of home computing. The first is my own one, I got it from my parents as a Christmas present back in 1985. The other one was owned by a friend. It was broken and couldn't be repaired, so he first intended to throw it away, but then gave it to me instead.

So here are the two sisters…

This article is about the restauration of my own ZX Spectrum. There will be a follow-up for the other one.

Let's have a look under the hood. This Speccy has a standard Issue Two board, with a floating transistor on the CPU as an usual factory modification of that board.

It looks alright so far. Let's find out if it is still working.

Composite Mod

All home computers of that era were designed to be connected to the "antenna in" of an ordinary color TV. The TV was tuned to UHF channel 36 to receive the signal. The picture quality was quite okay back in those days, but poor for today's standards.

Today, almost all TVs have a composite input, so there is no need for modulating the signal any more. Luckily the ZX Spectrum can be easily modified to give a composite signal. First, the two existing wires on the side of the TV modulator are unsoldered and just bent to the side (so the mod can be reversed if desired). Inside the modulator, the resistor is disconnected from the RCA jack. Then a new wire is connected from the outside's former signal pad straight to the RCA jack.

After that modification, the Spectrum can be directly connected to the TV's "composite in". The modification can be easily reverted, and there is no need to drill an additional hole into the case.

Testing

I have lost the original ZX Spectrum PSUs, but any stabilized 9 V PSU with at least 1.5 A will do as a replacement. It is very important to check the polarity of the barrel plug! Most modern PSUs have the positive pole at the inside of the plug, while the ZX Spectrum expects the positive pole at the outside:

Many Speccys certainly have been killed by using a replacement PSU with the wrong polarity.

I powered it up, and to my surprise it was still working!

All I would need to do now is giving it the usual technical overhaul.

Recapping

The first thing is to replace the electrolytic capacitors. The old ones dry out over the years, and lose their capacity. Some may even leak and damage the PCB.

To keep the old look, many people prefer axial caps with the classic shape and a blue (or at least black) color.

High quality axial caps are difficult to find and quite expensive. I chose to use Vishay capacitors with an expected lifetime of 2,000 h. However the reference photos of the retailer deceived me. The nice black "classic" caps turned out to have an odd shape, and an aluminum or plastic grey color. They are not of an inferior quality, quite the contrary, but they just don't look vintage. I still decided to use them.

There is a trap on the Issue 2 boards: the polarity of C46 is indicated backwards on the silkscreen. The capacitor must be installed with the positive end to the left.

After the recapping, I thoroughly washed the board with IPA and a toothbrush. Then it was time for another test.

Operation Successful…

…but the patient died. This is what I saw when I powered it up again:

Obviously I broke something. 😯 But what?

I first checked the voltages, but they were all right. No chip was getting hot, except of the ULA, but that's normal.

The ROM chip is socketed, so I removed it. Without the ROM, the CPU always executes the same instruction (RST 38h), which fills the memory and results in a distinct screen pattern.

The pattern was there, so the CPU was fine, but it had some noise in it. I suspected a broken RAM chip, and the signal on the data bus actually looked a bit strange on the scope. I started to replace a few suspicious RAM chips, but to make a long story short, it didn't change anything. I was clearly on the wrong track.

I tested the ROM, but it was fine. I swapped the ULA with the one from the other Speccy, but it didn't help either. I checked all the capacitors I had replaced, but they had the correct values and orientation, even that infamous C46.

What has just happened that damaged a previously working Spectrum so badly?

I noticed that, as the only standard chip, IC23 was socketed on that machine. It must have been from a previous repair, because unlike all the other soldering joints, the lead was yellowish there. When I touched the joints with the tip of my soldering iron, they were also sizzling. This was just scrap. I completely removed the old lead, and soldered in a new socket.

Could this have been the problem? I gave power to the Speccy. And yes, it was working again! 😄

I guess that when I cleaned the board with IPA, I partially dissolved the flux in these old soldering joints, making them cold. IC23 is used for the proper access timing of the upper RAM. With a bad timing, the upper RAM might just have disturbed the entire data bus.

Finishing Works

With the Speccy brought back to live, I did some final cleanups.

A defective TR4 is a common cause for a broken power converter. It was still working here, but I precautionary replaced it with a ZTX651, which is the more reliable successor type.

