The Model III to 4 Upgrade Kit

written by Matthew Reed

The TRS-80 Model 4, introduced in 1983, was 100% software compatible with its predecessor, the TRS-80 Model III. A cassette-based Model 4 behaved identically to a cassette-based Model III and could run all Model III software. But a disk-based Model 4 could also run new Model 4 software that exposed features unique to the Model 4, such as extra memory and a larger text resolution. For Model III owners who wanted to run the new software but were unwilling to discard their Model III, Radio Shack offered the Model III to 4 Upgrade Kit.

The Model III to 4 Upgrade Kit (catalog number 26-1123) originally cost $799.95 and was introduced at the same time as the Model 4. It could convert a disk-based Model III into a disk-based Model 4, capable of running all Model 4 programs. One early Model 4 advertisement stated that “Model III disk system owners can get all the new Model 4 features by adding our upgrade kit.”

The Model III to 4 Upgrade Kit consisted of several parts:

  • a Model 4 motherboard
  • a Model 4 keyboard with bezel
  • a new chassis and RF shielding
  • a TRSDOS 6 operating system disk and manual
  • a Model 4 label for the front of computer

Only the Model III case, video display, and disk drives were preserved as part of the upgrade. Everything else was replaced by the new Model 4 compatible parts.

Like most Radio Shack hardware, installation of the Model III to 4 Upgrade Kit by a technician was required, even though that installation wasn’t included in the price. It was possible to purchase the kit separately but installation was quite involved. The installation instructions included 56 different steps, certainly not a job for a beginner. One of the most important steps, recalibration of the video display, was barely mentioned by the directions.

Considering that a cassette-based Model 4 cost $999 in 1983, only $200 more than the Model III to 4 Upgrade Kit, it’s not hard to see that the market for the kit was quite limited. Radio Shack reduced the price of the Model III to 4 Upgrade Kit to $399.95 by 1986 and to $199.95 by 1989. It disappeared from the Radio Shack catalog in 1991, although resellers continued to sell the remaining kits for several years.

Categories: Hardware


Skip says:

Nice article. I upgraded my Model 3 to a 4 and I never regretted it. I already had Radio Shack disk drives and the upgrade went smooth. I lost a speedup board that was my one piece of non Radio Shack hardware but Model 4 mode was faster anyway so I didn’t care.

Probably because of that new shielding TV interference from my upgraded Model 4 dropped to about nothing. Quite a difference from the Model 3 which used to blast channel 4 and make it unwatchable. I used that computer it was destroyed in a fire in 1991.

Matt says:

There was some outfit in the mid to late 80’s that must have bought up old Radio Shack stock of parts. I bought two Model 3 power supplies for I think $4.95 each – they had Model 4 mainboards for $17.95 that I always kicked myself for not buying. It was just the mainboard so I don’t know how well it woulda worked to upgrade my Model 3 but woulda been fun to try.