Tandy 1400LT and 1400HD: A tale of 2 Tandies.

Disclaimer: As an experiment, I used an LLM (Large Language Model) to generate some drafts of this blog post. I ended up rewriting most of it anyway, but I tried to leave it in a generic style of a YouTube Retro Channel...

Greetings vintage computer enthusiasts! Today, I have an exciting restoration project to share with you - the resurrection of not one, but two Tandy 1400 laptops. These devices were produced by Tandy/Radio Shack, a well-known computer vendor from the 70's and 80's.  Tandy, with Commodore, and Apple, was part of the 1977 trinity that started the micro-computer revolution. 

These pieces of computing history ended up in possession of family member, that, very kindly sent them to me, and I was thrilled to take on the challenge of restoring them.

Tandy 1400LT with 2 floppy drives. It has a 8088 compatible V20 CPU and 768 KB of RAM.

Tandy 1400HD with one floppy and one hard drive. It has a external floppy port, but no composite video out.

The Tandy 1400LT and its later sibbling, the 1400HD, were the two laptops that I received. Both devices have CGA graphics, but the 1400LT has a little-known feature that allows for artifact color output, providing a greater color range for games than the 4 horrible CGA colors. The feature disappeared as most PCs were used with digital-input monitors that did not display the artifact color. A year later, in 1988, Tandy released the very similar 1400FD and 1400HD models, that do not have this feature. 

When I received the devices, I found that, while they were very well packed, the 1400HD's bottom case was cracked and broken into many pieces, likely due to old age and brittle plastic. Luckily, the top half of the case was pristine. The 1400LT was in excellent condition.

Broken bottom case of the 14000HD. The black plastic is extremely brittle, and there were shards everywhere in the packaging.

Unfortunately, I have only one original power supply - the one for the 1400HD. However, it did not power up. When I attempted to use it on the 1400LT, I realized that it used a different voltage and reverse polarity. I was very lucky that the1400LT was not damaged, and I was able to find a suitable replacement PSU to bring it to life.

The 1400LT worked right away after I found a suitable power supply! I connected the composite video output to the monitor in the background in case the LCD was faulty.

Now, onto the restoration of the 1400HD. I spent a considerable amount of time collecting all the little plastic pieces and gluing them back together. I used superglue to quickly bond matching pieces and added large amounts of hot glue and epoxy on the inside to reinforce the structure. I also used JB WELD to patch missing holes and coat the outside cracks before sanding and spray painting black. The plastic was incredibly brittle, and some pieces even cracked just by moving it on the table.

This is what the 1400HD looked after gluing and sanding the major pieces.

There were some holes resulting from parts where the plastic shattered intro bits too small to glue.

And here is after using JB Weld to cover some missing holes. Meanwhile, some more parts of the case broke and had to be glued.

During disassembly, I accidentally ripped both keyboard membrane connectors and cut some conductors. I repaired the membranes by applying sticky tape to the non-conducting side to provide mechanical stability. Then, I painted over the cut traces with a specialized conducting paint that contains over 70% silver in an organic flexible solution. This paint is quite expensive, but only a small amount is needed, and I've used it for countless membrane repairs.

Broken keyboard ribbon cable.

The other broken ribbon cable. Here I had already reinforced the ripped membrane with transparent tape.

Ribbon keyboard patched with conductive silver paint. This paint consists of 70% silver in a polymer base that allows it to flex substantially and cover small gaps. I held the membrane with aluminium tape while the paint cures.

I powered the 1400HD using its power supply, and the screen backlight turned on, but the device did not boot or display any text on the screen. After digging for information online, I found that it does not boot if the internal PSU has bad capacitors. I replaced a few dozen electrolytic capacitors in the PSU and could smell the telltale fishy odor of capacitor juice. They had leaked all right. After replacing the capacitors, the computer started, and I was surprised to find that it even booted from the hard drive. I did not expect the hard drive to work after all these years.

Backlight is on, but no other sign of life. The keyboard is away, still waiting for the conductive paint to cure.

This is the disassembled PSU with lots of electrolytic capacitors, and fishy dried capacitor juice. 

Some of the replaced capacitors.

Now it boots, and the hard drive still works!
Here we can see Norton Commander and some of the contents.

Repaired and re-assembled Tandy 1400HD.