I also preemptively replaced the 7805 voltage regulator by a fresh one, and used thermal paste for better cooling. It's common in the retro scene to replace the 7805 with a modern step-down converter that does not need any cooling, but I decided against it. I like to feel the heat of a working ZX Spectrum.

The ribbons of the keyboard membrane got brittle over the years, and already started to break. Luckily there are new membranes available on the market. And since I was on it, I also ordered a transparent replica case, a black rubber keyboard mat, and a chrome faceplate. I especially like the idea of a transparent case making the inside of this old computer visible.

The first of both sisters is restored now. The other one might be more difficult to restore though, as it was said to be "broken beyond repair". Let's find out.

After the test run was successful in the previous part, it's now time to put my Amiga back together.

Whitening

I've got the case parts and keycaps back from whitening at the CBM Museum Wuppertal. And the mandatory before-after photos proove that they did a great job:

Sadly the PSU case is made of a cheaper material, and could not be whitened any further without risking the dreadened "nacre effect", so it stills look a bit yellowed. (I forgot to use a ring light for the photos, but I did my best to make both pictures comparable.)

Removing the Rust

Due to my careless storage in the cellar, the metal shielding got a bit of rust over the years. I removed the worst rust spots with a fiberglass pen. Then I polished the shields with Nevr-Dull wadding polish, which removed a lot of the rust film. The result is still far from perfect, but it's hard to impossible to restore the original look of the shielding, so I decided to keep it that way.

PSU

I have already prepared the PSU in in the last part. Now that I also got the PSU case back, it's time to put it together. I've first replaced the power switch, as the old one got broken over the years. Luckily it's a standard switch that can still be found at electronic shops.

The Mean Well RT-65B fits into the case surprisingly well, almost as if it was made for it. I only had some difficulties to find a proper position for the wires. I tried different positions, and also tried to switch the sides of the mains and Amiga power cables.

The best result seems to be the original orientation of the cables (mains cable at the side of the power switch). With a bit of tweaking, I could finally route all the wires properly, make the RT-65B fit in, and close the PSU case.

I recommend to check the correct wiring (especially of the earthing) and the voltages once again. Also make sure that the terminals are firmly closed, and that no wires are overly bent, squeezed, or punctured.

GOEX Drive

The GOEX drive optionally comes with an OLED display that sits on top of the Amiga case and shows the currently selected file, and the current disk track. I can really recommend to order that display with the drive, as it makes selecting a file a lot easier. Unless of course you use the "GOEX on pills" version that offers a disk menu on the screen as OSD, but I wasn't sure how this version would interfere with the RGB-to-HDMI converter I am using.

The display is readily assembled, and already has crimp connectors at the end of the cable. The idea was to push them through an opening of the ventilation grille, but they just were too big for that. I had to cut them off, push the ribbon cable through an opening, and then solder new connectors to the cable.

HDMI Connector

Next problem was the HDMI connector. I use c0pperdragon's RGB-to-HDMI converter to connect my Amiga to a TV or monitor via HDMI. But where should I put the HDMI connector so I can reach it from the outside of the case?

My original idea was to cut a matching hole into the back of the Amiga case, but later I dismissed the plan and decided against disfiguring the case even further than I already did in the 1990s. A better idea was to print an expansion port cover with a hole for a HDMI cable, even though I disliked the thought that a cable would then loosely hang out of the case.

Then I noticed that the GOEX drive left a hole where the floppy drive's eject button has once been. And this hole is actually big enough for a HDMI connector, almost as if it has always been meant for that purpose.

I designed a 3D printed frame that holds the connector of a Delock 85463 panel-mount HDMI connector. You can find the STL file at Thingiverse for reprinting.

It looks a bit weird when a HDMI cable is plugged into that place where once the disk eject button has been, but it is working amazingly well, and saved me from cutting further holes into the Amiga case.

Completed

Now everything was mounted in its place. I couldn't close the upper shielding though, because the HDMI converter is in the way, so I decided to leave it out.

And that concludes the restauration project. Welcome back, Amiga 500. With this hardware upgrade, you should be fit for at least the next decade.

