TL;DR: It's the Olivetti P6060, introduced in April of 1975.
IMHO that's the first machine ticking all boxes while operating mostly like we would expect it from today's PoV
Desktop computer is a bit of a vague definition, even today as one may have his desktop box mounted beside a desk. Likewise including terminal based systems, as that would enable any pile of tech that fits on top of a desk as 'desktop'(*1). The mentioned Electronika 60 being one of them, as it's a rack mount mini (*2), not a desktop.
Also, anyone remember all those desk-mounted S100 boxes (*3). Plus all those machines that could be ordered with a desk dedicated to hold those components (*4).
Bottom line, the early computer world was way more colourful than a single term can distinguish. For a reasonable answer two additional restrictions may be helpful:
- The machine must fit at whole on top of a desk
- It must be intended to be placed that way
- Monitor and Keyboard are the only components that may be discrete and movable
With that we can see several distinct answers. All of the following systems have been called 'first desktop' and/or 'first personal computer' at some point:
Calculator Like Computers
Here foremost the 1965 Olivetti P101, eventually the first desktop computer. It included all components (keyboard, printer, magnetic card storage), although text capabilities as well as RAM were rather limited.
(Taken from Wikipedia)
The follow up 1967 P203 included an Editor 4ST typewriter-terminal, making it its own desk. Text manipulation was still rather limited. Usage was more about integrating calculatory results when needed.
(Taken from Wikipedia)
Desktop But Not Really
If mini computers are to be included (*5), then DEC's 1965 PDP-8 could make an interesting contender as it was clearly designed to be put on a table or sideboard (*6):
(Picture taken from the fine Computermuseum der Stuttgarter Informatik)
The size of the partially shown TTY does tell a story how 'practical' putting that beast on your desktop would have been. Also, it's only the CPU, so any storage also had to be placed somewhere.
Terminals That Could Be Computers
This is where the mentioned 1970 Datapoint 2200 and others, like the 1971 Cogar 4 (*7), fit. Those systems were all in one desktop units featuring some long term storage, a full card punch compatible keyboard and a CRT of limited size. Their purpose wasn't to act as stand alone desktop systems but replace Remote Job Entry (RJE *8) stations at lower price while offering more flexibility.
Use as stand alone systems only developed over time.
While third party software was available, it was at first mostly applications and mostly written in assembly, although both companies soon offered a COBOL alike development systems. The smallest possible configuration to run any development was 4 KiB and cassettes using CTOS and Databus, although only a cut down version. Any usable development environment would need at least 8 KiB to run DOS and an external (19" rack sized) 2,5 MiB Diabolo Disk system. In reality anyone developing would need the full 16 KiB of a 2200 Version 2
Such a development setup would end up past 25,000 USD - or the equivalent of 14 1972 model VW cars.
The Cogar 4 might be in addition noteworthy as its character set was fully ram based and redefinable, allowing bitmap graphics as well, a feature CTC only provided with the later 5500 model.
While HP still marketed 1972 HP 9830A as 'calculator', they did integrate a full keyboard, a 32 character display and a cassette unit with CPU and 4 KiB of RAM. in a single desktop case. With the optional full size HP 9866 thermal printer the hardware setup might fit the required definition.
(Taken from Computermuseum der Stuttgarter Informatik)
The picture shows nicely how unreal that tower get with disk drive and printer added.
Software wise it was all to be programmed in BASIC, though, a version 'optimized' for calculatory application. To handle text an optional ROM called 'String Variables' (HP 11274) had to be installed. It added string variables and functions to handle text to the build in BASIC. Further ROMS (Advanced Programming 1&2) added further basic functionality, like keyboard operation as with a typewriter and more string/database functions.
The late 1974 MCM/70 was not only the second micro processor (8008) based computer to be marketed (*9), but also the first desktop unit. It featured a single line display, up to two cassette drives and a typewriter style keyboard. Its APL environment was quite advanced for its time, but not exactly what one would use for text processing. Not that it wouldn't work (ignoring limited memory for a moment), it was just ... unusual.
The 1973 Wang 2200s are a remarkable intermediate. While early 2200 still had a CPU box mounted in/under a table their terminal did also contain its mass storage cassette drive.
All-In-One and All-On-Top
The 1975 Olivetti P6060 might eventually be the first desktop computer from today's point of view. It included a full keyboard, an LED display (CRT optional) and a full width printer together with the computer in a quite stylish 1970s (*10) case.
(Picture taken from this very detailed dissection of a P6060)
Most notable here is the very early use of floppy drives for data storage.
The Rest is History
In September 1975, Half a year after the P6060 IBM introduced the IBM 5100 desktop (*11). It featured a small CRT, a typewriter keyboard and a QIC tape drive (*12). APL and BASIC version were available. It wasn't until the 1978 5110 that a (fridge sized) floppy drive was added.
(Taken from Wikipedia)
By March 1976 Wang as well managed to fit the CPU into the terminal case, creating their first all in one desktop system, the 2200 PCS.
1976 will also be the year that all-in-one desktop micros like TRS-80 or PET became the prototypical desktop units.
*1 - Anyone remember North Star systems? With their wooden top cover they were marketed to fit 1970s office desks, but I've never seen one placed that way. Usually below or in a separate shelf.
*2 - By 1978 western rack mount systems of similar size could already have included mass storage as well as graphics output, marginally qualifying as desktop ... except, at that time real desktop were already standard devices.
*3 - Vector offered mounting sets for their boxes so they could be hung below the table or at the side (like a tower).
*4 - Wang (and others) had nice tables holding the CPU in some cabinet while the board had a hole to lower the terminal part, so the keyboard comes out levelled.
*5 - That is beside gluing rubber feet under an otherwise rack mount box to stay put on a table.
*6 - Though, more serious I'd say a pedestal (or sideboard) is rather what they had in mind for their new beauty :)
*7 - Or Singer 1500 or ICL 1500 as Cogar's company was the same year bought by Singer Business Machines just to be sold five years later to ICL. While Cogar developed it as stand allone system, it was as well associated with the remarkable System Ten.
*8 - RJE is in some way the first kind of terminal application as it allowed to locate all user interaction away from the computer by adding limited local processing capability. No wonder that the cross over between RJE and early terminals produced the first Desktop-PC like devices :))
*9 - The first would be the French Micral-N.
*10 - European Stylish that is, the US design was still stuck in pre-war round forms and fake wood.
*11 - Well, they called it IBM 5100 Portable Computer - which its 25 kg may only be considered when compared with their mainframes :))
*12 - Which gave it an enormous 20 MiB of Storage - slow but huge.