Machine Robo had been running since 1982, initially under the auspices of the legendary Popy and then Bandai. The line had always had minimal media support - some short manga storylines had been published (an example can be seen here) and a rough mythos was formed between these and the line's toy commercials. However, by the mid-1980s anime was becoming more and more necessary to support successful toylines and so Bandai decided to apply the treatment to Machine Robo.
They commissioned Ashi Productions (now known as Production Reed), a relatively young studio who had worked on Space Warrior Baldios, Special Armoured Battalion Dorvack and - with Bandai - Dancougar: Super Beast Machine God (they also contributed some animation to M.A.S.K.). Given the brief of working the revamped 1986 Machine Robo line into the series they chose to ignore the storyline of the original line (though considering the anime's later use of dimensional gates there are workarounds for inclusionists). Ashi's 'pilot' reel for the series - featuring unique animation not seen in the series - can be seen here.
Rather than the Machine Robo being benevolent aliens defending Earth from the extra-dimensional Devil Invaders they were instead relocated to become the population of Cronos. They were split into three 'tribes':
Other residents of Cronos were the Rock People, who - naturally - turned into rocks. These were based on the same figures issued for Tonka's Rock Lords line in the West. In the cartoon they were split up into two groups - those who helped the Machine Robo and those who helped their enemies. The Jewel Men and the Fossil Men were further subsets of Rock People.
In terms of adversaries the old Devil Invaders were replaced by the Gyandlar, a huge army of invading aliens led by Gadess. Gadess and his lieutenants Diondra, Grujios and Garudi were all invented specifically for the anime - as were the numerous Gyandlar captains which were a fixture of early episodes as one-shot villains Rom/Kenryû/Baikanfu could actually kill each episode. This wasn't unusual for even a toy-based anime like Revenge of Cronos as 'enemy' toys had traditionally fared poorly on the Japanese toy market. However, the large Devil Satan 6 combiner figure (recoloured in the West as Monsterous) was featured regularly as the Gyandlar's prime muscle. Initially it was joined by a menagerie of uglies devised by Ashi, and later the Gyandlar Devils - the old Devil Invader designs being incorporated in the show.
The series' tie-in line was a real hotchpotch featuring figures from a variety of different sources. Some were just repackaged reissues of older figures. Some - such as the new Double Machine Robo and Martial Arts Robo - were most likely planned for the original line but held over. Some - such as Baikanfu, Tough Trailer, Pro Truck Racer, the Dash Robo and the Varigale-X were drawn from other Bandai-owned lines. Some toys were new on the Japanese market - the likes of the Rock Robo and Devil Satan 6 - but may have been devised for or (less likely, considering their relationship) in conjunction with Tonka for their American Gobots line. Two non-transforming vehicles - Battle Flex and Land Giant 32 - were licensed from American manufacturer Galoob. The only moulds outright exclusive to the line were Triple Jim and non-transforming figures of Rom, Rod Drill and Blue Jet.
Revenge of Cronos began broadcast in July 1986 on the TV Tokyo network with a new episode each Thursday. Initially the show had a loose quest format featuring Rom and the other regular Machine Robo searching the planet for Wolf symbols and foiling various plans of the Gyandlar to kill them or learn the secret of the symbols themselves. They travel from settlement to settlement on Cronos, which seems to be largely modelled on feudal Japan. Guest characters in the early episode were largely drawn from the Cronos Tribe - and when Battle or Jet Tribe characters did show they were often given bizarre, arbitrary cameo roles. Early on it was also standard to see characters that weren't actually featured in the toy line (Ashi having seemingly been given character models for all extant Machine Robo).
Midway through the series this focus changed considerably. The show gained a semi-regular setting in the shape of the city of Yamilan, the plot was made more solidly arc-based and toy-based characters became the norm. Roles for the other Battle and Jet Tribe characters - notably the expensive Tough Trailer, Pro Truck Racer and Land Commander 5 figures - were beefed up, Cronos generics largely disappeared and the Gyandlar's Ashi-devised troopers were replaced almost entirely by the Gyandlar Devils. This was most likely at Bandai's behest, though without solid dates for the issuing of toys compared to air dates it's possible that Ashi just didn't really know who or what was actually going to be in the line. Rom, Leina, Jet, Drill, Jim, Gadess, Garudi, Diondra and Grujios remained the leads of the series during this period but their presence was less dominating.
The show tied up nicely after 44 episodes, with three clip show episodes rounding out the run in May 1987. Presumably the show did well enough for all concerned, being followed by two concurrent sequels. Rom and Leina's story continued in two OVAs - the three-part Wolf Sword Legend was released in 1988 and 1989 and the one-shot Leina and Laika - Lightning Trap in 1990. The Battle and Jet Tribe characters were folded into the soft reboot televised anime Machine Robo - Battle Hackers, Ashi making 31 episodes for TV Tokyo between June and December 1987 - broadcast following on directly from Revenge of Cronos.
Outside of Japan Revenge of Cronos has had a somewhat scattered release. By the time it was commissioned Tonka's Gobots line was winding down and there does not seem to have been any intention to import the anime for American broadcast. In France it was a different story - not only were the Robo Machine/Gobots figures produced by Bandai a solid hit but Hanna-Barbera's Challenge of the Gobots (as Les Defi des Gobots) was one of the most successful cartoons on its' broadcast. This led to the French importing it and dubbing the show as La Revanche des Gobots (Revenge of the Gobots). Solid information on the dub is hard to find but it seems the broadcaster made the bizarre choice of trying to mesh the continuities with Rom becoming Nick, Leina becoming AJ (or Agi as she was in France), Blue Jet becoming Leader-1 and Triple Jim becoming Turbo. How much actual continuity patching - i.e. if Bike Robo's role as a good guy was explained or if his name was just changed to Cy-Kill and that was it - took place I couldn't tell you.
