Reverse Engineering a walkie talkie

Wireless & IoT
1

Hello all! I’m new to these forums and I’m also relatively new to electronics. I want to share a project I’ve been working on for the past few months.

I have been reverse engineering a pair of walkie talkies that I’ve had since I was a kid. I finally have a circuit diagram (that part took the longest) but there is a “mystery component” that I need help indentifying, namely L2. I am not familiar with the layout of this type of FM transciever so if there is something that ‘clicks’ and makes sense to you please post! I would be very interested in what any information that would shed some light on the how and why of this circuit.

Here’s what I know:

  • Powered with a 9 volt battery

  • Classified by the FCC as DXC Part 15 Low Power Transceiver, Rx Certified

  • FM modulated

  • Upper frequency listed as 49.90000000 Mhz

  • Lower frequency listed as 49.82000000 Mhz

  • Uses the speaker as a microphone when transmitting.

  • Q1 is a C3192 transistor (High Frequency) http://pdf1.alldatasheet.com/datasheet- … C3192.html

  • Q2 and Q3 are C3198 transistors (general purpose) http://pdf1.alldatasheet.com/datasheet- … C3198.html

  • Q1 has a ferrite bead on its emitter

  • L2 is variable but comes glued in place from the factory

  • L2 seems to have the configuration of a centertapped transformer but I’ve only been able to test this with a multimeter since I don’t have a function generator. I did cut the traces to remove it from the circuit before measuring.

Pics!

http://farm6.static.flickr.com/5121/533 … e13c3e.jpg

The hardware!

http://farm6.static.flickr.com/5009/533 … 371c29.jpg

[<LINK_TEXT text=“http://farm6.static.flickr.com/5209/533 … b573_z.jpg”>http://farm6.static.flickr.com/5209/5335981281_182af8b573_z.jpg</LINK_TEXT>

[fm_circuit2 by [East_India_Tea, on Flickr

The 12 pin component is the push to talk switch and the middle unlabeled component is the On-Off switch

http://farm6.static.flickr.com/5009/533 … f59b1f.jpg

L2 is that strange white and black component on the left.

http://farm6.static.flickr.com/5087/533 … 34fc75.jpg

Another view of L2

[<LINK_TEXT text=“http://farm6.static.flickr.com/5248/533 … a861_b.jpg”>http://farm6.static.flickr.com/5248/5335933741_5388dea861_b.jpg</LINK_TEXT>

[Transmitting Circuit Diagram by [East_India_Tea, on Flickr

Transmitting

[<LINK_TEXT text=“http://farm6.static.flickr.com/5045/533 … 981a_b.jpg”>http://farm6.static.flickr.com/5045/5335933727_8b05e8981a_b.jpg</LINK_TEXT>

[Receiving Circuit Diagram by [East_India_Tea, on Flickr

Receiving

Eventually I hope to build a new pair from these schematics if I can understand this circuit.

Thanks! :)](About East_India_Tea | Flickr)](Receiving Circuit Diagram | After much work, an actual circu… | Flickr)](Receiving Circuit Diagram | After much work, an actual circu… | Flickr)](About East_India_Tea | Flickr)](Transmitting Circuit Diagram | This is the schematic for tra… | Flickr)](Transmitting Circuit Diagram | This is the schematic for tra… | Flickr)](About East_India_Tea | Flickr)](fm_circuit2 | Circuit board after all components are labeled… | East_India_Tea | Flickr)](fm_circuit2 | Circuit board after all components are labeled… | East_India_Tea | Flickr)

2

It might be an adjustable coil.

Does the center (core) screw in and out? It might be glued in place…

3

The circuit is a classic example of doing the most with the least. I tried to reverse engineer similar devices over 20 years ago! I guess once a classic, always a classic… The receive section, is a super regenerative (google) receiver. It normally uses a single transistor as an RF amp, and also an ultrasonic quench oscillator. The oscillator allows the transistor to operate in a very unstable region where it becomes a super high gain RF amp. Before it goes into self oscillation, the ultrasonic part resets it and starts it again. These usually produce a characteristic “rush” in the speaker. L2 is an RF coil tapped for maximum efficiency. Creating a receiver like this, is like balancing an egg on a basketball. But…I must say, I learned a huge amount in reverse engineering these, as well as other RF devices over the years. With the release of inexpensive RF ICs, these circuits fell by the wayside due to their inherent RF noise and difficulty to get stable.

Good luck and have fun,

Ron

4

Thanks so much Ron!

You’ve given me much to research 8)