Disassembly of an antique tape recorder Analysis of tape recorder circuit design

I got an old Panasonic radio, Panasonic RQ-517S, produced around 1975. It is a fully transistorized discrete circuit with no integrated circuit chips, with FM/AM dual-band radio, and single-card mono recording and playback. This machine was faulty when I got it. It made a continuous clicking sound when I turned it on, and I couldn't receive any sound. The buttons on the recording mechanism didn't work, so I repaired it.

The appearance is a classic of that era. This machine is portable and small in size. The distortion of the photo image is caused by the lens being too close:

back:

Nameplate, made in Japan. This radio is a global model, supporting power supply voltages in many countries, and the frequency range of the radio is also universal, not the domestic frequency of Japan:

The machine body has a striking factory voltage setting warning, 220-250V:

The buttons on the top, the 6 on the left are movement operation buttons, followed by the function switch, switching between FM, AM radio and recording and playback, and the two knobs on the far right control the pitch and volume respectively:

On the side is a large radio tuning knob, an external microphone jack, a monitor switch, and a headphone jack, and below are external DC and AC power ports:

Battery compartment, install 4 No. 1 batteries. The spring of the battery compartment has rusted and needs to be cleaned later:

This kind of old machine is usually easy to disassemble because the plastic mold at that time was simple and did not have the hidden clips that are common now. The front cover can be easily removed by unscrewing the screws:

The speaker on the front shell is made by Panasonic. It has an anti-magnetic structure to prevent the magnetic field from affecting the tape and the head. It looks like an internal magnetic speaker. This speaker has a very special 3.2 ohm impedance. This carefully designed low-impedance speaker can obtain greater output power at a lower power supply voltage:

The black thing on the front shell is the internal microphone, which is wrapped with foam rubber to reduce the noise caused by the vibration of the fuselage:

The lower right corner of the machine is the AC power supply:

The AC power supply can be removed separately, mainly including the fuse, transformer, rectifier filter circuit and switching circuit. The transformer is very small, estimated to be about 5W:

The black one above is the voltage switch, which can select AC voltages from all over the world:

Let's start with the mechanical part. The movement design in the 1970s was relatively primitive, with large size, simple parts, and large number of components. The magnetic head was also large, and this magnetic head showed no signs of wear:

Completely remove the movement from the machine for easy maintenance:

The bottom case after the movement and circuit board are completely removed:

The flywheel of the movement's main shaft is made of all-metal, with a large diameter and weight to ensure stable operation and reduce shaking. It is obvious that the belt has aged and broken and needs to be replaced:

A close-up of the movement's motor, which is quite large and is a Panasonic-made 6V motor with a built-in speed-stabilizing mechanism (it should be a mechanical centrifugal speed regulator, not an electronic speed-stabilizing circuit):

Close-up of the motor's metal pulley, which is screwed onto the motor shaft, and the installation process is complicated. The motor is fixed to the movement via four rubber pads to reduce the impact of vibration:

To replace the belt, you need to remove the main shaft fixing strip before inserting the new belt:

The new belt looks like after installation:

The problem of the movement button being stuck is easy to fix. It can be solved by using WD40 magic oil. After spraying the magic oil on the relevant parts of the buttons, repeatedly press each key and it will become flexible soon:

The movement is repaired and put aside for later use:

The following is the circuit maintenance, which is much more troublesome than the movement, and I took some detours.