Overall, I was pleased with the restoration of these Tandy 1400 laptops. It was a challenging project, but I learned a lot and had fun bringing these vintage devices back to life. I hope that this blog post has inspired you to take on your own restoration project and appreciate the history of computing.

Now, a retro flashback to the earlier days of the Internet and the immortal words of Kompressor:

Remember, you do not use a Macintosh - instead, you use a Tandy. 

KOMPRESSOR break your glowstick.

KOMPRESSOR eat your candy.

Someone very special to me won a Tandy Technology Scholar Award. 

A broken QL: How to find faulty RAM without a diagnostics ROM

I got a broken QL, the fault was described as white screen on boot, this seems a symptom of a failed RAM test, it should be an easy repair.

Nice looking Sinclair QL, UK version.

Once it arrived, I powered it up, and I saw the described white screen.

QL starting sequence. It freezes after clearing the screen. The initial random pattern should be in color, but this is PAL computer, and I am using an NTSC monitor, so, no color.

The QL's 128k of RAM is organized as 2 banks of 64k, each implemented as 8 64k x 1-bit chips (the 4164 that was also used in the Apple IIc, Atari 800xl, Commodore 64, IBM PCjr, and many other computers of the time). I suspect there is a problem in the top 64k of RAM. A problem in the lower 64k would be visible as a noise pattern on the screen.

The QL does a RAM check at startup that consists of writing random values to RAM, then writing 1's, then verifying the contents. The RAM test froze during the verification step, so we can assume that the RAM output will consist of mostly 1's.

To try to figure out which chip is to blame, I started by measuring the voltage at the output of every DRAM chip. A voltage much lower than 5V may indicate the faulty chip (i.e. the one also outputting 0s). I measured close to 5V everywhere so it is not a chip stuck at outputting 0s.

I let the computer run for a while, then touched each chip looking for one particularly hot, but they all felt similar.

Sinclair QL issue 6. View of the DRAM.

In a similar circumstance, Noel from Noel's Retro Lab used the Minerva ROM, which contains a diagnostics routine, to figure out the bad chip. But my Issue 6 board requires a non trivial adapter to be able to use an EEPROM (check Noel's video for instructions on how to build it).

Too bad the faulty chip is not in the lower bank, then the screen would display some error pattern to help me guess the chip. If only there was a way to swap the banks...

I looked at the schematic and figured something... the two banks seem to be connected to the same lines, except for the CAS0 and CAS1 signals from the ULA 8301. This ULA is also responsible for generating the video by reading from video RAM, maybe it can be fooled to use the upper bank...

I switched the RAM banks by swapping pins 9 and 10 of the ULA 8301 these are the CAS0 and CAS1 lines used to select each bank. I build a simple adapter with two stacked 40 pin sockets.

Crude adapter for swapping pins 9 and 10 of the ULA. Building instructions: 1) Cut pins 9 and 10 of top socket; 2) Solder 2 wires from the bottom to the top socket pins, swapping them; 3) Put some tape over the bottom pins 9 and 10 to prevent contact with the top cut pins. 4) Attach the 2 sockets.  Before using, verify all connections with a multimeter and look for shorts.

ULA 8301 on an adapter to swap pins 9 and 10 (CAS line for each 64k RAM Bank).

With the adapter installed, I can now see some scattered noise on the screen. The noise is sporadic explaining why I was unable to measure a significant voltage difference on the chips' output pin.

Once the top and bottom RAM banks are swapped, the the bad RAM is now visible as noise on the screen.

But now, how can I tell which ram chip corresponds to the noise ? The noise seems to be scattered along lines in columns that are probably separated by a multiple of 8 pixels, exactly what we would expect from a single broken chip. I could try to measure the number of pixels of some columns from the border... but it is hard to get an accurate measurement.

I made a simple probe to pull DRAM output pins to ground using a pick and alligator clips.

So, I decided to use a pull-down technique, that consists of briefly bringing each data line to ground, at the output of the RAM chips (PIN-14).

This should show a vertical line corresponding to the bit that is being turned to 0. We'll find the faulty chip once the line overlaps with the noise pattern.

I did this, and found that IC14 is the faulty chip. 

The vertical stripes caused by pulling the output of IC14 to ground overlap with the noise pattern. This means this is the chip responsible for the noise. 

I desoldered IC14, and replaced it with a socketed 4164. This solved the problem, and the computer boots to the starting screen.

View of replaced IC14.

QL Boot Screen.

