TRS-80 Model 4 Easy and Affordable Power Supply Refurbishing
One of the first steps in restoring a vintage computer like the TRS-80 Model 4 is to ensure the power supply is operating at peak efficiency. The power supply must be capable to deliver the required power to allow the machine to work as it was delivered from the manufacture. You can spend hours troubleshooting a machine only to find out there is low voltage from the power supply or it operates intermittently.
The TRS-80 series of machines is no different. At a minimum, replacing the RIFA capacitors should be at the top of your list. For a slightly higher cost, it is also advisable to replace the electrolytic capacitors in the power supply, although this is an optional step.
TRS-80 Model 4 Power Supply Schematic
The power supply used in the Model 4 is generic, being produced by Astec, and used in several different models of machines across various manufactures.
We have schematics and a complete parts list below for the power supply in Model 4 which for this unit was an ASTEC AA12090 (Tandy’s part number 8790043).
Immediately Replace RIFA Capacitors
One of the most common points of failure in a TRS-80 Model 4 is in the power supply. There are up to three suppression capacitors made by RIFA, which as they age, develop cracks allowing moisture in.
When the RIFA capacitors fail, they produce a large amount of smoke and generate a horrible odor. Although the capacitors go bad, they normally do not cause any other problems in the power supply and can be easily and inexpensively replaced with new capacitors.
There are approximately three models of power supplies for a Model 4. Depending on the model of power supply, there is normally one 0.1uF and one or two 0.01uF capacitors which varies based on the model of power supply in the system.
RIFA Replacement Capacitor Specifications
- 0.1uF 275VAC: PME271M610MR30 (available from PCBoard.ca)
- 0.01uF 275VAC: PME271M510MR30 (available from PCBoard.ca)
Recommended Capacitors to Replace on the TRS-80 Model 4
As your machine ages, the capacitors begin to dry out, losing their ability to function reliably and possibly failing in a spectacular fashion. The capacitors you should concentrate on after replacing the RIFA capacitors are the electrolytic models.
With the Model 4, there are seventeen electrolytic capacitors which to replace. For less than $30, you should be able to purchase replacements and install them in about two hours.
When it is time to purchase the capacitors, purchase quality models. Manufacturers such as Nichicon, Panasonic, Rubycon, or Chemicon produce reliable quality models and most electronic suppliers carry at least one or two of these brands.
When working with power supplies, care should be taken while servicing them. Capacitors can hold a charge for an extended period of time and can give you a nasty shock if not careful – you have been warned!
During the Spring of 2021, we undertook the replacement of capacitors on our TRS-80 Model 4. The end result was a power supply that produced solid outputs matching the original specifications while under three-quarters of the rated loads. Below is a list of the capacitors we replaced along with the models we used in the rebuild process.
Capacitor Replacement Models
Invest in DeoxIT Contact Cleaner
Other areas to look at servicing on your TRS-80 Model 4, are the trim-pots for the monitor contrast and brightness, along with edge connectors on the video driver. Look for a good contact cleaner and rejuvenator such as DeoxIT.
DeoxIT contact cleaner dissolves oxides and sulfides that form on metal surfaces which restores the contact’s integrity and leaves a thin microscopic layer that protects the metal. Special additives improve conductivity and prevent dissolved oxides from re-attaching, keeping them in suspension and allowing them to be easily dispersed by the mechanical action of the contact. We use DeoxIT and have only great reviews of how well it works and performs.