Radio Shack TRS-80 Model 100

Introduction to The TRS-80 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!

These computers saw broad adoption into business use cases that suit portable devices with usable, comfortable keyboards, barcode-based data entry, long battery life, built-in modems, and easy battery replacement. Such business cases include field journalism, inventory management, industrial control, calendar and task management, project management, and similar.

Notable features:

• Long battery life (up to 20 hours of active use on a set of 4 AA batteries, Ni-Cd backup for memory)
• Compact yet comfortable keyboard
• Built-in 300-baud modem
• 8k to 32k of static RAM, user-expandable
• RAM-based file system (no non-volatile storage on board)
• 32k of ROM space, with a very good implementation of MS BASIC-80 (including robust hardware-specific support)
• ROM-based TELCOM program for simple and robust terminal / modem connectivity
• 80C85 processor (low power consumption, compatible with intel 8085)
• Option ROM slot for addition of aftermarket ROMs
• Direct system bus access via underside connector
• RS232 serial port
• DE-9M port for optional barcode reader wand (not joystick or mouse compatible)
• Excellent technical documentation available
• Nearly 100% off-the-shelf parts used to design the mainboard
• Dedicated community of users and readily-available aftermarket products

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.

• Memory Power Switch: 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.
• No Ni-Cd Onboard? Okay, but Note This: 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.
• AC Adapter is Non-Standard: 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.
• Display Corruption Not Always LCD Problem: 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.
• Display Adjustment Oddity: 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.
• Not a Joystick Port: The 9 pin male D-subminiature (DE-9M) 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.
• Non-standard Peripheral Interfaces: 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.
• Unusual RS232 Port: The RS232C port is EIA standard, in terms of wiring and functionality, 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 will work fine with devices requiring 9 pin D-subminiature (DE-9) connections if attached via standard 25-pin to 9-pin adapters or cables. Depending on the application, it may require a null-modem type of adapter as well.
• Proprietary Printer Port: (TBD)

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 - assuming of course any leakage did not affect any other components or traces on the board.

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 original 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 (including all files stored on the computer) 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, C86, C90, C92). 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. (TBD)

Replacing the Electrolytic Capacitors

PCBs and Capacitors Overview

There are a total of 111 capacitors on the most common variants of this computer's PCBs. Thankfully there are only 17 electrolytic capacitors and they're all on the main motherboard PCB. 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. This computer's PCBs are well marked with component numbers in clear, easy to read white text. The polarized capacitors are marked as to which through-hole is negative. It seems as if the manufacturer understood component troubleshooting and repair should a feature and not a luxury. No doubt you'll find this helpful.

The main motherboard PCB does not have any surface mount capacitors or resistors. Therefore, capacitor replacement on this board is a relatively simple affair. However, do note that the main PCB is jam-packed with components including large numbers of 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 double-verify your solder points before de-soldering and make notations (perhaps with a Sharpie pen) before you start replacing things.

The LCD daughterboard PCB has a number of surface mount capacitors (C1 through C20) that are almost all ceramic. A couple of them (C4, C8) are tantalum. None of these should be problematic. If they are, it should only affect the operation of the LCD and won't otherwise affect the main PCB's operation at all. Therefore, don't bother troubleshooting this area unless you know you have LCD display issues and nothing else seems to have helped, or once you've exhausted all other troubleshooting with no apparent resolution.

Problem Children

It's sound advice to replace all of the electrolytic capacitors on the main PCB, as they're all at least somewhat prone to leakage or other time-related damage. If you're not keen on doing that, at least examine each one of them carefully for signs of leakage, bulges, or other damage. Because these boards tend to have a lot of residual flux left over from the manufacturing process, it can be hard to distinguish leaks or bulges at a casual glance - even harder given the tiny size of some of the capacitors. The most leaky ones tend to be the smallest. There are a few common offenders, however:

• The DC to DC power supply/regulation/conversion circuit capacitors on the right middle side of the board near the AA battery housing and power switch
  • C82 (4.7µF) seems to be the most likely to fail and/or leak of all capacitors in this computer, and you can expect the computer to operate very erratically or not at all after all these years no matter the observed condition of it - do yourself a favor and replace it even if it seems okay
  • Meanwhile, C83 seems to leak or fail less often of all the electrolytic capacitors in this area of the board
  • C84 is larger in diameter and soldered very close to the board, so it can be very hard to tell if it has leaked or failed - assume it has
  • C85, C86, C90, and C92 are in this area as well - of these it seems C90 might be the most prone to leakage, likely causing some damage to traces and other connections around it
• The electrolytic capacitors near the middle left side that are part of the modem circuitry
  • C49, C50, C54, and C55 (all 10µF) seem to be the worst offenders in this area of the board - look closely, and you'll see they've probably all leaked and caused some minor corrosion damage to the PCB and surrounding components
  • C52 doesn't seem to leak as often - note the lower capacitance and higher voltage rating of this capacitor relative to its 10µF neighbors
  • If any of the capacitors in this are have leaked, you likely won't notice it during operation of the computer until you try to use the built-in modem

Meanwhile, the remaining electrolytic capacitors don't seem to leak, bulge, or show any other signs of damage nearly as often:

• C75, C76, and C77 up near the RS232C port are usually fine
• C78 near the reset momentary switch is usually fine as well
• C103 up near the barcode reader port (upper left) is usually fine, too

However, since they're all quite old, even if they don't show signs of damage it is possible after all of this time that their internal chemistry has aged enough to affect their capacitances and/or tolerances. So again, it is still a good idea to replace every single electrolytic capacitor in this computer.