The pull-down technique is very general and can be used in other computers as long as the fault affects the screen RAM. Some errors may require a pull-up instead. Next time I try this trick I will connect the probe through a resistor to be safer. What value of resistor to use ? Start with a high value, say 10k, if that is not enough to change the signal, try gradually reducing the value i.e. 1k, 100 ohm, etc. 

Resources:

• QL Issue 6 Motherboard Schematic: http://www.dilwyn.me.uk/docs/hardware/qlissue6.jpg
• Noel's Retro Lab QL Repair using the Minerva ROM.
• QL Roms, including the Minerva Rom.

 

Repairing Two Sad-Looking Apple IIc

I got a lot of 2 broken Apple IIc for cheap (the lot was almost 1/2 the usual price of a single Apple IIc in untested state), it was unclear from the listing if it included 1 or 2 computers. First, one arrived. Then, to my surprise, the other arrived a few weeks later.

The first Apple IIc to arrive. No, it did not cost me $4.95. It was quite dirty.

The second Apple IIc. No, it also did not cost me $2.99.

Both computers were very dirty, yellowed, and had missing keys. 

Disgusting dirty machine.

Cleaning

I gave them a superficial cleaning, so I could touch them without gloves...

 Notice the glass cleaner, IPA, Q-tips, paint brush, tooth brush, and microfiber cloth I used for the superficial cleaning. I also used a magic eraser to remove tough spots. But now I can touch it.

Keyboards

Three keys are missing in the 1st computer ("9", "S", and "Caps Lock"), while the second one is missing  "Caps Lock". I downloaded a design from Thingiverse and 3D printed replacement keys for the "9" and "S". I started designing a replacement for "Caps Lock", that uses different switches, but I have not finished yet. I printed the keys with both a filament and a resin 3D printer. The resin keys turned out better, as expected, but even the PLA plastic keys are very usable (I printed the keys on a 45 degree angle to maximize the resolution). I applied a spray primer and spray painted the keys gray, still, the color is not a good match.

Trying a 3D printed key (this one was printed with a filament printer).

Removed keys, and rotting rubber membrane.

I deep cleaned the keyboard by removing and cleaning all keys, and I discarded a rotting rubber membrane.

Keyboard switches ready for lubrication.

These keyboards use Apple hairpin switches... The Alps keyboard used in later revisions of the Apple IIc is one of the best keyboards I've ever used, but this earlier one is pretty horrible. Even worse, the keys have dirt or corrosion and stick frequently. I used first WD40 (some water may have gotten inside during cleaning) and then DeOxit on the key switches to improve their action.

Retrobrighting

It is April, and I am finally starting to get a few hours of direct sunlight in my north-facing back yard. This is the start of the retrobrighting season. After cleaning the cases, I retrobrighted them and the keys. 

Retrobrighting the two Apple IIc. I disassembled one of them but not the other. I am also retrobrigthting  two mice (one beige and one platinum).

Some time later, halfway though the process, and the difference is clear. An Apple IIc external drive is also getting the retrobrighting treatment it had been waiting for since the end of Summer.

To retrobright the keys, I put them in a closed container with liquid 3% hydrogen peroxide, and shuffled them around every 30m.

My technique for the cases is to brush thick cream bleach (peroxide) then spray 3% hydrogen peroxide (often reused from other applications), or just water, to melt the brush lines and achieve a consistent coating. I achieve better results with this technique than with plastic wrap to keep the cream bleach from drying, but I need to monitor the process every 30m, reorienting the parts, and spraying more liquid as needed.

Repair

When I tried to start the 1st computer, I got the following screen:

This does not look right.

Using an oscilloscope I looked for problems in the clock and reset lines of the CPU. I also looked for suspicious signals in the address and data lines, but all looked normal.

Since I have 2 computers, I systematically swapped socketed chips with the other computer, until I found the broken chip: It is the ROM (issue 256). This is an easy fix: I burned a 27C512 Eprom with 2 concatenated copies of the latest version for this motherboard (issue 0), and modified the board by cutting a trace and bridging a jumper, as described in bigmessowires.com.

Then I serviced the 5 1/4 floppy-drive: I cleaned the head with IPA on a cotton swab, then cleaned the rest of the drive and lubricated the drive mechanism.

The 1st computer is fixed and I was able to play a couple games.

It is working now. It is shown with the drive that is missing the latch spring. Also, note the two resin printed keys.

