Radio Shack TRS-80 Model 100

Introduction to The Model 100 and Variants

The TRS-80 Model 100 is a Tandy Corporation 8-bit portable computer released in 1983. It is actually a rebranded Kyotronic 85 designed and produced by Kyocera for the early 80s Japanese computer market, but the most popular SKU of this machine by far is the Tandy / Radio Shack version. It was a very successful model for Tandy, selling over 6 million units, no doubt due to its compact & portable size, excellent keyboard, built-in modem, and the fact that it runs on standard AA batteries. As a vintage computer, it is still widely available on the used computer market (e.g. eBay), and still sought-after for its charm, size, portability, usability, and "fun factor." In fact, with readily-available non-destructive modifications, the computer can be expanded greatly beyond its original capabilities and can even run CP/M v2.2!

For more information, see this article's corresponding Wikipedia page: https://en.wikipedia.org/wiki/TRS-80_Model_100.

Do This First, and Always

Regardless of the operating condition of the computer, the original internal Ni-Cd battery must be removed and/or replaced to ensure the motherboard is not eventually damaged by battery leakage. Even though it's usually a good quality battery, and even if it's not currently leaking and/or seems to be working well, it's very old. It's usually just a matter of time before it fails or leaks. The acid from a leaking Ni-Cd will most certainly destroy nearby components and traces. It can just be removed if no replacement is desired. The computer will still work with no replacement battery installed, but since this battery is used to keep the internal memory from being wiped (there is no internal hard drive or other non-volatile storage beyond the memory chips) when replacing the main AA batteries, for best results it is recommended you replace it with a new Ni-Cd or Ni-MH battery of equivalent size, capacity, and voltage. See "Replacing The Ni-Cd Battery," below.

Quick Troubleshooting and Quirks

The Model 100 has a few interesting quirks that often confuse those inexperienced with it. Here are some notes about use, quirks, and basic troubleshooting.

• The unit will not power on when the "Memory Power" switch is off, regardless of the position of the "Power" switch. Since the computer does not include any built-in non-volatile storage, the internal memory of the device is continuously powered by the batteries. The internal Ni-Cd battery is used to "keep alive" the memory for a while (1 to 3 weeks, depending on the memory configuration) when the AA batteries are being replaced or are dead or removed. You can find the "Memory Power" switch on the underside of the computer near the AA battery compartment.
• Even if the internal Ni-Cd battery is non-functional or removed, the computer will still work, but you must still make sure the "Memory Power" switch is on or it won't power up. Just know that if the AA batteries go dead or are removed (and the computer is not using an AC adapter), the entire memory of the computer will be erased if there is no functional Ni-Cd or equivalent battery on board. All programs and data previously stored on it will disappear and the computer will be returned to a "factory reset" condition upon restoration of power.
• An AC adapter (6v, 200mAh) will power the device in place of AA batteries, but there are a couple of quirks to be aware of:
  • If you're not using the original AC power supply that originally came with the device, note that the barrel plug requirement for this computer is "center-negative," which is a non-standard plug type.
  • You can use a power supply of slightly higher voltage, but since the computer has an internal DC to DC power supply/conversion circuit and transformer, make sure to keep it at or below 8v so as not to damage the circuit over time. In tests, it seems the computer requires at least ~5.5v to power up.
• The 40x8 character (240x64 pixel) LCD is not often prone to complete failure, but individual columns or groups of pixels failing to appear can be a sign that some of the LCD circuitry or driver chips are faulty or damaged. However, it can also be a sign that the unit is receiving insufficient power or uneven power regulation - usually caused by a failing Ni-Cd or failing electrolytic capacitors. If the LCD or its driver circuitry is faulty, though, unfortunately replacements are very tough to find and the repair/replacement process is relatively difficult. See notes about the LCD in the "Solutions & Repairs" section below.
• Note that the LCD's "DISP" adjustment control (rotary dial near the power switch) not only adjusts the brightness of the pixels/screen, but it also adjusts the viewing angle. You will need to continually adjust it to best suit whatever position you're viewing it from if you move around while using the computer.
• The 9 pin male D-subminiature port on the left side of the computer (when viewed from the front) will fit an Atari-style joystick, but it is NOT a joystick or mouse port. It was designed for a barcode reader, specifically the sold-separately Tandy 26-1183 "Digital Wand" accessory. Some of this computer's common use cases back in the day were inventory/materials tracking and data entry.
• The computer does work with common Tandy or Tandy-compatible peripheral devices of the era, including the Tandy cassette drives, standard cassette recorders, and Tandy Portable Disk Drives (TPDD and TPDD2). There was even a docking peripheral for this device with an external monitor/terminal interface. However, the connections to these devices are proprietary or industry non-standard and these devices come with quirks of their own. This article will not cover troubleshooting of these use cases.
• The RS232C port is EIA standard, but it's the non-standard female 25 pin (DB-25S) connector type, requiring a female-to-female adapter for use with non-proprietary serial port gear. It can be converted down to work with devices requiring 9 pin D-subminiature connections with standard 25-pin to 9-pin adapters or cables. Depending on the application, it may require a null-modem type of adapter as well.

