The TRS-80 Microcomputer RFI Testing Report

By | June 6, 2020

Soon after the release of the TRS-80 Microcomputer in 1977, it became apparent that the computer was emitting significant amounts of RFI (Radio Frequency Interference). A common story at the time is that you could generate primitive sound effects for games by simply placing an AM radio next to the computer. This was a hot topic at the time for many of the new microcomputer manufacturers, including Radio Shack’s competitors TI and Apple, because the FCC soon became involved. Complaints poured in from amateur radio operators and television viewers who were being negatively affected by their neighbor’s new computers.

The TRS-80 Microcomputer with Expansion Interface and two disk drives.

To get a handle on the issue, in 1979, Radio Shack contracted Southwest Research Institute (SwRI) to do a thorough analysis of the power line and radiated emissions of the TRS-80. It turned out that the anecdotal evidence was correct as the report indicates significant emissions. In fact, in one case the computer even exceeded the commonly accepted RFI limits at the time, which were detailed in CBEMA/ESC5/77/29. Knowing that the FCC would soon enact even stricter limits, the company moved forward with the design of the TRS-80 Model III which had encapsulating metal shielding that would significantly reduce RFI.

Of course, Tandy Radio Shack did not want to get on the wrong side of the FCC in any way due to the significant business they had in radio related merchandise. As a result, the Model I was soon removed from the market well before the new FCC regulations were enacted.

Here is the report from SwRI that was provided to Radio Shack in 1979.

For more details about the premature death of the Model I due to RFI, check out this article.

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