Siemens CHR 531

Manufactured by Siemens, Siemens AG Radio & Radar Systems Group, D-8044 Unterschleissheim.

In 1984, Siemens brought out another sophisticated communications receiver, the CHR 531, which also met the highest demands in the reception of radioteletype transmissions and which faced competition from Telefunken (E1501) and Rohde & Schwarz (EK070).

Siemens CHR 531

Technical data

Power supply


  • 448 x 153 x 470 mm, weight 23 kg


  • optional with built-in F1 demodulator: FSK reception 50, 100, 200, 600, 1200 baud; ± 42.5, 85, 300, 400, 1000 Hz; a teletypewriter can be addressed via the corresponding output with ± 20 mA double current or 40 mA single current.
  • optional with A3B - demodulator (6A3B or 6B8E): ISB (independent side band) telephony

Siemens CHR 531


The Siemens CHR 531 is a solid state general coverage receiver that covers all frequencies from the VLF / longwave range of 10 kHz to the upper end of the shortwave range.

With the built-in FSK demodulator, F1 and F4 radioteletype signals can be decoded and a teletypewriter or fax machine can be directly controlled. ISB reception („independent side band“, two different audio channels in two side bands of a signal) needs an optional demodulator.

Via an optional control interface, the CHR 531 can be accessed from a computer and the most important functions can be remote controlled. With the Siemens FSE 401 telegraphy demodulator, F6 (F7B) transmissions can also be demodulated; depending on the configuration, various diversity functions are also available. With the ARQ 1000, the receiver allows reception of ARQ transmissions.

The antenna connector is of BNC standard; an antenna with an impedance of 50 ohms is to be connected here.

The mains switch at the bottom left of the front panel lights up when the unit is switched on, right next to it are the sockets for loudspeaker and headphones and the volume control; when the knob is pulled out, the internal monitor loudspeaker is switched off.

Right next to it are the controls for the RF Gain and the switch for the AGC, the automatic gain control.

In the two right positions of the RF GAIN switch, automatic gain control with short (AM) and long (CW, FSK) control decay time is selectable, in the two left positions, mixed automatic / manual control and in the position on the far left, manual control of the RF gain is selected. In all operation modes with manual gain control, the S-meter indicates the level at which the automatic gain control intervenes or distortion occurs. The RF ATT lamp lights up at high RF input voltages when the signal in the preamplifier needs to be attenuated before processing.

The next two switches are the operating mode switch MODE and the selector switch for the IF filter bandwidth.

The operation mode switch is used to call up the corresponding operation modes, provided that the corresponding filters and options are fitted. For CW reception, A1 is selected, the BFO control is used to set the pitch of the local oscillator. Single sideband reception is selected in the Operation mode A3J, the Obere resp. the Uunter sideband (you are sitting in front of a German set, so German abbreviations accordingly) are switched with the filter switch.

In position F1/F4 the FSK modulator is activated, on the far right the controls for the baud rate (usually 50) and the shift (distance between the mark and space signals) are located. The LED line assists with the correct setting; the polarity of the received signal can be selected with the Π / U control, in the centre position STOP the teletypewriter is at a standstill.

Below the large LED frequency display is the tuning knob, which is also large, and next to it are the control LEDs for tuning. The top LED signals that the decadic tuning is activated. With a button below each digit of the frequency display, the frequency is switched up by one digit. Thus, the button below the 1 MHz display increases the reception frequency in steps of 1 MHz. However, if you want to tune down by one MHz, you have to press the button accordingly many times.

Alternatively, the receiver can be tuned with the tuning knob. The three buttons 1, 2, 3 to the right of the tuning knob can be used to switch between slow (200 Hz per revolution), medium (2 kHz per revolution) or fast (20 kHz per revolution) tuning steps.

Alternatively, the MEMORY rotary switch located at the left of the frequency display can be used to switch to memory mode. The switch selects one of twelve memory channels. Pressing STO and RCL at the same time stores a frequency in the corresponding memory slot; pressing RCL recalls it. A NiCd accumulator retains the memory contents during three months of non-use.

The analogue instrument shows different operating states, in the normal position „RF“ the RF level is displayed, the instrument acts as S-meter. In the „AF“ position, the level of the audio signal is displayed between -10 and +3 dBm. In position ∼ the mains voltage is displayed by measuring the voltage after the mains transformer, in position = the 5 V DC internal voltage.

To check the sideband filters, tune the set to 10 MHz, set the instrument switch to position BITE, with RF gain at the right stop and the upper or lower sideband switched on, tune between 9997 - 9999.7 or 10000.03 - 10003 kHz and make sure the instrument pointer is in the green sector. The AM filters are similarly tested by tuning up or down half the filter bandwidth from 10 MHz and the instrument pointer must also remain in the green sector.

In the TP position, voltages are checked at various test points in the set.

A built-in tester allows checking the receiver function and error analysis. With the cover open, only one test lead is required. With the instrument switch in position TP, the test lead is connected to various test points in the set, the instrument must deflect into the red range.

Technical principle

The receiver is built completely with semiconductors in solid state technology.

The signal first passes through a 30 MHz low-pass filter and then through a preselection module. Inside the preselector module, an LED indicates which frequency range is switched: 0.01 - 1.5 / 1.5 - 2 / 2 - 3 / 3 - 5 / 5 - 7 / 7- 10 / 10 - 13 / 13 - 17 / 17 - 22 / 22 - 30 MHz.

After an HF amplifier, the signal is converted to the intermediate frequency of 70.030 MHz, this high first IF avoids mirror frequency reception, and the oscillator oscillates between 73.030 - 103.03 MHz. After a ± 10 kHz wide filter, the first IF signal is mixed with the signal of an oscillator oscillating at 73 MHz to generate the second intermediate frequency of 30 kHz. The IF filters, which make up the selectivity of the receiver, are active on this second IF; seven filter positions are available.

Inside the unit there are two LEDs that are connected to the phase discriminators and light up when one of the two phase-locked loops is not synchronised due to an error.


The set is solid state.

Technical documentation


Further information

en/chr-531.txt · Zuletzt geändert: 2022/12/27 10:25 von mb