IIUC, common mods/expansions of the TRS-80 Model I (or so) weren't pre-assembled.
A guy I worked for as a kid one time said/joked he got his company's start by finishing people's botched attempts at assembling their TRS-80s. (Though his company also dealt in S-100 bus CP/M systems around then, and he might've been speaking more/also about those.)
Probably the latter. According to wikipedia[1] the Model I was modular but preassembled while the later Model III was an all-in-one integrated system, and Radio Shack provided upgrade and repair services (presumably installing things like RAM upgrades or expansion boards like an RS-232 interface.) Neither machine seems to have included the sort of user-accessible slots that S-100 machines, the Apple II, or the IBM PC had.
> In February 1977 they showed their prototype, running a simple tax-accounting program, to Charles Tandy, head of Tandy Corporation. The program quickly crashed as the computer's implementation of Tiny BASIC could not handle the US$150,000 figure that Tandy typed in as his salary, and the two men added support for floating-point math to its Level I BASIC to prevent a recurrence.
I mean common mods/expansions atop the base pre-assembled system.
For example, the Model I with and expansion box that I used circa 1980 had, for example, a hole drilled in one of the chassis with a homebrew toggle switch in it. I never learned what it did, but I recall the whole thing with the ribbon cable looked like there was still a homebrew/DIY aspect to it.
Perhaps an alternate ROM, memory upgrade, 80 column or lowercase board, graphics upgrade, or??? I wonder!
Radio Shack (and Apple, for that matter) actually made schematics for their computers available, enabling people to potentially repair or modify their own machines – and this was actually practical because of the discrete (and often off-the-shelf), through-hole soldered components.
Talk about "right to repair!" Personally I'd feel a bit queasy potentially bricking a $2700 computer, but then again you could also possibly replace a component if you damaged it.
I think Apple and IBM also included firmware listings. In general vintage PCs seem to be very hacker-friendly, and they're also simple enough so that you can understand the entire system. The closest thing now might be something like an Arduino or bare-metal programming on a Raspberry Pi.
Pi's are fun but they don't really have the spirit of those 8 bit machines. For starters it's way too powerful compared to those machines.
Some of the machines had communities (offline!) who flat out encouraged hardware hacking, from simple things like piggybacking EPROMs and switching them with a toggle switch (as above) right up to hacking together sound processors. Often the documentation was non-existent so curious hackers would trace all the lines on whatever expansion was available and document it themselves.
This was generally on the "also ran" computers (thinking specifically of the Australian Microbee) since expensive US things like Apple 2 or TRS-80 were mostly unaffordable here.
If you want the real deal, the closest thing would be single board kit computers based around a Z80 or a 6502.
Back in the 80s I was told by a university employee that the football coach had three different accounts in the old mainframe based accounting system because his salary exceed the maximum allowed by the salary field, which probably seemed like a reasonable limit when the thing was first written. Bet there were lots of hacks back then to work around what seemed like reasonable constraints made in the name of performance or storage.
