Frost on a roof

ZX Spectrum "Recoiled"

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.

Another Issue 2 board.

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

This one is having a tiny daughterboard though.

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.

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.

This isn't looking good.

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 isolation plastics was completely melted and got an almost black color. The left photo shows this coil, the right one shows a good coil for comparison.

The isolator is melted. The primary and secondary side are shorted.This is how a good coil is supposed to look.

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 isolator plastics. I was hoping that I could just unwind the old coils and count the number of windings, but the isolation 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.

The battlefield.

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 isolated transformer copper wire of the same strength. 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 on the ferrite, then wind the wire firmly to the other pin. The windings on the ferrite don't need to be perfect, but should still be as tight as possible.

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 if both coils are wound clockwise or anti-clockwise, as long as you use the same direction for both coils.

First the inner coil with 13 turns, then the outer coil with 39 turns. Start with the pin marked red. Use the same direction for both coils.

Finally, use a lighter to remove the isolation 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.

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

I could only get 0.35mm wire, so my coil got a bit too "fat".

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.

The restored ZX Spectrum 48K.

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

ZX Spectrum "Chrome"

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…

Two Sinclair ZX Spectrum 48K

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.

My own Speccy. The ROM, ULA, and a 74LS532 are socketed.

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.

The old two wires are removed. A new wire is connected to the left "signal" pad.Inside the modulator, the resistor is disconnected, and the wire is soldered 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!

Almost 40 years old, and 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.

Shiny new caps that should last for much longer.Sadly these ones don't have a classic look, but they will do their job.

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.

C46 (the upper one) is correctly installed with the positive end to the left, while the silkscreen claims it's on the right.

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:

This doesn't look good...

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.

Screen pattern without ROM, indicating that the CPU is working.

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 restaured ZX Spectrum 48K "Chrome Edition"

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.

Amiga 500 Restauration, Part 3

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:

Before whiteningAfter whitening

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.

The upper shield after removing the rust and polishing.

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 RT-65B in its new housing. The photo shows an early attempt with swapped mains and power cables.

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.

The HDMI connector fits amazingly well!The holder frame (white) sits within the frame of the GOEX drive (grey) and does not need any screws.

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.

Everything is completed and in its final place.

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

Welcome back, Amiga!

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.