The Kickstart Dilemma

Well, not quite. There is a Kickstart 2.04 ROM installed in this Amiga, and while playing some old games, I found out that a surprisingly large number of them crash on that version of the AmigaOS. It was indeed a big problem of the Amiga back in these days. In the previous generation of home computers, the operating system was static and was never updated, so programmers assumed to find certain routines at fixed positions. When the Amiga got popular, programmers continued to use the operating system like that, ignoring documented rules. Then in 1990, Commodore released Kickstart 2.0, a major AmigaOS upgrade. And suddenly, many poorly programmed games crashed, but instead of the programmers, the Amiga was blamed for it.

I still have an original Kickstart 1.2 ROM here, along with a ROM switch. Maybe I will use that board again. Luckily, I haven't yet closed the holes that I once drilled into the case, so I can use one for the switch again.

In the first part, I dusted off and checked my old Amiga 500. I've also ordered all parts for the project, and they have been delivered by now. Let's start with the restauration!

Recapping

All home computers of the 1980s and 1990s have been designed for common households, and thus had to be cheap. Commodore did not expect that the Amiga would become an "old dame" some day, so they used standard components. A common problem is that electrolyic capacitors dry out over the years, losing their capacity. Some may even leak and, in worst case, damage the board. So the first restauration step is always to replace all electrolytic capacitors, even if they should still look fine.

As I don't want to repeat that process in a decade or so, I ordered premium capacitors with an expected lifetime of 10,000 h, which is probably tenfold of the standard caps' lifetime. At the bottom of this article, I have listed all the caps on my Amiga 500 Rev 6A board.

On the first sight, all capacitors seemed to be fine. Still, after removing one of them, I found traces of dried electrolyte on the board and at the bottom of the component. It really shows that a superficial inspection can be deceiving.

I also removed the wires of my self-made NMI button, and looked for cold solder joints and other potential problems. As final step, I carefully washed the mainboard with IPA.

HDMI Output

Let's start with the next construction site. As it gets more and more difficult to find TVs with a SCART connector (and I never liked them anyway), I want to connect my Amiga 500 via HDMI. Luckily there is a project by c0pperdragon that adds a pixel-perfect HDMI output to the Denise chip. The converter cannot be bought at shops, but you need to assemble it yourself. It consists of a few standard components that can be bought at good electronic shops, while the board can be ordered at PCB manufacturers. The assembly requires some fine-pitch SMD soldering though. If you don't feel comfortable with that, you maybe find a private seller for a readily assembled board.

To install the adapter, the Denise chip is removed from its socket first. Then the adapter is placed into the socket, and Denise into the adapter. A button can be connected for changing the configuration and taking screenshots, but that is not really required for operation.

The RGBtoHDMI firmware needs to be extracted to a FAT formatted MicroSD card. Make sure to use release 20210322_f771e51 or later. Earlier releases will not work, but only show four colored rectangles.

Finally the Raspberry Pi Zero is plugged into the socket. Be very careful here! The pin header will also fit if not aligned properly, and may then damage your Raspberry or (even worse!) your Amiga.

New Power Supply

In the first part I found some liquid at the bottom of the power supply's PCB. I suspected it was capacitor liquid, but my contact at the CBM Museum Wuppertal explained me that it was just a lot of flux. Still, the old power supply would need a technical overhaul, which I can only recommend to people who know exactly what they are doing.

WARNING: Switched power supplies may still contain high voltages hours after they have been disconnected from mains. I strongly advise against attempting repairs yourself. If you decide to keep your old PSU, please ask a trained technician to restore it for you!

As I am not "trained personnel" myself, I decided against restoring the original PSU, but for replacing it by a Mean Well RT-65B. It has sufficient power, and also fits nicely into the original Amiga PSU case.

Before I removed the wires from the old PSU, I took notes on what color is connected to what voltage. Then I removed the wires, put crimp shoes on the wire endings, and connected them to the corresponding output of the new PSU.

My original PSU had a separate SEN wire. It is connected to +5V at the power plug, and is used so the old PSU could compensate wire losses and provide exactly 5V there. The replacement PSU does not provide a sense connector, so I just connected the SEN wire to the +5V line to increase the total cross-section and reduce losses, but it can also just be left open.

The "shield" wire must be connected to earth.

ATTENTION: There were different variants of Amiga 500 power supplies on the market. Your number of wires, and the wire color code, may be different. Do not just rely on my photos!

This is how it looked like after the wiring:

Do not connect your PSU to mains yet! First switch your multimeter to continuity test mode, and test if your earthing is connected to the PSU case, and to the shield and the shield pin of the Amiga power plug. After that, test if the power lines are properly connected. Now you can power up your PSU, and use your multimeter to check the voltages at the Amiga power plug.

