I bought an AM transmitter kit for $20 from AliExpress just to test the AM performance of my various radios during daytime, as most AM stations can only be heard at my location in the evening. The kit arrived after a week, complete with circuit schematics and assembly instructions. Below is the circuit schematic, which is partly in Chinese:
The schematic does not fully match the PCB. For example, a power switch is shown on the schematic but there is none on the PCB. Despite this, the schematic is still perfectly usable and within an hour I was able to assemble the complete circuit board:
The kit includes almost all components, except for a missing 10K trimmer resistor (R15) which is needed to adjust the modulation depth. Luckily I managed to find the exact same resistor with exact same value and form factor from an old radio. Despite looking a bit rusty, the resistor worked well and my AM transmitter worked right out of the box. After connecting the transmitter to a 9V power supply, an audio source and a short wire antenna for testing, I set R15 (Modulation Depth), SW2 (Audio Quality) and SW1 (Input Volume) to roughly 50%, put my radio to an empty frequency and adjust C15 (Transmitter Frequency) until the audio could be heard on my radio. Take note that C15 can only be adjusted from the other side of the PCB. Once then, SW1, SW2 and R15 can be further adjusted for the best possible audio quality. If the output audio still sounds a bit distorted, try to adjust the volume from the input source. The output voltage level on the headphone jack can be anything from 0.3V to 1.5V depending on the device, so some adjustment will be needed to get things to work just right. Adjustment of the IF transformer B3 is not needed but you can always try that if all else fails.
The frequency coverage is between 530kHz – 1600kHz, which can be adjusted by using a different value for C15 or by connecting capacitors in series/parallel to adjust its capacitance. Connected to a good long wire antenna with no obstructions, the distance is around 500m in my tests. You can run at 12V to increase the distance even further. If you do, please make sure that the heat sinks on the D882 transistors do not get too hot, else the transmitter might stop working.
This transmitter is not without issues, however. For one thing, no voltage regulator is used, so if the transmitter is run on a battery, the frequency will drift significantly (and the distance will also decrease) as the battery voltage drops. You can improve it by running at, let’s say 12V or 15V, and use a 78L09 or 78L12 as voltage regulator. Alternatively, operate it with a transformer-based power supply to ensure that the voltage always stays constant. Obviously, a switched mode power supply cannot be used because switching noises will affect the transmitter operation. Still, even with a stable power supply, the frequency might still drift a little within the first five minutes as the components heat up but will settle to a constant value once the temperature stabilizes. To overcome this issue, expensive devices such as function generators or frequency counters have integrated cooling fans or even a feedback mechanism to cater for frequency drift. All of this is out of the capabilities of this low-end hobbyist transmitter.
Another minor inconvenience is the inability to reach an exact frequency, which is understandable given that this is an analog transmitter. Although this should not matter if you are using an analog receiver, digital receivers may not find the transmitted station during an auto-scan if the transmitted frequency is not at the frequency expected by the radio based on the AM spacing setting (9kHz or 10kHz). For example, if the transmitter peaks at 608kHz, my Tecsun PL-360 will not find any station at 612kHz in 9kHz AM mode, unless the radio is very close to the transmitter. This is despite the fact that the station can still be heard clearly at 612kHz. This is most likely because the signal-to-noise ratio (SNR) at 612kHz is below the detection threshold of the auto-scan algorithm, even though the station sounds great at 612kHz. The Tecsun SNR calculation is very conservative and will return very low value or zero if the signal is only slightly distorted, even when the station is still clearly audible. This issue does not affect me at all, as I can simply navigate to the wanted frequency manually without using auto-scan.
Overall, this is a very good transmitter for the price and you can definitely consider purchasing one from Aliexpress just to play with. The scanned copy of the circuit schematics and English/Chinese instructions, with some useful information on how to adjust the transmitter for the best signal quality, can be downloaded here.