Solved - Help with 27Mhz R/C toy

[rodcastler]

Hi! complete noob to radio transmission, trying to troubleshoot my 3yr old son's Go MINI Remote Controlled Wolf

Issue:
Everything works fine as long as the transmitter antenna and receiver antenna are 1 inch away.
At any further distance it all keeps working (even up to a few feet away which is good) however the behavior becomes intermittent. For example the motor -that should run steady- starts jumping at somehow 4 pulses/second which ends up being super-anoying. As soon as the antennas get really close together, it goes nice and smooth and very steady. This happens when the motor turns in either direction.

It came like that from factory. Too late to return it.

What have I tried:

1.- Check the antennas. Both are well soldered in place.
2.- Fresh batteries on both Rx & Tx (this should have been step#1 but you know what they say on the EEVblog....)
3.- Removed the wax from the variable inductance thingy and turned the ferrite while running the motor in order to find a sweet spot. This improved my range to 2 inches. The reason why I tried this is because since it came bad from factory, I thought this is the only human-error related step.
4.- Added a capacitor to the motor in the hopes of filtering noise coming from there. Also tried capacitors to an unpopulated cap on the board (they cut corners sometimes and who knows...)
5.-Tried a longer antenna.
6.-Tried different places in the house to avoid external interference.

None of these have helped.

Questions for you:

- What is the correct next step?
- Where should I be poking my scope at?

The board seems simple: A micro controller on a daughter board, an H-Bridge and a few discrete components. Here are some photos for you. The Rx antena came soldered to a copper sticker on the inside roof of the car. The red wheel is the transmitter.

[electr_peter]

Simple and cheap RC toys are known to have very poor range. This could be due to badly matched RX/TX antennas, low power, poor transmit/receive circuitry, mismatched frequencies, etc. - in short, hard to tell and difficult to improve.

Possible actions:

• buy/trade/return a better RC toy or better RC radio. Decent RC radio should give at least 50m (which is plentiful), hobby grade radios - 300m or even 1km.
• or get a oscilloscope and/or spectrum analyser and start checking frequencies and power levels. Adjust matching circuits and antennas, boost power in extreme case (power should be more than enough for 10m)

[rodcastler]

Simple and cheap RC toys are known to have very poor range.

I know I shouldn't expect a lot from these, but this is a licensed toy and I can see some signs of decent quality on this particular toy.

So yes, I am aware of that. But 3cm of range? That can't be right.  I've seen the exact same toy working perfectly at a 5 meter range. That's why my guess is that something must be wrong and can be greatly improved.

[4cx10000]

Hi!

Looks like the slot on the ferrite slug - T1 - is rounded. Have you tried to tune the transformer? Using a metal screw driver will most probably crack the slug and detune the circuit as well. Guess the 2SC1815GR work as an RF-amplifier. Check the transistor itself and the surrounded components 

[brevor]

Study the receiver board and find the trace where the data signal from the receiver circuit connects to the daughter board. Put your scope here, hold down a button on the transmitter and adjust the tuning coil for the strongest signal.

[rodcastler]

Study the receiver board and find the trace where the data signal from the receiver circuit connects to the daughter board. Put your scope here, hold down a button on the transmitter and adjust the tuning coil for the strongest signal.

Cool! that helped. Also, the scope revealed a power loss at the Rx chip while the motor was running. I replaced a resistor that was limiting the current to the circuit and boom. The kid has a winner car. 

Wouldn't have made it without you guys thanks!!!!

[Psi]

First thing I would do is look at the antennas and make sure they are actually connected. Cheap China soldering on toys is not the best