ATTENTION: Please be very careful when you test the PSU outside the case. Make sure you cannot accidentally touch the live terminals. Also do not open the shielding of the PSU.

Test Run

After I made sure all voltages are correct, it was finally time for a first test run.

As I don't have many disks any more, I had ordered a GOEX drive as a replacement for the original floppy drive. It emulates a disk drive, but uses ADF files from a SD card. It even emulates the mechanical noises of the head stepper motor, which actually sounds much better than expected. I connected it to the floppy header of the mainboard, and used a cardboard box to make sure it won't cause short circuits.

Then I checked everything again. Is the mainboard okay? Is the HDMI converter properly connected? Is the GOEX drive connected? Is the Raspberry properly seated and the MicroSD inserted? Is nothing there that might cause a short circuit? Is the PSU protected against accidental touching the live terminators?

And then, after almost 30 years, it was finally time to wake up my Amiga 500 again.

I first booted an Amiga Test Kit disk and checked the RAM and the CIAs. After that, I ran some demos. The picture quality of the HDMI converter is stunning!

Meanwhile I also got the case and keyboard back from the whitening service. I cannot wait to put it all back together, and enjoy my shiny new Amiga 500.

Capacitor List

The caps list of my Amiga 500 Rev 6A, and the replacement parts I used:

Notes:

• All capacitors are radial and have a lead spacing of 5mm.
• C401, C402: Height should be 24mm or less to fit under the shielding.
• C324, C334: Bipolar caps may enhance audio quality. I haven't tested that though.
• You can use any fitting capacitors with the same capacity and the same (or higher) voltage.
• Temperature rating should be 85°C or, even better, 105°C.

My first Amiga was an Amiga 500. A classmate already had one, and when I visited him and saw the Amiga for the first time, I just had to have my own. So I pestered my parents until they gave in and bought one for me. With this computer I learned 68000 assembler, programming in general, and blind ten-finger typing.

A few years later, I bought an Amiga 4000 from the pay I got doing my civilian service. My good old Amiga 500 spent a few grace months in a closet, and was then stowed away in a box in the basement, forgotten for about three decades.

I would like to restore this machine, make it beautiful again, and give it a subtle technical overhaul. This project is a work in progress. I will report in my blog whenever there is something new. And in the end, I can hopefully share a feeling of success with you.

These are my goals for the restauration project:

• The case has likely got a yellow tint over the years, like all white computer cases of that era. The yellowing has to go, the case should look as good as new again.
• I will give it a technical overhaul. The electrolytic capacitors may have leaked and will be replaced. But after all the years, there may be even more technical problems that need to be fixed.
• Since I have almost no floppy disks left, the floppy drive will be replaced by a drive simulator.
• The old 1084 CRT monitor is long broken and disposed of (I hated these old flicker boxes anyway). I want to connect the Amiga to a modern TV instead, preferably via HDMI, so I will have a closer look at this Raspberry Pi Zero based converter that will be connected to the Denise socket.

After the restauration, the Amiga should feel (more or less) like it did when I got it in 1988. This means that there will be no accelerator card or harddisk controller. Apart from the HDMI output and the floppy simulator, the only acceptable "tuning" is the chip RAM extension to 1 MByte.

Okay, so let's get the computer out of the box and have a first look at it:

The case is not quite as yellowed as I had feared, nevertheless the ugly "nicotine yellow" has to go. On the right side of the case I had added four switches. At that time the Amiga was rather a commodity, and I didn't care much about it. Today I scolded my old self for drilling holes into that beautiful case. So there's one more item on my to-do list: fill the holes.

As for the case, I couldn't decide if I should have it dyed black, or have it bleached at the CBM Museum Wuppertal (CMW). I couldn't find a paint shop who would dye plastic though (they only offered varnishing), and the contact to the CMW turned out to be nice, so the case will now be bleached by the professionals there.

I have to admit: My fingers were itching to plug the Amiga into the wall socket and the TV right away. Fortunately I didn't do that. The circuit board of the power supply looks like it has been soaked in coke:

There are no traces of that liquid inside the case, so my guess is a leaked capacitor. Maybe it was this one:

A homemade repair is totally out of the question if mains voltage is involved. Modern switching power supply modules are simply too cheap for that. So the next item on my to-do list is: get a new power supply.

