Transceiver A412. Description

The “A412 Transceiver” was the most popular home made HF rig ever built in YO. Since his appearance, the early 80’s and till now, hundreds of models were built, some after the original project, and others with improvements made by hams. The project is the work of YO3NP Nicoara Paulin, YO3JX Dan Voiculescu and YO3DP Stefan Bordeanu, all known as the LIXCO team.

The constructive model that I choose to build is not far away from the original block. My goal from the beginning was the keep as much as possible the originality of this rig. The main differences are:

–          I used a digital scale with microcontroller and LED display, instead of the original mechanical wheal scale;

–          Relays switching for the VFO and LPF.

Block diagram

A412 Transceiver Block Diagram

Working principle

When receiving, the signal from the antenna is passes through the band pass filters, with a band pass of approximately 600 to 1200 kHz, at 6dB. After the BPF, the signal goes to a diode balanced mixer, which mixes it with the VFO, generating a suitable IF. The IF signal obtained is amplified by a bidirectional low noise amplifier. It follows then the crystal filter, an IF amplifier stage, product detector and AF amplifier. One part of the signal after the product detector is used to produce the AGC voltage and to control the S-meter. The AGC signal is used on two IF amplifier stages.

When transmitting, the audio from the microphone is amplified and sent to the diode balanced modulator. On the other side, it is going also to the VOX system, which triggers the diode switch, bidirectional amp, DSB amplifier, the AF amplifier Tx blocking and the antenna relay. The DSB signal after the balanced modulator is first amplified, then applied to the crystal filter which selects the desired side band, and then amplified by the bidirectional amp. The signal gets to the diode mixer and then to the selected working band followed by the wide band amplifier who brings the level up to 100mW, enough for the 5W PA.


Bands 80, 40, 20, 15, 10
Sensitivity 0.5μV for 10dB s/z Power 5W (3W @ 28Mhz)
Stability 150Hz/hour Power consumption 10W
Image rejection better than 60dB Carrier attenuation minim 40dB
IF attenuation better than 60dB Side band attenuation minim 60dB
Internal noise 1μV Harmonics attenuation better than 40dB

Board A412-A, Frontend
Here are the: band pass filters, common Rx-Tx mixer, IF preamplifier, Tx IF amplifier and the wide band Tx preamplifier stage.

A412-A Board


The signal from the antenna is passing through D13 and by one of the BPF diode switching group. It goes then to the balanced mixer, diodes D21-D24, which transposed the receiving signal to 9MHz IF and then to the variable gain bidirectional amp. At receiving, T3 is working, which is controlled either manual, by RF Gain pot, either automatic with AGC voltage. So the signal now obtained on the pin 16 will go the pin 4 on the F board.


The signal from pin 4, board F, is applied to pin 16, in reverse way as when receiving and amplified by T4, where the gain is variable by the Tx Gain pot. When T4 works, T3 is disconnected, and vice versa. The signal is the applied to the mixer and then through the filters. At the filter’s output, the signal passes through D15, D13 is closed, and then applied to the wide band amplifier, T1 and T2. In CW, both transistors are emitter controlled by the C54, C65 and D20 group. The signal now obtained at the output of TR13 (pin 21) is about 50-80mW.

Board A412-B, Power supply
The design of the supply is one with transistors and zener diodes. This delivers 13.5V for most of the circuits, and an extra 10V for the VFO.

Board A412-C, VFO
This board is the local oscillator. The variable capacitor must have two sections of 12 and 24 pF. The main section is the 12pF, and the 24 pF one is used only for 20m band, where the LO frequency is very low. The oscillator is common base type. The D1 varicap does the RIT dial.

Board A412-D, LPF
This is the low pass filters board. The impedance is 50Ω, both input and output and the harmonics rejection is minimum 30dB (only the filters, plus the 20dB attenuation of the PA)

Board A412-F, Mainboard
Here are the rest of the circuits: AF stage, both Rx and Tx, product detector, balanced modulator, BFO, IF amplifier stages, CW tone generator, filters, VOX and the Rx/Tx switch.

