Completion of the e-book Texet TB-840HD. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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The article describes how, by simple means, to increase the volume and improve the quality of the sound accompaniment of the files played by the named e-book and increase the convenience of using it in the mode of receiving VHF radio broadcasts.

Electronic books (EC) with a color TFT touch display, sold under the brand name "Texet", belong to the class of inexpensive multimedia devices, but, in fact, these are not simple mobile "readers", but multifunctional multimedia devices that "understand" a huge number of formats of various text, graphic, audio and video files, work with FAT, FAT32, ExFAT, NTFS file systems. Unfortunately, both in this model and in all other tested EC "Texet", the sound of the built-in dynamic head did not please with either quality or volume, noticeably yielding even to the sound of small-sized mobile telephones.

Initially, it was thought that the relatively poor and quiet sound of these ECs was due to the use of a low-quality dynamic head. To check this version, the EC had to be disassembled - for this it turned out to be enough to unscrew four self-tapping screws, and the aluminum back cover of the case sitting on the latches was easily removed (a view of the installation of the device is shown in Fig. 1; it also shows the new parts installed during revision, about than will be discussed later). But then an unpleasant surprise awaited - the touch screen stopped responding to touch. It didn't take long to find the reason. The assembler of the device glued a rechargeable battery (AB) on top of the display backlight power wire. The back cover pressed on the unevenly lying AB, and that, in turn, pressed on the back metal wall of the display, due to which it was deformed. In this state, the device manufacturer performed the touch panel calibration...

Rice. 1. View of the installation of the device

To eliminate the malfunction that occurred, the interfering wire was fixed with adhesive tape away from the battery. Due to the wire that fell under it during assembly, it was held on only by one strip of adhesive tape, ready to come off at any moment (a malfunction often discussed on the manufacturer's Internet forum). To prevent this from happening, two strips of thin double-sided tape were additionally glued to the AB case, after which the AB was evenly pressed against the back wall of the display with the entire plane, and for greater reliability, it was also glued to it on the sides with a small amount of Quintol-Lux glue. After that, imitating the eliminated defect by pressing a finger, he entered the touch panel calibration menu and performed a preliminary calibration. (In the absence of forced deformation, it is necessary to perform a preliminary calibration, the final one should be done after assembling the structure, with the back cover closed. Make sure that the display is placed exactly in the latches that compress it).

As for the quiet sound, it turned out that the main reason for this was not in a bad dynamic head, but in the use of an insufficiently powerful AF amplifier: the output signal swing when playing audio files reached a maximum of 1,5 V (and even less when playing video files). This is very small in comparison with modern UMZCH in other mobile devices that are capable of developing a voltage with an amplitude of 1 ... 2U at the load, where U is the device supply voltage.

There was no desire and technical capabilities to produce a new built-in UMZCH, so it was decided to replace the dynamic head with another, more sensitive one. Having tried more than a dozen miniature heads with 8 ohm coils from various mobile devices, I settled on the KSJ (R) head from the mobile navigator "Explay" (it turned out to be the best result). True, the dimensions of its diffuser holder are larger and it is 1,5 mm thicker than the “native” one, so it had to be glued (with a thin double-sided adhesive tape) not to the back cover of the EC, but to the circuit board (Fig. 2, top left). After checking the performance, it is also glued to it with a small amount of hot glue. The old head (see Fig. 1 is located below the AB) was additionally pressed against the lid with a 1,5 mm thick rubber gasket glued to its magnetic system, which is no longer required. With the new dynamic head, the sound volume has increased by 2...4 times and speech intelligibility has noticeably improved, which is important when watching movies, listening to audio books, radio plays (the old head did not reproduce high frequencies at all).

