Radio Shack TRS-80 Model 100

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For general information, see more on Wikipedia: TRS-80 Model 100
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
Hardware Specifications
CPU 80C85
Memory Capacity 8k-32k
Memory Type SRAM
Storage 32k ROM + Option ROM slot
Graphics HD44102CH
Power Requirements
Input Voltage 6V DC
Current (Amps) 600mA (Battery) 180mA (PSU)
Power Connector Barrel Jack 5.5mm x 2.1mm Center Negative
Power Pinout

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.

Known Issues

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 North American 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

  • 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).
  • You can replace the Ni-Cd battery with a 5 volt, 1F super-capacitor to maintain memory when replacing the batteries. DGH105Q5R5 is a viable choice

Capacitors Leaking

There are only 17 electrolytic capacitors and they're all on the main motherboard PCB. Four (4) of those are non-polarized.

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)
  • C82 (4.7µF) seems to be the most likely to fail and/or leak

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

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).

Cause

  • Memory backup switch in the "off" position

Solution

  • Flip switch

Causes

  • DC to DC power converter circuit failure
    • Leaking or failing electrolytic capacitors in the DC to DC circuit
    • DC to DC transformer

Solutions

  • Replace failing components in power regulation area

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)
  • 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

Missing Characters or Pixels on LCD

Causes

  • 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.
  • It can also be a sign that the unit is receiving insufficient power or uneven power regulation

Original Parts

Motherboard

PLX110CH1X

Capacitor Specifications and Footprint Information
RefDes Qty Capacitance Voltage Mount Diameter/Size Height Lead Spacing Temp Type Brand Series
C49, C50, C54, C55 4 10 µF 16 V Through Hole Radial 4 mm 7 mm 2 mm 85°C Aluminum Electrolytic Panasonic
C52 1 1 50 V Through Hole Radial 5 mm 10 mm 2 mm 85°C Aluminum Electrolytic Non-Polar Panasonic
C75, C76, C77 3 47 16 V Through Hole Radial 8 mm 10 mm 5 mm 85°C Aluminum Electrolytic Non-Polar Panasonic
C78 1 3.3 50 V Through Hole Radial 5 mm 10 mm 2 mm 85°C Aluminum Electrolytic SHOEI W
C82 1 4.7 25 V Through Hole Radial 4 mm 7 mm 2 mm 85°C Aluminum Electrolytic Panasonic
C83 1 470 10 V Through Hole Radial 10 mm 10 mm 5 mm 85°C Aluminum Electrolytic SHOEI W
C84 1 470 6.3 V Through Hole Radial 10 mm 10 mm 5 mm 85°C Aluminum Electrolytic SHOEI W
C85 1 33 10 V Through Hole Radial 5 mm 10 mm 2 mm 85°C Aluminum Electrolytic Panasonic
C86 1 100 6.3 V Through Hole Radial 6 mm 10 mm 2 mm 85°C Aluminum Electrolytic SHOEI W
C90 1 1 50 V Through Hole Radial 4 mm 7 mm 2 mm 85°C Aluminum Electrolytic Panasonic
C92 1 0.47 50 V Through Hole Radial 5 mm 10 mm 2 mm 85°C Aluminum Electrolytic SHOEI W
C103 1 220 10 V Through Hole Radial 8 mm 10 mm 5 mm 85°C Aluminum Electrolytic SHOEI W

Replacement Parts

Motherboard

Capacitors
RefDes Qty Compatible Part Number Order Links Notes
C49, C50, C54, C55 4 ECE-A1EKS100 Digikey
C52 1 UVP1H010MDD1TD DigiKey Mouser Too tall, must be folded over
C75, C76, C77 3 ECE-A1CN470U DigiKey Too tall, must be folded over
C78 1 EEA-GA1H3R3H DigiKey Board lead spacing is 5mm, original cap was smaller
C82 1 ECE-A1EKS4R7I DigiKey
C83, C84 2 UHE1C471MPD6 DigiKey
C85 1 UPW1A330MDD DigiKey
C86 1 EEU-EB1A101S DigiKey Tall, may be folded over
C90 1 REA05111R0M050K DigiKey
C92 1 URS1HR47MDD1TD DigiKey
C103 1 EEA-GA1A221B 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.

Related Links


References

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