A412-F Board


The 9MHz IF from pin 4 passes through one of the filters (CW or SSB) switched by D1-D4, and then amplified by the T1-T3 stage. Then goes to the product detector, D10-D13, which takes the BFO signal from T5. The selection of the desired side band is done with D18 and D19. The AF signal is now obtained on pin 33, from which goes to pin 13. The AF mixing with the signal from the tone generator (T10 and T11) is done on R58 and R24. From pin 14 to pin 11 a volume potentiometer is connected to adjust the level of the AF amp (T15-18).

VOX system

The source signal is either from the microphone amplifier, either from the tone generator. Both voltages are applied to T9, then rectified by D20-D21, then applied by mean of a external time delay potentiometer, to T12, a buffer for TTL switching block.

Rx/Tx switching system

First IC1 is a trigger Schmidt followed by an inverter. This way, two logical levels are obtained, 0 and 1. This logical signals goes then to a series of inverters of IC2. T13 is the AF amplifier Tx blocking.

AGC System

One part of the AF voltage goes through T6 buffer, then the signal is rectified and filtered and controls the AGC amplifier and S-meter (T14). The AGC amplifier is supplied by the +R line, which makes the IF stage to be disconnected on Tx.


From the microphone, the signal is amplified by T7 and T8, and after is applied to the balanced modulator D6-D9. After the T4 IF amplifier, the DSB signal goes through the filters, thus the SSB signal can be found on pin 4. In CW, the microphone amplifier if switched off, the IF amp is switched by D5, but D16 opens and D17 goes closed, so a small RF voltage from the BFO goes to pin 4.

To be updated!

See also related articles:

A412 Transceiver. Construction Part 1

A412 Transceiver. Construction Part 2

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3 Responses to Transceiver A412. Description

  1. Ameer Khan says:

    Dear OM,

    I am interested in homebrewing this transceiver A412. Kindly let me know from where I can get all the details including PCB layout and all circuit diagream etc…
    Please treat this most urgent. I am from India. So I require materials in English.


  2. Adrian says:

    Hi Ameer,
    Thank you for your interest in my project. Unfortnatelly what is on this site right now is what I can give to you. All the necesary infos I have is in jpeg/scanned images from the original book of the rig, but they are all in Romanian (anyway I will upload them after I comment here, because I saw that the link to them is broken now). In time, as I will keep posting the evoulution of my project, I will add all the necesary data in English, including PCBs, connection diagrams etc.

    In other words, I want to say that I am buliding this rig for fun and practice ONLY. The design is to old and obsolete for these days, so is really not a good ideea to build it as your base station. There are many components hard to find these days and some stages inside it are not so performant (i.e. AF amp with germanium transistors, Tx/Rx switch, balanced demodulator, VFO needs an extra buffer stage etc) You can find on the web a lot of easy to build rigs and with good performances like Bitx, Forty, Taurus and many many others. If you need any asistance please feel free to contact me.

    73 Adrian

    • Ameer Khan says:

      Dear Adrian,

      Thank u for the reply.
      I do not propose to use this as a base station now. I am having one ICom rig with me for mail use. While surfing the net net some designs of QRP i came to your blog and A412. i lime the simplicity of the circuit and some of the design principles in it. Then I thought of adopting and modifying them for a new design of QRP i wish to home brew. It is true that components get obselate, even with the ICs the same is the case. we can find some substitutes for the components or we can get some new good ideas from old designs.
      I have subscribed for your feeds to this blog. You have mentioned about Bitx20. It is a version of all transistor in major sections QRP similar to A412. With wide popularity and sufficient support from people and forums it developed to this stage. Actually it is a circuit similar to A412.
      hoping to hear from you regularly regarding the developments

      VU3SQM Ameer khan

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