Old Lego Robot Teardown/Repair - Page 1
Forum section: electronics / general

author: Cyberdragon
author_rank: Super Contributor
timestamp: September 18, 2017, 01:33:14 am
content: I've mentioned this kit before on a different thread, but here's finally a teardown. We don't get many electronic toy teardowns here, we should do more.The set is the 4095 Record N Play kit, which consists of a control brick with three axles, two on the back with motors that can be programmed for a certain speed and number of turns, and a third as a rotary switch which restarts the unit when moved. The original fault with it was that it was making static noises even with the unit switched off. However, it uses a RAM based memory so it still has power when it is "off" (IE, sleep mode). The device has two sections, electronics and mechanical, with the mechanical components in a separate module. This module consists of two motors, one on top and bottom, which drive two parallel gear trains which connect the axles and optical rotary encoders. The encoder wheels pass through slots in the PCB on either side, which have an IR LED and sensor on either side of the slot. The reset switch is simply a camshaft that pushes two large contacts together. On the top of the board are strangely multiple contacts for each button top, I wonder if this is to trigger multiple inputs together for some reason, or just to make it more durable/reliable. There is also a piezo, and the motor control IC, which is an LB1836M. It's for low voltage, but a minimum of 2.5V, so they still had to use three batteries to make it work with rechargables. Unfortunately, no photos of the underside of the PCB yet due to the wires being glued down with that awful rubbery yellow stuff often found in older equipment. But from what I can see, the MCU is a cob chip, and there are two discreet SMD transistor H-bridges for the motors. I won't be able to see more until I find a way to remove the glue without causing damage (any suggestions?).The cause of the static problem was really stupid. When I poked the red piezo wire with my scope probe needle to check for noise, it popped and crackled. Upon closer inspection, the stripped part of the red wire was so close to the pad where the moving contact for the cam switch is riveted, that any pressure or even expansion would cause it to touch. This contact appears to be permanently live for some reason, not sure why, but it's probably because the "power switch" is really just an input to the controller IC that puts it into standby mode.Funny thing is, there are also several more design flaws besides those piezo connections being too close to an always live power connection. There is a gear on the left drive train that passes at close tolerance through a slot cut in the mount for the encoder wheel. The biggest on is that robot they show gloriously on the box and the manual...doesn't actually work. You can tell that the technical department and the toy design department had a miscommunication. The large head of the robot is mounted directly to the cam switch on the front of the unit, which usually restarts the unit when moved. So as soon as it tries to move, the head swings, causing a reset loop to occur until the jerking causes the head to just fall of which it then proceeds to push it around trying to run over it!Also, any tips for safely removing that nasty glue without breaking off wires or damaging things? Usually that stuff just pulls off, but this stuff is extra nasty.Links to high-res photos: