This is a child's toy to play with it in bath. This Yellow Submarine is 10 cm long and 8 cm wide. It is powered by two size AA cells. The remote control unit only has two buttons: straight forward and turn backward.
One day it stopped working. I checked the batteries but these were ok. After replacement, the sub still did not work. Replacing the batteries of the remote control did not bring any improvement, either. I inquired whether the sub had been dropped, opened in the bath? No.
I checked if any signal was coming from the remote control. My scope told me it was transmitting something in the 27MHz band, so that seemed to be OK.
Then I opened the sub. I removed the 4 screws that hold together the two halves of the hull. Using some force, I could separate the two halves. It turned out that the two “engines” on both sides did not actually contain separate motors, but just a couple of gears to drive the propellors. Each engine has a shaft, that goes into the inner hull. When the sub is put into the water, the room between the inner and outer hull is flooded. A piece of styrofoam in the "tower" gives the sub just enough buoyancy and stability. A small weight at the bottom of the inner hull gives additional stability.
Then I opened the grey plastic inner hull. The batteries are inserted into the inner hull. The lid of the battery compartment is closed watertight onto the back of the hull. I was able to separate the front and rear part of the inner hull. There was a rubber ring between the two halves, amply greased with Vaseline. After this, I could separate the (gear) driving assembly from the rear inner hull. The electronics were on a small (3 cm) PCB, that could be taken out of the front inner hull after removing a small screw.
I was disappointed by the perfect state of all the components. No oxidation, no moist, no broken wires. So there was no easy clue to the problem.
I took a close look at the circuit board. It has about 8 transistors in TO92 case, a number of SMD transistors and other SMD components, a couple of small electrolytics and an inductor and antenna. A daughterboard was soldered perpendicularly to the PCB, with a chip mounted directly on the board, covered by a drop of black plastic compound. Apparently the chip does the "intelligent" functions of the RC receiver.
I connected the device to a 3V power supply and connected my scope. I found that half of the circuitry on the PCB is a radio circuit. When the RC transmitter was off, my scope only showed a lot of noise delivered to the daughterboard. When the RC transmitter was actuated, the noise was replaced by a more defined signal and a pulse train was seen. The other half of the circuitry on the PCB looks like a bridge that drives the motor. Two connections from the daughterboard go to this bridge. The bridge makes the motor run forward or reverse. When it is in reverse, a gear mechanism makes the two propellors turn in opposite direction so that the sub will turn. As I saw, no signal was coming from the daughterboard to turn the motor on.
I did a lot of additional checks and resoldered the joints between the PCB and the daughterboard, but finally had to admit that the daughterboard is just not working. Without a circuit diagram or reference data on the signals on the board, I wasn't able to fix it.
But anyway, it was interesting to see how this sub was built.