A DIY build of a two-tube FM receiver kit
I recently received a DIY two-tube FM receiver kit from one of my Chinese component suppliers for review. The circuit is elegantly simple, designed around a 1A2 heptode and a 2P2 pentode. The kit includes all the necessary components for the build, featuring a single-sided FR4 PCB with dimensions of 100mm × 100mm.
The primary hurdle when starting this project was the complete lack of proper assembly instructions. To assist others, this article documents the exact steps I took to assemble and test the receiver. I have also reverse-engineered the schematic based on the PCB layout; a high-resolution PDF version is available for download here.
The radio is designed around two main stages to handle frequency conversion and audio amplification: The 1A2 heptode - functions as a super-regenerative detector. The 2P2 pentode acts as the audio power amplifier, which receives the demodulated signal and drives the headphones via an output transformer.
I began by soldering the resistors, capacitors, and 33µH inductors. During assembly, I noticed that several component footprints did not perfectly match the provided parts, specifically regarding lead spacing and component boundaries.
The most significant mechanical issue was mounting the tuning capacitor. The capacitor pins did not align with the PCB holes, requiring me to slightly trim the sides of the pins to fit. To secure the capacitor firmly to the board, I used two M2.5 × 5mm screws, which were not included in the kit.
For the inductors L1 and L2, I used 22 SWG enameled copper wire. Both are wound on a 14mm former: L1 - 2 turns and L2 - 4 turns.
Interestingly, the kit utilizes a standard 230V to 6V step-down transformer as the AF output transformer. The primary (230V) winding connects to the 2P2 pentode, while the secondary (6V) winding connects to the audio output jack.
The circuit operates on two distinct power supplies: a 1.35V DC source for the filaments and a 67V DC source for the anode/plate. Power consumption is notably low, with the filaments drawing about 80mA - 85mA and the 67V line using under 10mA.
For the audio output, I used standard low-impedance (32Ω) stereo earphones.
Tuning this receiver is not a straightforward task. I used an RF signal generator to calibrate it to known frequencies. Overall, the performance is modest; it struggles to clearly receive standard FM broadcast stations. Achieving the best possible results requires a "balancing act" between the C2 variable capacitor and the R2 potentiometer. The potentiometer adjusts the regeneration level, effectively acting as a sensitivity control.
During my tests, I could only pick up a few stations, even when coupled with a 50Ω outdoor FM antenna. However, the receiver performed well with FM microphones and low-power FM transmitters over short distances. A notable limitation is the frequency coverage; sensitivity deteriorates significantly at the higher end of the band. While it performs adequately between 75MHz and 99MHz, it fails to cover the full 88MHz - 108MHz broadcast range effectively.
I would not recommend this kit for those looking for a reliable way to listen to FM broadcasts. However, it is an excellent choice for educational or experimental purposes. It successfully demonstrates the principles of FM reception with a minimal component count, though it understandably trades sensitivity and stability for simplicity.