TRS-80 Model 100

Preliminary Information
Release Year	1983
Leak Risk	High (capacitors and Ni-Cd)
Batteries	Four (4) removable AA for primary power, one (1) PCB-mounted Ni-Cd (3.6v 50mAh) for volatile memory backup
Mounting Technology	Through-hole on the motherboard PCB, several surface mount components on LCD daughterboard PCB
Capacitor Types	Aluminum Electrolytic, Tantalum, Ceramic, Mylar, Poly Film
Destructive Entry	No

Known Issues

The following issues have been observed with this computer. Most of these issues are caused by leaking/failing capacitors and/or a leaking/failing internal Ni-Cd battery, so the replacement of these components will very often bring a Model 100 back to operating condition with no further repair necessary.

Leaking or Failing Ni-Cd Battery

This is a very typical issue on the Model 100 and its variants. The original Ni-Cd battery (3 Ni-Cd cells sandwiched together and shrink-wrapped in a plastic coat), if still installed, is very likely to have leaked due to age. The originally installed Ni-Cd is usually a Yuasa 3-51FT-A (3.6v 50mAh), at least in USA variants. It is soldered by its legs to the motherboard, one leg on each side of the battery. When it leaks, the leaking material tends to spread across the board and infect other components, corrupting the board traces and through-holes as it goes. Most of the original Yuasa batteries are of good quality, however, and amazingly some of them still function normally all these years later. Either way, to avoid risk of damage to the motherboard, any original Ni-Cd battery in the Model 100 must be immediately replaced.

Causes

• Age. These batteries were not meant to operate beyond a few years. Most owners of Model 100s never replaced these batteries, as the skill and effort required to do so would have been beyond the average user's ability or patience.
• Long-term exposure to humidity or higher-than-room temperatures would most certainly hasten the breakdown of the battery's outer casing and internal chemical composition.

Solutions

• While it is possible for the Model 100's original Ni-Cd batteries to still work seemingly fine all these years later, they should be replaced immediately to prevent eventual damage to the motherboard due to leakage. See "Replacing the Ni-Cd Battery" under the "Solutions and Repairs" section below.
• The battery can just be removed, if no replacement is desired, but note that the computer's memory will be completely lost when replacing or removing the AA batteries (if the AC adapter is not also used).

Symptom: Won't Power On - No Apparent Activity

A Model 100 that doesn't seem to be powering on could be experiencing one or more of many different issues. Consider this a basic guide to power-on troubleshooting for the Model 100. Some of these steps are covered more in depth in the Model 100 Service Manual (see Related Links section).