I started the 2nd computer, but it shows some garbage on the screen, and fails to fully boot. The garbage consists of scattered "!". This seems to be a RAM problem, the "!" (hex #21) differs only on bit-0 from space " " (hex #20), so it is probably a fault in a chip responsible for bit-0.

Some garbage on the screen that points to a bad RAM chip.  I am still using the same keyboard, parts get mixed up when working on more than one machine.

Looking at the Apple IIc schematics it is either chip UF19, or, less likely, UF11 (Apple identifies chips by their coordinates on the board, chip F19 is the one on coordinates F-19, bottom right corner).

Socketed and replaced RAM chips.

I desoldered both chips, installed sockets, and found that UF19 was indeed the culprit.  I replaced the bad chip with a compatible 4146 64k x 1bit DRAM chip. There is very little space between some of the RAM chips and the floppy drive, and I was afraid that UF11 would not fit with the socket, luckily it did fit thanks to some empty space in the drive mechanism. Later, I added some insulation tape over the chip as a precaution against shorts with the drive.

I also serviced the drive of the 2nd computer. This one is missing a spring in the drive closure mechanism, I tried replacing the spring with one from a ball point pen, but It was not strong enough. The drive still works fine, but the latch needs to be pulled to close it. I'll fix it once I find the right spring.

Both computers are now in working order, and (mostly) repaired. To fully finish the repair, I need to find original replacement keys, and a spring for the drive.

It is alive!

Resources:

• Apple IIc ROM replacement: https://www.bigmessowires.com/2015/05/29/apple-iic-rom-upgrade
• Apple IIc schematic: http://www.applelogic.org/files/IICSCHEMATIC.pdf
• Apple IIc keycap design: https://www.thingiverse.com/thing:4223485

 Thinkpad 240x: A Risky Upgrade

Thinkpad 240x between a 14inch T60 and a 12inch Lenovo X61 tablet. The 240x is one of the smallest Thinkpads, with a 10.4inch screen.

Profile view of the Thinkpad 240x. Very thin for a 1999 laptop.
Great keyboard, better than any modern Macbook.

The Thinkpad 240x is one of the smaller, and rarer, IBM Thinkpads ever released. It is the grandfather of the modern X1 Thinkpads and the X series.

I had one in the early 2000's that I had bought second hand from a Georgia Tech Student. It was my first Thinkpad. I used it extensively, I loved its portability. One summer, I took it as my only computing device in a long backpacking vacation across Eastern and Central Europe. I remember using it to remotely resolve technical issues at work from internet cafes, or from open WiFi networks. I sold it cheap to a colleague when I left to the US. I regret this sale.

Recently I saw one listed on eBay with a non outrageous price. I jumped on the chance, I made a reasonable bid, and I got it!

The computer arrived in excellent conditions. With the exception of the 3 horizontal lines on the LCD, everything looks great. There is some keyboard shininess, meaning the computer was well used. The outside is very nice. This computer does not have the mate rubbery finish of most Thinkpads. That finish degrades with age and scratches, and tends to become sticky, looking and feeling terrible. The finish on the 240X is glossy, and mine came scratch free. It is quite surprising that the original battery is still able to hold about 1h of power. The computer came with the original external floppy drive, and an aftermarket PCMCIA cdrom drive.

I think this is an excellent machine to run an old OS and interface with more vintage machines. While small, it has full size ports: VGA, USB (1.0), Parallel, and Serial. The serial port is nice to upload disk images to Apple II computers using ADT Pro, or to 8-bit Atari using an SIO2PC cable. The parallel port can be used to program old CPLD and FPGA chips using a simple cable. 

This Thinkpad has a 500Mhz Pentium III. Enough to run anything up to Windows XP.  It has a 12GB hardrive, that can be replaced by a modern compact flash or sdcard of higher capacity. Its main limitation is the small amount of RAM.

Dangerous RAM Upgrade

The chipset (82443MX) supports a maximum of only 256MB. The computer comes with 64MB soldered on the mainboard, and a single PC100 sodimm socket. This means that the maximum practical amount of RAM is 196MB (64MB + 128MB sodimm). If a 256MB sodimm is installed, the BIOS shows 256MB but the computer won't boot. Part of the sodimm is conflicting with the onboard memory.  On the internet, there are mentions of a special 196MB sodimm, available only in Japan. Still, I found it impossible to obtain such specialized memory.