I first found the circuit diagram of the machine on the Internet - I only found the circuit diagram, but no printed circuit board diagram, so it was still very time-consuming to actually compare the components and wiring. This radio recorder is composed of 14 transistors, most of which are silicon tubes, but the low-power part is also mixed with 4 germanium transistors, which is very unusual in the 1970s when Japanese silicon planar process transistors and even large-scale integrated circuits were already mature. Analyzing the circuit, I think these germanium tubes were deliberately selected. The purpose is to use the extremely low saturation voltage characteristics of germanium tubes at a low power supply voltage (6V) to obtain greater output power and less distortion, so as to take into account the portability and performance indicators of the radio recorder product at the same time. The circuit of the whole machine is very mature and classic. Tr51 is the FM radio high amplifier, Tr52 is responsible for FM local oscillator mixing, and Tr53 constitutes the first stage FM mid-amplifier. Tr54 is the local oscillator mixer for AM radio, and then Tr55 and Tr56 are responsible for two-stage intermediate amplification of FM and AM at the same time, and finally sent to AM diode detection and FM double diode balanced frequency discrimination to become audio signals. Tr1~Tr3 are audio preamplifiers, which are shared by radio and tape recording and playback. Tr5 is an active filter that provides a stable power supply to the preamplifier circuit. Tr7 and Tr8 are a pair of complementary medium power tubes made of germanium to complete the output of the post-stage OTL push-pull amplifier. Tr4 and Tr6 provide stable bias current for Tr7 and Tr8 to reduce distortion.

First take a look at the circuit board:

Some details on the circuit board are quite interesting and worth seeing, such as the following.

This circuit board is single-sided, but pay attention, there is high technology on this single-sided circuit board - the white lines printed on the component surface are not the routing marks on the copper foil surface, but the real circuit. It is printed on the component surface with silver paste to form the circuit on the other side, thereby realizing the routing flexibility of the double-sided circuit board on the single-sided circuit board. Moreover, it is not only the circuit that is printed, but also the components. Note that the place with yellow circles is the resistor number printed on the silk screen, and the resistor inside the circle is printed directly on the circuit board with carbon film! This was high technology at the time. The resistor was directly printed on the circuit board using the printing process, which reduced the number of soldered parts and improved the reliability of the system. This technology of printing circuits and resistors directly on the circuit board with silver paste and carbon film was popular in the electrical appliances of Japanese companies such as Panasonic and Sony in the 1970s and 1980s. It was high-tech, but later it withdrew from the stage of history due to process cost issues:

Take a closer look at the carbon film printed resistor, which is more clear under the close-up (circled area):

Although the high-tech silver-paste printed circuit board has been used to achieve the performance of a double-sided circuit board on a single-sided circuit board, this circuit board still cannot completely avoid the use of jumpers. In addition, the Japanese engineer who designed this circuit board did not speak pidgin English very well. The jumpers on the circuit board were incorrectly spelled "JAMP" instead of "JUMP" in the silk screen (circled in the picture):

On the circuit board you can see some glass-encapsulated point-contact diodes, a bit like the 2AP9 commonly used in Chinese ore machines, but without the black paint:

What is more interesting is the small and medium power germanium triodes in metal tube package (indicated by the arrow in the figure below), which are very common in old Chinese radios, but are very rare in Japanese products in the mid-1970s. The reason for using germanium tubes has been analyzed above, which is to improve the undistorted output power of the power amplifier under low voltage:

The power output germanium tubes of two of the amplifiers are wrapped with aluminum sheets to help dissipate heat from the tubes:

Take a power tube out of the aluminum heat sink and see that the transistor is not directly soldered to the circuit board, but connected via a soft wire:

What’s even more bizarre is that although a high-tech circuit board is used, some parts are welded together, such as this electrolytic capacitor (this is the original one, not due to repair):

Now we start the arduous repair. Because the fault phenomenon is a clicking noise, common sense suggests that the electrolytic capacitors, which are nearly 50 years old and are the most vulnerable to damage, may have failed. However, after removing the capacitors for testing, we surprisingly found that the capacity of the electrolytic capacitors is in line with the nominal value, and half a century of aging has not caused them to lose capacity. No wonder, even the smallest electrolytic capacitors in this machine are all ELNA, which is a high-end product and very reliable:

Connect a pointer multimeter to the power supply of the audio preamplifier circuit and find that the voltage is unstable and swings back and forth: