1
\$\begingroup\$

Trying to fix an old transmitter from some rc toy, I may have removed the crystal oscillator some 4~5 years ago so I can only guess it was either 27Mhz or 40Mhz.

I tried 27Mhz crystal oscillator (I don't have 40Mhz) but the transmitter didn't work on any toy that have an rc receiver (tried many), working transmitters read high (2.xx volts) on antenna, mine does not.

Transmitter uses SM6136B ic, very similar to TX-2B and have same pinout. I have seen circuits that use 27Mhz oscillators with these ics but the circuits are entirely different, anything I can do to make/modify it to work? IC appears to be fine as it does output voltage on SO pin (pin 8) when any of the switches are pressed.

Schematic: (Added missing ground and some fixes) Schematic

PCB Front and Back: enter image description here

Congested part with components: enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Voltage Spike Dec 5 '20 at 23:32
2
\$\begingroup\$

The teal colored 'resistor' that you call R3 is actually an inductor, and the missing 'resistors' R7 and R8 probably are too.

Looking at some other transmitter schematics may give you a clue as to what values might work. I got a number of good hits from googling "toy rc transmitter 27Mhz schematics", including this...

enter image description here

I tried 27Mhz crystal oscillator (I don't have 40Mhz) but the transmitter didn't work on any toy that have an rc receiver

Apart from being on the correct frequency, it must also use the same transmission protocol. If the receiver uses the matching decoder chip or equivalent (eg. RX2, SM6135, HES3017) then there is a reasonable chance of the controls working, though they may be mixed up. Most toy RC receivers of this vintage use a superregenerative front end which is broadly tuned, so the exact transmitting frequency doesn't matter - it just has to be on the same band (27 MHz, 40 MHz etc.).

Get the transmitter working by adding the missing inductors, then try each of your toys again. You might get lucky!

\$\endgroup\$
2
  • \$\begingroup\$ WoW! I never would have guessed that it was an inductor :O \$\endgroup\$ – asim Dec 5 '20 at 22:32
  • \$\begingroup\$ You had no need to guess. I told it in a comment which said it partially compensates the shortness of the antenna. But the given answer is fine. \$\endgroup\$ – user287001 Dec 6 '20 at 1:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.