1981 Tandy Radio Shack TRS-80 Model III Repair

I few months ago, a friend of mine asked me to take a look at their 1980s Tandy TRS-80 Model III, a unit with level II BASIC, 48KB RAM, and dual 5.25″ disk drives. They had bought it sold-as-seen assumed non-working online and had it imported from the USA, so it was a 110V machine and needed work.

The first step was to disassemble the unit and check over everything inside.

Disassembly

Large-case TRS-80s are all fairly easy to dismantle with basic tools, taking care not to damage the neck of the CRT when you’re removing the upper case piece.

With the case opened, everything seemed to be original and pretty much untouched.

I wanted to do perform some upgrades and servicing on the machine, including:

• Replacing the original internal power supplies.
• Replacing the original mainboard ribbon cables.
• Servicing the internal 5.25″ floppy disk drives.
• Upgrading the onboard ROM for use with an external HDD emulator.

PSU Replacement

The TRS-80 Model III has two separate early switch-mode power supplies fitted internally, one for the mainboard, display, and serial card (if fitted), and another for the disk controller and disk drives (if fitted).

At the very least, these original PSUs need to be serviced – they have metallised paper mains filter capacitors which will very likely let out a cloud of acrid smoke if you try to power them up in this condition, without removing or replacing them first. That’s obviously what happened for the original owner of this machine, as the RIFAs had all burst – probably why the machine was being sold for spares/repairs in the first place.

However, power supply design has advanced a lot in the last four decades, and the owner wanted to replace these two original 110V PSUs with a single MeanWell RT-50B SMPS using a nice kit available from Jay Newirth of newSOFT.

The original PSUs are easy to remove, each is only held in place with four Philips screws.

The 2021 variant of the PSU replacement kit includes: the SMPS itself, which is attached to the lower case using strong double-sided adhesive, and wired into the mains from the output of the power switch like the original PSUs; a power breakout board, which is attached to the rear of the case using strong double-sided adhesive, wired into the SMPS using an extension cable, and connected to the up to six power connectors for the various parts of the system using pin headers.

The power breakout board also has an LED indicator for each power rail – 5Vdc, +12Vdc, and -12Vdc – to show that they are working. I found that these were incredibly bright, so I covered them up with electrical tape.

With the new power supply fitted, I tried powering up the machine for the first time – I got a bright screen with good deflection and raster, but no noticeable text.

The display brightness and contrast controls on the left-hand side of the machine were completely frozen up, a common issue on the TRS-80. I used some regular WD-40 spray lubricant to carefully lubricate the control shafts, and some contact cleaner to flush out and clean the potentiometers themselves – after doing this, they worked fine.

I adjusted the display until scrambled text was visible – the machine wasn’t booting.

Ribbon Cable Replacement

The TRS-80 uses a set of ribbon cables: two on the mainboard to jump traces across different parts of the board; one to connect the mainboard to the serial card (if fitted); one to connect the mainboard to the disk controller card (if fitted).

These are a common point of failure, as the adhesive ages and the cables delaminate, and they don’t make a proper connection any more – for the two mainboard cables, this can cause all kinds of problems.

Ian Mavric (The Right Stuff) sells a kit of replacement ribbon cables for the Model III.

I replaced the ribbon cables on the mainboard, reflowed some of the jumper cables which had cold solder joints, reseated all of the socketed ICs and cleaned their sockets using contact cleaner, and cleaned up some flux residue on the back of the board where it seemed to have had some rework in the past.

I also replaced the ribbon cables to the disk controller and serial cards.

With this work performed and the system reassembled for testing, I powered the machine up, and I now got a proper “Cass?” startup prompt, followed by a prompt for the memory size and then the proper BASIC startup message.

All of the keyboard keys registered correctly, so I tried to load some 5.25″ system disks that the owner had kindly provided – the disk drives spun up and the activity LEDs lit, but the drive stepper motors wouldn’t move, another common problem on the TRS-80.

FDD Servicing

The 5.25″ FDDs in the TRS-80 commonly seize up, and require proper servicing including cleaning and lubrication. Texas Peripheral drives like the ones in this unit also have common problems with their drive doors – the plastic hinges that hold these often break, but it’s possible to 3D-print replacements using suitable hard resin.

I removed the two drives for proper access, which is just a case of unscrewing the four screws holding them in and disconnecting the power and data cables.

Both drives fitted in this unit are Texas Peripheral drives, which are apparently a cheaper and less reliable version of the earlier Tandon TM-100 drives.

The drive board need to be removed to gain access to the head and stepper rails – I cleaned the read/write head using a cotton bud and 99% IPA, cleaned out any dust on the disk detection diodes and the rest of the drive using compressed air, and cleaned and lubricated the head stepper rails using lithium grease – each of these was completely frozen up, so I gently freed them and worked them up and down until they ran easily and smoothly.

After servicing both disk drives, I reassembled the computer for testing – with an LDOS 5.25″ boot disk installed in drive 0, the computer booted correctly into it.

SD Card Interface Installation

The owner of the machine had bought a FreHD hard drive emulator, which uses an SD card to load TRS-80 disk images, and has an enclosure so sits on top of the machine.

This came with a modified U106 auto-boot ROM which I fitted to the mainboard, a data cable which I fitted to the hard drive expansion port on the underside of the machine, and a power cable which I fit to one of the disk drive power connectors and fed through the underside of the case.

This also has an onboard battery-backed real-time clock, so I fitted a CR2032 battery.

Switching on the machine now boots it into the FreHD menu, unless there is a boot disk in drive 0, in which case that will be booted instead.

Reassembly and Testing

The computer reassembly is just the reverse of its disassembly.

After its modifications and preventative maintenance, the TRS-80 Model III looked great and worked perfectly! The owner was very happy with the outcome.