Yoshiya's Planet Robot has long been regarded as the poor man's Robby. This is particularly true of the black clockwork Planet Robot which exists in considerable quantities and is relatively easy to obtain. The blue remote controlled battery operated version is much harder to locate and, unless you are lucky, won't be making its way into any poor man's home. The name clearly comes from "Forbidden Planet".

• The black version can be seen to change slightly during what must have been a long production run. I can't put any firm dates on its availability, though I have always regarded it as a robot of the 1960s rather than the 1950s. The first versions are all tin. The head is a smooth, heavy gauge pressing. The ear cones are chromed metal. The hands are red tin claws on short arms, just like the KO Moon Explorer. This all metal version now commands a higher price than the later versions. Only recently I noticed something that I had previously missed: the earliest tin-head Planet Robot has tiny metal ears. The antenna perch uncomfortably in these little components. A later tin-head version has larger ears, and the antenna (particularly the vertical one) pass right through. These are a much better size, more to scale.

• The next change was the introduction of rubber hands. This must have simplified one stage of production, but it has left collectors with a component that is susceptible to perishing. These light coloured rubber hands are easily damaged, and they decay relentlessly.

• The tin head went next, to be replaced by a hard plastic casting that incorporated ear cones into the design, though the metal hoops remained. There is considerably more detailing in this head, in particular there is a grill effect on the side of the head. There's no denying that the loss of those tin ears does affect the look of the toy. The tin and plastic heads are not interchangeable, there is a modification of the body to allow the plastic head to sit in place.

• The last change was the introduction of the stock red plastic three fingered hand. This hand sees service on the High Wheel robots and, although it looks reasonable, is not as desirable as either the tin or rubber versions. For some reason kids seem to love chewing those hands, and they can become unsightly. They are a vigorous push-twist fit, and easy to replace.

To spark or not to spark: Inevitably a well-used robot will lose its spark as the flint wears down. Replacement is easy enough, but involves dismantling the robot. I would suggest that you tolerate the no-spark unless you are sure of what you're doing. Mint in box means that it should still spark! The other recurrent problem with this robot is the final gear in the motor. This is a tiny milled wheel which regulates the speed of the motor and provides the friction for the spark. The shaft it is attached to frequently acquires a groove caused by wear. The crafty repair involves removing the gear, turning it around and tapping the wheel a few millimeters along.Now a new portion of the shaft can come into play. It never fails. (While you're there, replace the flint.)

It has to be admitted that the box art is nothing special. There's a large picture of Robby, but it lacks either the beauty or the refinement of many other boxes. The quality of the printing is often hit-and-miss. In addition the robot's hands tend to protrude from the box during storage, making damage to the lid a common sight.

Large numbers turned up in 1999 - all mint in the box. This has become a very common, very affordable robot that no collection should be without.