9
\$\begingroup\$

I got an old RF circuit (I guess), probably used for pedagogical purposes. Here's the schematic:

Schematic

I chose the positive and ground rails as it seems right to me, looking at transistors and resistor values. But I could be very wrong, because there are no wires/labels at all. The three unnamed transistors are 2N2923s. Resistors at the output of 3a and 3b are 470k.

It looks like an RF transmitter to me, maybe FM.

Is the block n°1 a known topology? If so, what's its name? Where should the antenna be connected? Is the crystal oscillator fixing the carrier frequency at 27.120 MHz?

Moreover, I'm puzzled by the blocks 3a and 3b. Are they input(s)? Why are there two nearly (different resistors values) symmetrical blocks? Again, I have absolutely no clue on where the inputs should be (no wire or obvious terminals).

Finally, if I'm right on the power rails, is there a safe voltage I could apply to test the circuit? The 20 µF capacitor is rated for 10 V.

Here's the physical circuit:

Physical circuit

Edit:

Regardless of the applied voltage (between 2V and 8V) I can't manage to make the two phase-shift oscillators oscillate.

But the RF oscillator works fine! I measure a frequency of \$28\pm2 \text{MHz}\$ with an oscilloscope. This is what I get when I directly probe the 2N1711's collector (0.8 Vpp when powered at 8 V, with the variable capacitor properly tuned):

Output RF signal

However, I can make it modulate when I apply a signal on the base of the transistor in block n°2 through a resistor. At best (adjusting signal frequency and amplitude), I manage to get a pretty good modulation (at the bottom, the TTL output of my generator):

Modulated RF signal

Edit 2:

After replacing the 22 µF capacitors, the phase-shift oscillators works. At 6V, the 3a oscillate between 2.86 kHz and 2.30 kHz, and the 3b between 3.08 kHz and 5.08 kHz.

\$\endgroup\$
  • 1
    \$\begingroup\$ First time here, is the hand-drawn schematic an issue ? Should I post a picture of the physical circuit ? \$\endgroup\$ – Félix Faisant Jul 28 '18 at 18:20
  • 1
    \$\begingroup\$ looks like a remote control transmitter with a joystick (or two pots) \$\endgroup\$ – jsotola Jul 28 '18 at 18:26
  • 1
    \$\begingroup\$ please post some pictures \$\endgroup\$ – jsotola Jul 28 '18 at 18:28
  • 1
    \$\begingroup\$ @FélixFaisant, if you have the actual circuit, why did you not provide more information, like voltage ratings of components? \$\endgroup\$ – jsotola Jul 28 '18 at 18:38
  • 1
    \$\begingroup\$ For the phase shift oscillators, ensure that the collector voltages are around half of what's on the other side of the collector resistor, which tells you that the transistor is part way on. Only then can it oscillate. But more likely, the capacitors went bad. Together they may make some sort of DTMF (touch tone telephone) signal. \$\endgroup\$ – Alex Cannon Jul 29 '18 at 2:21
6
\$\begingroup\$

It's Citizen's Band (about 27MHz) AM transmitter which sends a dual frequency tone - maybe it's intended to activate or turn off some machine.

The crystal stabilizes the frequency, it cannot be FM. Two phase shift oscillators generate the modulating signal. It's possible to adjust the frequencies of the modulating signals.

The modulation isn't especially effective. It reduces substantially the output RF power, but it can still be ok for the intended purpose. At least it's simple.

Better modulation principle is to swing the operating DC voltage of the RF oscillator by adding an AC component to it with a transformer.

The antenna would be connected through few pF capacitor to the collector of the leftmost transistor. Because it's an oscillator, not much power can be sucked out of the circuit. Otherwise the oscillator stops. The antenna definitely cannot have max. power transmission matching, it can be connected only very loosely to the circuit.

After seeing the photo: A circuit this big with no shielding very likely is easy to detect with a normal CB receiver at 10 meters. An antenna probably has never been used.

\$\endgroup\$
6
\$\begingroup\$

It looks like an RF transmitter to me, maybe FM

It smells of amplitude modulation rather than FM because of the way 2 modulates the base bias in the oscillator.

Moreover, I'm puzzled by the blocks 3a and 3b. Are they input(s)? Why are there two nearly (different resistors values) symmetrical blocks?

3A and 3B are two independent phase shift oscillators that amplitude modulate at two different frequencies simultaneously thus giving the transmission some degree of integrity. The receiver will likely demodulate the transmission and only operate (maybe a garage door or lock) when it is getting both baseband frequencies.

Is the block n°1 a known topology? If so, what's its name? Is the crystal oscillator fixing the carrier frequency at 27.120 MHz?

The oscillator is a type of hartley oscillator that uses a crystal to lock-in the frequency of operation.

Where does the antenna should be connected ?

It quite possibly uses the split coil to generate just a localized magnetic field rather than a full EM field. This works at much less distance than an EM radiation because only one field is being transmitted. If you don't have an antenna connection then that settles it - garage door opener is the likely product.

Finally, if I'm right on the power rails, is there a safe voltage I could apply to test the circuit? The 20 µF capacitor is rated for 10 V.

It's likely it will work at round about 6 volts. You could try using a bench supply to raise the voltage from 0 volts to about 8 volts but don't go higher given the capacitor is rated only at 10 volts.

I can't manage to make the two phase-shift oscillators oscillate.

Quite likely is that the 22 uF capacitors on each have gone very leaky and are taking enough current through their respective pull-up resistors to render each phase shift oscillator "silent". Try measuring the voltage across them and report back.

\$\endgroup\$
  • \$\begingroup\$ Thanks. Given the size of the circuit, it's indeed probably radiating enough without an antenna. \$\endgroup\$ – Félix Faisant Jul 28 '18 at 21:16
  • \$\begingroup\$ There is 0.2V across the 330Ω pull-up resistors, when power with 6V. So it seams fine ? I replaced a 22µF cap and the phase-shift oscillator works. I measured the leakage resistance of the old cap, and it is high enough. I have nothing to measure its capacity right now. \$\endgroup\$ – Félix Faisant Jul 29 '18 at 12:46
  • 1
    \$\begingroup\$ If the 22 uF had gone high impedance also, the phase shift oscillator would have no AC 0 volts hard reference so this would also cause failure. Old skool electrolytics were never really expected to last more than ten years! \$\endgroup\$ – Andy aka Jul 29 '18 at 13:03

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.