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Tech Musings

106.92 MegaHertz.

Purty nigh but not plumb. So they fixed it with a little bit of varactor dc bias that is channel dependent.

X5 or X6 multiplier +2V

FM band power stage +2V

That FM multiplexing portion of the circuit is shown in figure one. It seems to be a grunt "me to" BA-1404 circuit. But lacking any internal RF oscillator connections.






100 pF



from varactor bias network

17.78 MHz



82 pF

17.82 MHz


220 pF 120 pF LF FM modulator


Fig. 2 – THE RF MODULATOR PORTION of the Radio Shack 12-2051 FM Stereo Transmitter. 5X or 6X multiplication linearizes crystal pulling.

circuits. Creating the older Pioneer CD-FM-1 and the Sony XA-7A FM Stereo Modulators.

When you can still locate them, the Pioneer unit is single channel but is easily modifiable and worked upon. The Sony version is dual channel but smaller and difficult to adapt. Sony actually manages to linearly pull the special crystal over two adjacent FM channels. Using switched bias.

These small FM modulators were intended to let you add a CD player to your existing car radio. Simply by unplugging and replugging the radio antenna. We did see a circuit and a detailed description of their innards in the Hardware Hacker IV reprints and in HACK52.PDF

All of which just may be ancient history now. Thanks to…

Radio Shack’s 12-2051

This new FM Stereo Transmitter sells for as little as $24. It has four channels. It is crystal stabilized. The intended use is to route audio from a CD player or any other mono or stereo signal a few feet into a hi fi or a car radio receiver.

The unit can run off an internal battery or from an external 3 volt DC supply. Which powers an internal two volt regulator ic and a special temperature compensation circuit. Current is 9.5 mils on low channels and 7 on the high ones. 100 hours of operation are claimed per pair of AA batteries. 800 millivolts (the usual "line" level of 0 DBM) is needed for full modulation.

Distortion specs are not all that great at one to three percent.

Several sneaky tricks are used to let you pick four stable channels. A switchable pair of crystals gets used. Their frequencies get multiplied by either five or six. Which means the crystals now only have to be pulled one-fifth or one-sixth as far. Greatly improving upon their linearity and simplifying the circuit design.

Uh, they did cheat a little as their numbers don’t quite work out exact. The 17.78 MHz crystal hits 88.9 right on by using its fifth harmonic. But it slightly misses 106.7 by landing at

    • 106.68

      with its sixth. Similarly, the

    • 17.82

      MHz crystal hits 89.10 exact,

but misses its 106.9 by ending up at

That box I have labeled varactor bias circuit is rather obtuse. What this all passive resistor-diode-switch- capacitor circuit does is combine the multiplexed audio with a temperature compensated master dc level and a custom switchable bias offset for each selected channel. Needed audio preemphasis is also increased a tad on the high two channels.

The net result is a dc varactor bias value appropriate for your selected channel, with properly emphasized audio superimposed. See the RS docs if you really need the gory details.

The discrete RF part is shown in figure two. That varactor diode gets used as an electronically variable capacitor to pull one of two selected crystals. The voltage on the varicap is the sum of the L+R mono audio channel, an L-R signal on a 38 kHz subcarrier, and possibly a dc fine tuning value.

The first stage oscillates in the 18 MHz region. The second stage then multiplies by five for the low two channels or by six for the high two channels. The final circuitry is a grounded base linear amp which operates at FM band frequencies.

Switching can add extra tuning capacitance to the multiplier and output stages. Shifting your tank resonance accomodates either the high or low channels.

I’ve simplified Figure 2 a tad for clarity. The hi-lo switching is really done with NPN transistors. Further, each of the stages is independently decoupled from +2 VDC by its own RC filter network. See the RS docs if you need additional detail.

Two mechanical switches are used for frequency selection. One picks the crystal; the other peaks for low or high frequency channels.

Note particularly the 1K resistor in the final output stage. Radio Shack apparently added this in their "-A" version to sharply reduce the range. This may have been done to meet an


Copyright c 1998 by Don Lancaster and Synergetics (520) 428-4073 www.tinaja.com All commercial rights and all electronic media rights fully reserved. Reposting is expressly forbidden.

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