Replacement Process

Aside from any other damaged components, for a full electrolytic recap you will need to replace the following capacitors. You'll note that the most common leakers in this list don't seem to have corresponding original Radio Shack part numbers, as indicated in the "TRS Part No." column. Curious, right?

Ref No.	Capacitance	Voltage	Tolerance	Type	Polarized	TRS Part No.	Mfr Part No.	Notes
C49	10µF	16v	+-20%	Electrolytic	Yes	N/A	CEAD100ADN	Especially prone to leakage
C50	10µF	16v	+-20%	Electrolytic	Yes	N/A	CEAD100ADN	Especially prone to leakage
C52	1µF	50v	+75-10%	Electrolytic	No	N/A	CEAG010NLN
C54	10µF	16v	+-20%	Electrolytic	Yes	N/A	CEAD100ADN	Especially prone to leakage
C55	10µF	16v	+-20%	Electrolytic	Yes	N/A	CEAD100ADN	Especially prone to leakage
C75	47µF	16v	+-20%	Electrolytic	No	N/A	CEAD470NLX
C76	47µF	16v	+-20%	Electrolytic	No	N/A	CEAD470NLX
C77	47µF	16v	+-20%	Electrolytic	No	N/A	CEAD470NLX
C78	3.3µF	50v	+-75-10%	Electrolytic	Yes	ACC-335XJAP	CEVG3R3ALN
C82	4.7µF	25v	+-20%	Electrolytic	Yes	N/A	CEAE4R7ADN	Extremely prone to leakage!
C83	470µF	10v	+30-10%	Electrolytic	Yes	ACC-477RCAP	CEAC471ACX
C84	470µF	6.3v	+30-10%	Electrolytic	Yes	ACC-477RBAP	CEAB471ACX
C85	33µF	10v	+-20%	Electrolytic	Yes	ACC-336MCAP	CEAC330ADN
C86	100µF	6.3v	+75-10%	Electrolytic	Yes	ACC-107XBAP	CEAB101ALN
C90	1µF	50v	+-20%	Electrolytic	Yes	N/A	CEAG010ADN
C92	0.47µF	50v	+75-10%	Electrolytic	Yes	ACC-474XJAP	CEAGR47ALN
C103	220µF	10v	+30-10%	Electrolytic	Yes	ACC-227RCAP	CAEC221ACX

With replacements in hand, follow the usual tips for vintage computer electrolytic capacitor replacement:

• Use high quality capacitors - avoid the inexpensive Chinese manufacturers that mass-produce and list capacitor "variety kits" on sites like Amazon. Go with the tried and true Japanese (preferred), USA, or German/EU manufacturers. Non-Chinese manufacturers do not generally package their capacitors in variety kits - they are usually listed as single components or in low quantities of 2-10. Capacitors rated for audio and/or RF filtering use are among the best, but really any Japanese-manufactured capacitor will server you better than the Chinese variety. This is not meant to read like a dig on Chinese manufacturers, it's just a general statement on the overall quality of component manufacturing and subsequent QA/testing based on experience. Japanese capacitors are more expensive, more thoroughly tested for quality, and their tolerances are practically guaranteed to be within spec. Chinese capacitors are mass-produced on an epic scale to be inexpensive and highly available. Fine, but experience shows the quality control is not as high. Furthermore, at-home manual soldering tools tend to apply greater heat for longer periods to a component, and cheaper, lower-quality capacitors can be more easily damaged by heat exposure.
• The solder joints on vintage computers can be quite "sticky" and require a lot of heat to loosen away from components. It is good practice to add a small blob of new solder to each joint you want to de-solder in order to help loosen it up. The less physical pulling you need to do on a de-soldered component to remove it from the board and traces, the better.
• Always clean or wipe away any old flux, grime, debris, etc. on or near the solder joints with isopropyl alcohol or other electronics-safe solvent meant for cleaning PCBs. Electrolytic capacitors that have leaked will leave a lot of leakage/grime on the board and in the through-holes.
• Always apply new flux when soldering new components in place to help the solder flow into the holes and joints. The syringe-style of flux applicator makes this process more pleasant and simpler than, say, a tub of paste or even liquid flux pens.
• Tin your solder iron with a small bit of solder before soldering any parts. Dab the excess off on a wet sponge or on steel/copper wool as needed. Yes, everyone knows it's a pain to get the excess off.
• Heat the component leg with the solder iron and then, when it's hot enough (a second or two), apply the solder directly to the component's leg and not the iron - if you heat the solder directly with the iron and then dab that onto the part it may cause what is known as a "cold solder joint," which may be far less electrically conductive and may not fuse the part to the traces very well if at all, causing "loose" parts and low or no conductivity. It also usually results in blobs that are bigger than they need to be, and in tight spaces that's more likely to cause shorts across other components or traces.
• Opinions differ on when the leg of a component should be snipped to final length - either before soldering or after. There's no wrong answer here as long as you snip only the leg and not any solder you may have applied to it. It's much easier to solder if you get it to "almost final" length before soldering.
• Avoid bending component legs to help them sit into place better while you're soldering. The temptation is great, but if you solder a joint with a leg at an angle, your solder blob may spread out in an undesirable way as you apply it.
• In general you can replace electrolytic capacitors of a specified voltage with higher voltage rating capacitors, as long as the replacement has the same capacitance and tolerance.

Replacing the LCD Screen

(TBD)

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