Causes

• Not powered on (obviously)
• Low AA battery power
• Not receiving (enough) power from the 6v 200mAh AC adapter power supply (original or alternative)
• Memory backup switch in the "off" position
• DC to DC power converter circuit failure
  • Leaking or failing electrolytic capacitors in the DC to DC circuit
  • DC to DC transformer (

Symptom: Won't Power On - Low Battery Light Flashes

With known good batteries or known good 6v power supply attached, the unit doesn't seem to power on, with one small exception: the "low battery" LED near the screen flashes for a very brief moment when the device is turned off.

Causes

• Leaking or failing capacitors, most commonly in the DC to DC power supply/regulator circuits (C82, C83, C84, C85). The small 4.7µF and 10µF electrolytic capacitors spread across the board seem to be the worst offenders (C82, C49, C50, C54, and C55) - these guys almost always leak and corrupt traces and other components when they do
• Internal Ni-Cd battery leaking or failing (low voltage)
• Defective/damaged DC to DC circuit or transformer

Solutions

• Replace the leaking or failing capacitors - you might as well replace all of the electrolytic capacitors on the board, and any other component or trace damaged by leakage
• Replace the original Ni-Cd battery - this should always be done regardless of any observed issues

Symptom: Garbage Characters or Pixels on LCD

When powering on, non-sensical garbage is displayed on the LCD, either as characters or pixels or groups of either. Sometimes some portions of the screen will display normally while other portions will not.

Causes

• Leaking or failing capacitors

Solutions

• Solution to the problem

Symptom: Missing Characters or Pixels on LCD

Causes

• Description of cause

Solutions

• Solution to the problem

Symptom:

Causes

• Description of cause

Solutions

• Solution to the problem

Disassembly Notes

Warnings or tips to disassemble the device

TBD: go through a photo-based teardown.

Solutions and Repairs

The following solutions and/or repairs correspond to the "Known Issues" section above.

Replacing The Ni-Cd Battery

Thankfully replacing the internal Ni-Cd battery is relatively straightforward.

Replacing the Electrolytic Capacitors

There are a total of 111 capacitors on the most common variant of this computer. Thankfully there are only 17 electrolytic capacitors. Four (4) of those are non-polarized. The rest of the capacitors (ceramic, poly film, tantalum, etc.) should not need to be touched unless one of the electrolytic capacitors (or the Ni-Cd battery) leaked and damaged them or their traces/holes. Unfortunately this is not uncommon.

The main motherboard PCB does not have any surface mount capacitors or resistors. Therefore, capacitor replacement is a relatively simple affair. However, do note that the main PCB is jam-packed with components - resistors, capacitors, diodes, and small-package ICs (mainly C-MOS logic chips of various kinds), so determining which underside solder point matches which topside component can be tedious. Definitely consider verifying your solder points with continuity checks and make notations (perhaps with a Sharpie pen) as you start replacing things.

Aside from any other damaged components, you will need to replace these electrolytic capacitors:

C49 10uF 16v

C50 10uF 16v

C52 1uF 50v NP

C54 10uF 16v

C55 10uF 16v

C75 47uF 16v NP

C76 47uF 16v NP

C77 47uF 16v NP

C78 3.3uF 50v

C82 4.7uF 25v

C83 470uF 10v

C84 470uF 6.3v

C85 33uF 10v

C86 100uF 10v

C90 1uF 50v

C92 0.47uF 50v

C103 220uF 10v

Ref No.	Capacitance	Voltage	Tolerance	Radio Shack Part No.	Mfr Part No.
C49	10µF	16v

Original Parts

Basic PCB Name (call out all the variants)

Note model numbers and board revisions here

Capacitor Specifications and Footprint Information

RefDes	Qty	Capacitance	Voltage	Mount	Diameter/Size	Height	Lead Spacing	Temp	Type	Brand	Series
		0 µF						0 °C

Replacement Parts

Basic PCB Name (call out all the variants)

Capacitors

RefDes	Qty	Compatible Part Number	Order Links
			Digikey

If parts are not available or different selection is preferred, you can use the values in the Original Parts section to perform a parametric search.

Kits

• Digikey BOM: https://www.digikey.com/

Related Links

• Videos or other things that might be helpful to check out