Let's have a look inside the Amiga. The metal cage is a bit rusty here and there, but all in all still in good shape. The circuit board smells a bit of cellar mustiness, but everything is in its place and nothing seems to be damaged. It all certainly does look very good!

For recapping I inspected all the electrolytic capacitors and listed their values. After that I found out that someone else did that work already. I will replace all the capacitors with premium ones, which sounds more dramatic than it is for cent items. I chose Panasonic caps with a lifetime of up to 10,000 h. The old capacitors were standard types with maybe a tenth of that lifetime.

What's next? The case is now on its way to the CMW for whitening. I've also placed multiple orders for the new power supply, the capacitors, the HDMI adapter and the floppy drive simulator. So let's wait for the deliveries.

The one thing computers are really good at is calulating. You might now expect that all CPUs are capable of the four basic arithmetic operations, but that isn't the case. The first 8 bit processors were only able to add and subtract numbers, and even the subtraction was performed by adding the negated subtrahend. Multiplication and division instructions first appeared on 16 bit processors, albeit they were still very slow in the first generation.

Simple multiplications and divisions with powers of two can be achieved by shifting a value bitwise to the left or right, respectively. This is, shifting a value by one bit to the left is the same as multiplying it by 2, while shifting by two bits to the left multiplies it by 4, and so on.

But how could you multiply any two numbers? It had to be done step by step, by using basic operations like addition or bit rotation. This article will explain how it works on a Z80 CPU.

Back at school, we have learned to multiply large numbers by long multiplication. Basically, we break up the problem by multiplying the multiplier with each digit of the multiplicand, and then summing the products. For example, if we want to compute the product of 27 and 12, we compute 27×2 = 54 and 27×10 = 270, and then sum the products 57+270 = 324.

  27 × 12
———————————
       54
 +    27∙
———————————
      324

We can use the same algorithm on a computer. But wait, wouldn't we still have to multiply, even if with smaller numbers? Actually not! Since computers use binary digits, we only need to multiply either by 1, giving the value itself, or by 0, always giving 0.

This is the the same long multiplication of 27 (11011) and 12 (1100) with binary numbers:

  11011 × 1100
————————————————
         00000
        00000∙
       11011∙∙
 +    11011∙∙∙
————————————————
     101000100

The steps can be executed in a loop. At the beginning, a result register is initialized with zero. If the rightmost bit of the multiplicand is 1, the multiplier is added to the result register. After that, the multiplicand is rotated to the right by one bit, and the multiplier is rotated to the left by one bit. This loop is repeated until the multiplicand is zero, because the result won't change after that anymore.

The following Z80 assembler code example multiplies the values in the BC and DE register pairs, and returns the product in the HL register pair. If an overflow occured during multiplication, the Carry flag will be set.

The multiplicant is kept in the BC register pair. To rotate it one bit to the right, we first use the srl b instruction. It rotates the B register, moving the value of bit 0 to the Carry flag, and inserting a 0 to bit 7, so the multiplicant is filled up with zeros with each rotation. After that, rr c rotates the C register and moves the content of the Carry flag to bit 7. Both instructions combined rotate the BC register pair one bit to the right, insert a 0 to the highest bit. The lowest bit is moved to the Carry flag, where it can be tested.

We essentially do the same with the multiplier in the DE register pair, but in the opposite direction. As a rotation to the left by one bit essentially just doubles the value, we also could have used add de, de. Sadly the Z80 does not offer such an instruction.

multiply:	ld	hl, 0		; clear the result register
.loop:		ld	a, b		; is BC == 0?
		or	c		;   (also resets carry flag)
		ret     z		; then we're done!
		srl	b		; logical right shift of BC
		rr	c		; bit 0 goes to carry flag
		jr	nc, .zerobit	; unless bit 0 was 0
		add	hl, de		; add multiplier to result
		ret	c		;   return on overflow
.zerobit:	sla	e		; shift multiplier to the left
		rl	d		;   topmost bit goes to carry flag
		ret	c		;   return on overflow
		jr	.loop		; next iteration

The example only multiplies positive integers. To multiply negative integers, we first need to change all factors to positive numbers and do the multiplication. The result then needs to be negated if one of the factors was negative, but not both.