The show did get a partial American release when Central Park released three DVDs each containing five episodes of the show with carefully translated subs. For copyright (and possibly marketing) reasons no connection with Gobots was made. It would seem poor sales curtailed these; the third volume is notable for featuring a massive spoiler for the last couple of episodes of the series (note the entire series, not just the episodes on the disc) on the front cover. There is also an Asian bootleg set (most likely originating from Hong Kong) containing the whole series with English subtitles, though these are poorly translated and often outright confusing. Some variants have Cronos artwork bit claim to contain Machine Robo Rescue (something of a success at the time they were pressed); they don't.
With the advent of the internet clips and screenshots of the show have become more widely distributed and raise a bit of interest - mainly comparing the show to Challenge of the Gobots and speculating that Gobots would have been more of a hit if Cronos had been used in its' place. This is somewhat harsh on poor old Challenge. Comparing both series is difficult as it's hard to imagine two more different series based on the same basic bunch of toys - even Transformers' most diverse contrasts (probably between Masterforce and Beast Machines) have more in common than these two.
The main attraction of Cronos is its' generally slick animation and solid designs. The latter are largely very heavily based on the toys - I've always felt it was a big mistake for Hanna-Barbera to change them so much from what were a decent enough set of figures and here the Ashi show scores big points. In terms of visual looks it's no competition - the anime aesthetic is easily preferable to Hanna-Barbera's style. However, Cronos is one of the most clichéd anime series I've ever seen. All the usual hallmarks are there. Big standout problems are the massive overuse of stock footage - Devil Satan 6's combinations along probably genuinely take up about an episode's worth of the entire run time; Baikanfu (despite using a much shorter sequence later in the series) and Land Commander 5 also provide ample opportunities to nip out of the room and put the kettle on. Perhaps more jarringly battle scenes are often used multiple times in the same episode, often within mere minutes. There are a number of stock sequences such as Rom's big intervening speech that will similarly grate.
Story-wise it's harder to pick a clear winner. Challenge has many outright clunkers but I don't think it's ever quite as meandering, repetitive and dull as the first 20 episodes of Cronos. But then the second half of Cronos picks up and is much better than anything Challenge manages. Crucially the first 20 episodes of Cronos feature minimal development so can be largely skipped; on the other hand most Challenge episodes are stand-alones and can also be skipped (though you'd be left with very few). One thing Cronos does suffer from - again like a lot of mecha anime - is a massive dose of over-seriousness. Only Rod Drill avoids being completely po-faced for most of the run and provides the first half with most of its' more memorable moments. A little humour is added to the mix later on - mainly Leina's jealousy of Rom's attraction to Minmei - but it's still a lot more straight-laced than it needs to be. The Gyandlar especially suffer, falling into a repetitive and predictable pattern of squabbling sub-commanders that's less fun that the inept goonery of Cy-Kill and his clowns.
Character-wise again Cronos is a tale of two halves - most of the regulars are stale and boring early on and develop into something better later in the series. Rom and Jet lose their joylessly solemn personas to some extent - Rom especially benefits from not being so overexposed; both Leina and Jim become less useless; Drill remains superb. Beyond them, though, the Machine Robo are often kitted out with an interchangeable 'brave, noble, heroic' basic personality - while this can also be said of Challenge, Cronos confuses the matter by never really explaining whether there are multiple versions of every type or not. This undermines the drama in places - Phantom Robo can get killed in one episode and then turn up the next without anyone saying anything. The Gyandlar are much more stale. While Ashi wisely keep their powder dry with Gadess the bickering between Garudi, Diondra and Grujios gets old fast. Both Garudi (who we're presumably meant to see as the most honourable of the bad guys, though it's rarely executed well) and Diondra threaten to break into decent character arcs but always seem to reset their personalities each episode.
I unashamedly approach Cronos (like Challenge) as a fan of Machine Robo figures rather than mech anime and in this respect the second half of the show is much more successful about giving a wide variety of guys useful roles. However the later episodes also benefit from a much more focused storyline. The early ones are very vague about what exactly Rom and his friends are trying to do exactly. Possibly the most baffling aspect of the anime is that the universe just isn't mapped out very well at a base level.
It's never explained, for example, why some Machine Robo are transforming robots and some are basically humans (the Cronos Tribe really do show very little signs of being robots aside from when they're killed; they're regularly shown to eat, drink, breathe, blush, cry, have family units and so on). There's absolutely no attempt to explain why there are Lockheed Blackbirds and Nissan Skylines capering around an alien planet or why a race of advanced robots don't seem to have any real communication or even infrastructure. This lack of groundwork means a lot of stuff is pulled out of nowhere, often causing confusion. The thing could have done with a Western-style infodump pilot along the lines of "Battle for Gobotron" or "More Than Meets the Eye" but the early stuff is more interested in martial arts mysticism and the groundwork's never put in. It really can't be emphasised how much of a shot in the arm the mid-series revamp gives the show. I can't recall any other single season of a TV show that features such a drastic change of gears. Most animes tend to start off as slow burners especially after the first couple of episodes set out the basic format but none are as disinteresting as this one is and then get this much better for the end. I'd cautiously recommend it to watch but warn that early on - something that sadly includes nearly all of the only officially released material with English subtitles - it really is very hard work. Ultimately it's worth the slog.