Cassette restoration. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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A cassette recorder that has been lying idle for a long time, morally and physically obsolete, can find a second life and serve as a quite decent cassette player with good characteristics. The modern elemental base makes it possible to make such an alteration quite easily, with little time. What about record mode? At the present time, when audio cassettes with recordings are sold on almost every corner, at prices only slightly higher than the cost of "clean", it is simply a pity to waste your time on "writing". If someone has doubts about the quality of the Soviet-made tape drive mechanism, then try measuring the detonation coefficient of a Chinese player after a year of operation and compare the readings.

For modernization, from a wide variety of imported microcircuits, BA3521 f.ROHM was chosen, which is a stereo playback amplifier with electronic volume control and built-in output stage overload protection. Direct connection of the magnetic head to the playback amplifier of the microcircuit without a decoupling capacitor allows you to expand the dynamic range and improve the sound quality. Electrolytic capacitors give specific distortions, and their absence in the signal path circuits is a rather tangible advantage over similar microcircuits. The microcircuit is made in a standard 18-pin DIP package. The purpose of the IC pins is given in Table 1, the electrical parameters are in Table 2, and the player circuit is in Figure 1.

Fig.1. Schematic diagram of the player (click to enlarge)

Table 1. Pin assignment BA3521

number	appointment	DIP	number	appointment
1	General		18	UPete/2
2	Input 2 channels		17	Input 1 channels
3	Frequency response correction		16	Frequency response correction
4	Frequency response correction		15	Frequency response correction
5	"Mute"		14	Volume
6	-		13	-
7	2 channel output		12	1 channel output
8	"Mute"		11	-
9	General		10	UPete

Variable resistor R3 provides electronic volume control in both channels simultaneously. Chains C5-R2-C4 and C8-R5-C9 correct the playback frequency response according to the NAB standard for magnetic tape Fe₂O₃ (type 1). When output 5 of the IC is connected to output 8 by the SA1 switch, the output is blocked - the "Mute" mode (shutdown). If this mode is not needed, you can leave these two outputs free. Elements R6, VD1, C10 form a parametric voltage regulator, C11 - an interference suppression capacitor of the "Mute" mode. It is set to eliminate clicks when the output is enabled/disabled. Chains R7-C12 and R8-C13 limit the current at the control outputs (non-regulated outputs).

Table 2. Electrical parameters of the player

There are no special requirements for accessories. Suitable for almost any public, including imported ones. Resistors - type MLT, C2-33 with a tolerance of at least + 5% and a power of at least 0,125 W. Electrolytic capacitors must be with a voltage of at least 10 V, for example, type K50-35. Their capacity can be safely increased by 2...5 times. The remaining capacitors are ceramic, K10-7v, K10-17, KM with a tolerance of at least ± 10%. Any low-power zener diode VD1 is suitable for any low-power stabilization voltage of 4,7 ... 6,2 V. It is permissible to replace the BA3521 microcircuit with the BA3520 or CXA8008R, which are made in a similar package, have an identical switching circuit and slight differences in electrical parameters. When you install known-good elements, the circuit starts working immediately, no adjustment or selection by rating is required.

The main problem that may arise is the magnetic head (its overall dimensions and method of fastening). Therefore, it is better to install a domestic magnetic head (ZD24N080, ZD24N091, etc.) in Soviet-made equipment, in an imported one - similar to the one that stood. Replacement of the magnetic head and its adjustment (height and tilt adjustment) must be done without fail! You can control the correct installation, if there is no special measuring equipment, by ear, focusing on the loudness and purity of the sound of high frequencies in the reproduced phonogram. Accordingly, the cassette must also be selected with high-quality recording and with a large number of high sound frequencies.

In portable models of tape recorders and radio tape recorders of Asian assembly, there is more than enough free space, so there are usually no problems with installing the assembled board in the case. In Soviet-made tape recorders, despite the decent dimensions of the case, it can be difficult to find a free place near the magnetic head. I can suggest such a way out: find the playback amplifier zone on the old board, remove all now unnecessary radio components, carefully cut out a place for a new one, install it in the cut hole and fasten both boards together. Be careful and take your time during this operation - you can accidentally damage other necessary components of the tape recorder: the engine speed control, autostop or power supply. The erase and bias generator, as well as the recording amplifier, are removed from the old printed circuit board. If you do not want to mess around, just remove the erase head and cut the power path to the generator.

When upgrading, the main condition is to place the assembled playback amplifier as close as possible to the magnetic head and connect it only with a shielded wire with high-quality soldering of the shielding wire (braid).

As a resistor for the volume control, you can try to use the one already in the device. It is connected with a conventional mounting wire. If the wires turned out to be too long, or the resistor has a slight but acceptable wear, and rustles and crackles are heard during adjustment, connect a blocking capacitor with a capacity of 0,01 ... 100 uF between its middle terminal and the case. The sound volume is adjusted simultaneously in two channels, so there is no need to look for a dual resistor.

The output voltage level of the playback amplifier is quite high and allows using the tone control of the device being upgraded, both active and passive. Most often, the elements of passive regulators are soldered directly to the terminals of variable resistors, and there are no difficulties here. You can also use a microcircuit like TDA1524A, TDA1526, K174XA48, which provides electronic volume control (not used in this case), balance and treble tone, LF with a single resistor in both channels simultaneously.

As practice has shown, even the obsolete UMZCH TDA2003 (K174UN14) microcircuit "sounds" much better than many standard microcircuits in Asian-assembled devices or Soviet-made transistor-transformer amplifiers. The choice of power amplifiers in a microcircuit design is quite wide. You need to focus on the voltage and power of the power supply of the tape recorder, the availability and price of the microcircuit. Installing a two-channel amplifier in portable equipment and making a completely stereo version does not make sense - the batteries will run out faster, and, as a rule, there is only one speaker. And for home listening it is better to make a separate remote high-quality power amplifier with good acoustics.

If after the upgrade one more variable resistor remained unused (it used to be a recording level control), you can additionally install a printed circuit board of a VHF receiver with frequency tunability on varicaps and use this resistor for electronic tuning to a broadcasting station. In this case, the "Record" button is used to switch the input of the power amplifier from the output of the playback amplifier to the output of the receiver.

Author: S. Gvozdev

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