Rice. 2. Circuit board

EC "Texet TB-840HD" can work as a VHF radio, but in order to be able to receive radio broadcasts, you need to insert a wire-antenna into the headphone / TV jack (the dynamic head is turned off). The possibility of equipping the EC with a small built-in antenna was tried, but the experiment ended in failure: since the EC circuit board is not shielded, the level of noise emitted by it turned out to be too high. Then it was decided to install an additional socket for connecting an external VHF antenna, which would allow listening to radio broadcasts through the built-in head. To do this, the central core of a thin coaxial cable was soldered to one of the contacts for connecting telephones - the "common" wire - (Fig. 2, in the middle on the right) (visible in Fig. 1 and 2 as a white wire with red heat-shrinkable tubes worn at the ends) .

The braid of the cable is soldered to the common wire (power minus) at one point near the audio-video output jack, opposite the power switch. As a socket for connecting an external antenna, a contact from a socket (panel) for microcircuits was used. It is installed next to the "UP", "DOWN" buttons, for which a hole with a diameter of 1,2 mm is drilled in the plastic front cover on the side, and a small groove is cut with a file opposite this hole in the rear aluminum cover (to the depth of the grooves for fixing case latches) about 4 mm. To protect the device from possible damage when connecting the antenna, a simple assembly was assembled, the diagram of which is shown in Fig. 3, and a view of the installation - in Fig. 4.

Rice. 3. Node diagram

Rice. 4. Assembly assembly

As an antenna, you can use a piece of wire about 1 m long, it is even better to make an external telescopic antenna for mobile devices, described in the author's article of the same name in "Radio", 2013, No. 8, p. 20. You can also connect the central core of the coaxial cable to reduce from an external MV or MV-UHF television antenna (for this case, you will have to make an adapter).

The pin for connecting an external antenna is made of a half-watt MLT resistor with a resistance of 0,33 Ohm, one end of which is shortened to 4 mm, and a stranded wire 1 m long in soft PVC insulation is soldered to the other (Fig. 5). The resistor and part of the wire are covered with a yellow heat-shrinkable tube.

Rice. 5. Connecting an external antenna

Capacitor C1 (see Fig. 3) is a high-voltage ceramic capacitor with a capacity of 22 ... 100 pF. Resistor R1 - any small-sized, R2 - preferably high-voltage resistance of 1 ... 40 MΩ, but you can also use the usual one.

In the process of finalizing the EC, another assembly defect was discovered - one of the two quartz resonators (Fig. 6) was smeared with glue, but not glued to the printed circuit board. It is necessary to glue it, otherwise, due to vibrations during shaking, the resonator leads may break off. It is recommended to use "instant" cyanoacrylate glue, but do not apply it to the resonator leads. If it is mounted obliquely (as shown in Figure 6), do not straighten its leads. The socket for connecting the antenna is additionally fixed with the same glue. Then make sure that the glue does not flow under the display.

Rice. 6. Quartz resonator

"Texet" is a trademark of the product, the "stuffing" is produced by the OEM, so similar ECs are sold under other trade names. In case you encounter the same problems, the described solutions can also be applied to the modernization of "twins" made by other companies.

Of the unrealized ideas, the installation of an additional 1500 mAh battery (built-in capacity is about 2700 mAh) remained, which would increase the duration of the EC by 1 ... 2 hours at the maximum brightness of the display backlight in movie viewing mode. Unfortunately, the available lithium-ion batteries did not fit either in width or thickness.

With the rear cover removed, power the EC only from the built-in battery to prevent damage to the device from leakage currents of the mains voltage in case of accidental contact with the terminals of the parts. In no case do not close the terminals of the AB with each other. If you notice the slightest traces of swelling of the battery, it must be replaced, otherwise, with an increase in the degree of swelling, it may "break" the display. High-capacity Lithium flat batteries can be made up of several cells connected in parallel, which means that the bulge may not necessarily be in the middle of the battery. Any lithium-ion battery with a nominal voltage of 3,6 ... 3,8 V with a built-in controller that suits the capacity and size is suitable. Lithium batteries of the same or similar size, weight and design cannot differ significantly in capacity - be critical of the sometimes fabulously large numbers of ampere-hours indicated on the cases of flat batteries.

Literature

1. A. Butov, External telescopic antenna for mobile devices. - Radio, 2013, No. 8, p. 20

Author: A. Butov

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