There is a risky alternative, first demonstrated on the Thinkpad 240z: Remove the internal RAM, so that it does not conflict with the sodimm.

View of the computer after removing the keyboard. The motherboard RAM chips are behind the single sodimm socket to the right. On the left we can see the modem mini-PCI card.

The internal RAM chips are behind the sodimm socket. They are easily accessible once the keyboard is removed. 

Close up of the RAM area. I tried to remove a sticker that was over the RAM, but it also remove the chip markings. I think there is enough space to remove the RAM, if I am careful.

I saw that there is sufficient space, so I went about removing the RAM without disassembling the laptop. 

The easiest way to remove the RAM is to use hot hair. I started by using aluminum adhesive tape to isolate the RAM chips. This is the kind of tape often used to repair HVAC vents.  I find that aluminum tape is very effective for isolating the area, and avoiding damaging to the case or melting nearby components, such as the sodimm socket. The aluminum deflects the hot air, and works as a heat sink for plastics and components, limiting heat damage.

Aluminum tape protecting the case and nearby components from the hot air.

I set the hot air temperature to ~380 degrees Celsius, and I started by slowly warming the whole area of the board. This reduces damage due to thermal expansion. After a while, I lowered the hot air gun closer to one of the ICs, and I focused the hot air to its legs, quickly cycling around its perimeter. I used a pick to softly tap the sides of the IC, so that it came off the moment that solder melted. Then I repeated the procedure on the next one, and so on.

Removed RAM chips. You can see one of the capacitors sticking to the legs of one IC.

After removing the 4 ICs, I cleaned the whole area with IPA. I then noticed that 3 of the 4 decoupling capacitors had been blown away. I soldered then back.

This is where RAM used to live. A sodimm is now required for the computer to boot.
I managed to damage one of the pads (left most chip).
I'll have to add a botch wire if I ever solder the RAM back.

Time to test the computer. I installed the 256MB sodimm and I turned it on... 

Nothing showed on the screen, but the computer beeped once, then 3 times in sequence.  

I tried the original 64MB sodimm that came with it, but the same think happened. 

Great! I just destroyed the computer...

Computer with 256MB sodimm installed. The Thinkpad 240 series requires low density chips. This 256MB module has 8 RAM chips on each side.

I calmed down, and inspected the work. I checked continuity on the terminals of the decoupling capacitors, and the multimeter beeped. I have a short! I removed all 4 RAM decoupling capacitors (they are no longer needed), but this did not fix the short.

I took the computer to the microscope and inspected the area in detail. I saw that solder on some of the pads of the removed RAM were touching. I took some desoldering braid and removed excess solder from the pads.

I put the 256MB sodimm back, and this time the computer started to boot. Victory! 

Computer starting after fixing the short.

Network Upgrade

The Thinkpad 240x does not have Ethernet or WiFi on board. It has a CardBus Slot (i.e. PCMCIA) that can be used to install a network card. I had an WiFi card in my old one. But now, I have a better option. The Thinkpad 240x has an internal modem installed in a mini-PCI slot. I don't think I will ever use a modem, so I can replaced it by an WiFi mini-PCI card. This frees the CardBus Slot for other uses. 

I installed an Atheros WiFi card I had in my parts bin. It requires 2 antennas. Once I salvage some (from a broken T61 display I have somewhere, or from an old access point) I'll install then in the display housing.

Modem removed from mini-PCI slot, and an Atheros WiFi card ready to be installed in its place.

Screen Repair

As you can see from the pictures, the LCD panel is slightly damaged, with a few permanent horizontal lines. I plan to eventually replace it, once I find a cheap enough replacement. I though of replacing the SVGA panel (800x600, usually a Hitachi TX26D32VC1CAA) by a XGA one. The Japanese 240z version has a XGA panel, however, it uses a different cable than the SGVA model, and I don't think I'll ever be able to get that cable. Thinkpads of this era use the cable to distinguish between LCD models. I have some experience replacing and upgrading panels on other Thinkpads, for example I've added a QXGA panel (2048x1520) first to a T43 and later a T60. Only starting around the time of the T60 did motherboards automatically detect the panel.

Thinkpad 240x running a period appropriate copy of Windows 2000.

Resources:

- Thinkpad 240x Service Manual.

- Thinkpad 240 Service Manual.

- Thinkpad 240z internal RAM removal and 256MB sodimm upgrade.

- Thinkpad 240x Drivers and Utilities.