Audio signal level indicator for ILT6-30M. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The article proposes the use of the Mayak MP-240S cassette recorder assembly as a two-channel signal level indicator for an amplifier, sound card or other amateur audio equipment.

When disassembling outdated or faulty industrial radio equipment for spare parts, various functionally complete modules may be available, which are more profitable not to be divided into spare parts, but used for their intended purpose [1], giving them a "second life". So, when disassembling outdated cassette tape recorders of the Mayak-23x and Mayak-24x series, you can get a working module of a two-channel two-color indicator of the recording and playback level, assembled using a vacuum electroluminescent indicator (VEI) ILT6-30M.

Indicators with displays such as in fig. 1 usually look more impressive than simple LED signal level indicators. Such a device in modern multimedia designs will resemble the era of transistorized magnetic recording equipment, and for some, lamp luminescent "eyes" used in old tube radio equipment. For amateur radio use, the most convenient level indicators are assembled on KR1534PPZ or KR1534PP4 microcircuits. Such indicator modules require only three supply voltages for their operation: +15 V, -15 V and a galvanically coupled voltage of 4,25 ... 5 V DC or AC. It should be noted that the level indicators assembled using the KR1534PP2 microcircuits (Mayak-233 stereo tape recorder [2]) require four supply voltages for their operation.

Rice. 1. Signal level indicator display

The indicator module with VEI ILT6-30M has a printed circuit board size of 140x70 mm, and the mounting height from solder tails to the display mounting spring is 50 mm. The current consumption in the +/-15 V circuits does not exceed 18 mA, the filament supply current is about 100 mA at a voltage of 4,5 V. Such a module can be easily integrated into most audio frequency power amplifiers with bipolar power supply. The dimensions of this module allow you to install it in a "five-inch" compartment of a computer case, but then for power supply you will need to make an additional voltage converter +12 V to -12 V and a 5 V DC or AC galvanically not connected to them, preferably DC due to easier control effective voltage value.

To use the level indicator module from the Mayak-240C cassette recorder, it needs to be slightly modified. On fig. 2 shows the complete scheme of the modified module. Designations of additionally installed parts begin with the prefix "1". When finalizing, it is necessary to install blocking capacitors 1C1 and 1C2, as well as chokes 1L1 and 1L2. The manufacturer saved on these details, as a result of which, when playing audio recordings, interference from the dynamic switching of the VEI scales is possible.

Rice. 2. Scheme of the modified module (click to enlarge)

It is also mandatory to install resistors 1R5 and 1R6. The input resistors create a galvanic connection with a voltage source of -12...15 V and minimize the difference in the potentials of the filaments that act as the SEI cathode.

Resistors 1R1 and 1R2 are protective for the input circuit of the DA2 microcircuit. Trimmer resistors 1R3 and 1R4 can reduce the input signal level if it exceeds ~1 Veff. Capacitors 1C1, 1C2 are installed in place of the connector XP1, and the interfering printed conductors are cut. The remaining additional parts are mounted on free sections of the circuit board.

A photo of the installation of the upgraded signal level indicator module is shown in fig. 3. Inductors can be used with any small-sized inductance of 47 ... 100 μH. Capacitors 1C1, 1C2 - K50-35, K50-68 or their imported analogues. Fixed and trimmer resistors - any type of general application.

Rice. 3. Installation of the upgraded signal level indicator module

Sensitivity simultaneously for both channels can be adjusted by trimming resistor R5. When connecting the signal inputs of the indicator module to the line output of a computer sound card or the output of the preliminary UZCH, trimming resistors 1R3, 1R4 can be omitted. When connecting the inputs of the indicator module to the outputs of the UMZCH with a power of more than 6 W, the resistors 1R1, 1R2 must be 47 ... 150 kOhm in order to avoid accidental damage to the DA2 chip.

If the indicator module is built into the case of some UMZCH, which, for example, has an unstabilized bipolar +/-40 V power supply, then to power the built-in module it is useful to make two simple parametric voltage stabilizers +/-12.15 V, for example, by analogy with the construction [1]. To power the VEI filaments, you can use the 6,3 V AC voltage available in the network transformer of some amplifiers to power indicator lamps, which is fed through an additional current-limiting resistor to pins 1 and 2 of the XP1 connector.

In the absence of a suitable voltage to power the filaments, the simplest solution would be to wind an additional 0,18 ... 4,25 V AC winding on the coil of a network transformer with a thin wire (for example, MGTF-5) at a load of up to 100 mA. For network transformers of power supplies in powerful UMZCH, 2 ... 4 turns are usually needed for every 1 V of AC voltage. Also, a good way to obtain the missing voltages can be the use of compact upgraded power supplies or pulse transformer converters from accounting desktop electronic calculators or cash registers.

The device does not have protection against polarity reversal of the supply voltages, therefore, when connecting to a voltage source, errors must be excluded. If, in the absence of an input signal on the indicator, more than one double element of the scale lights up in one or both channels, then any interference penetrates the input of the device, for example, from a switching power supply, a mains transformer, or a nearby electric soldering iron. If necessary, connect the level meter module to the audio signal source with shielded wires.

Literature

1. Butov A. Electronic clock from the timer "Electronics VM-12". - Radio, 2006, No. 1, p. 39.
2. Tape recorder-prefix "Mayak-233 stereo". - User guide.

Author: A. Butov

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