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The idea was to repair some problems, calibration, replacing the old parts that grow old and change their parameters. The original playback frequency response of the machine is previously measured with IEC Prague Teac test tape MTT and it shows full comparison with the current standard that is why the playback frequency response will not be modify.
The machine comes from Portugal in very good condition without any shipping problems. The mention problem was about periodic electric discharge on playback.
At first look the other functions were in normal working condition. Compared the sounding signature with one rebuild Nakamichi ZX at first look the sounding was no so interesting. In the past few years I made rebuild process of several A and that is why I decide to publish with detailed explanation why it is required. First of all, the electrolytic capacitors are 35 years old. Original components are good quality Elna capacitors.
Aluminum electrolytic capacitor is the shortest life parts in power supplies. This component is the most important parts in terms of reliabilty. Expected lifetime of power supplies can be estimated by calculating the estimation of expected lifetime of aluminum electrolytic capacitor. Expected lifetime of aluminum electrolytic capacitor is determined by amount of excess electorlyte, speed of electrolyte evaporating through sealed rubber and temperature that determines the evaporating speed.
Life expectancy is determined by a coefficient depending on temperature, based on Lo. If the capacitor is used in temperature of 25 degree its life will extend with a ratio of Is is about 15 years. Noticed that several biggest manufacturers of audio equipment like MBL high-end audio and Accuphase Laboratory Inc use the same type of capacitors in their designs and products. I also decided to replace some critical caps with bipolar depend on the circuit. In this article will be explained the oversize and the critical part replacement for every component in the machine.
Looking at the mechanism of the TCD I noticed that the flat belts are in working condition but the square belts for the take up and supply hubs are weak. Please follow this link to the correct page on our website to Purchase Thanks Again, Krista Making a combined shipping I was able to get it for 60 USD including the shipping.
The main reason to order full belt kit is that flat belts on the others machines that I have done were in bad condition. The replacement is not so easy and it is good to be done when the mechanism is rebuild. Meanwhile it is time to diagnose the electrical discharge problem and some improvement into the circuit. Note that there exist some places that the replace is not suitable and I will note about it. The power supply electrolytic capacitors are one of the most important part in the audio.
It is better to oversize their capacity. One reason of the fact it is not done in the design of the machine is that the higher capacity caps are too expensive for regular production.
The measured ripple from the power supply using the original capacitors was about mV 0. It can be seen also on the snapshot of the scope. When replacing the power caps over their capacity the ripple was reduced up to mV. It is significant improvement because some modules are powered from the non regulated supply for example the variable output amplifier and the headphone amplifier.
The second part of this post will describe the schematics and when it is good to oversize a capacitor. There is another yet grounding problem in the machine.
The construction consist of two main PCB connected together with detachable connectors. These connectors are used for the power supply exchange and when the contacts are dust with oxide the noise from the power supply take a interference. It is good to solder with hard connections between the two main boards as shown on the photo.
The reference ground point of the PCB is where the rectifier filter caps for the positive and the negative are connected together. It is good to set this point for the chassis ground. It can be seen how it looks like on the scope. The investigation show that it comes from the playback amplifier and exists on both channels. I suspect some filter caps on the power supply. As can be seen it looks like DC charging the output and in some time this small DC charge clears in a oscillation process.
One can decide that it can be due leakage transistors in some high impedance circuit. After the replacement the problem still remains but the sounding of the PB amp stage and the unity gain buffer amp improves its slew rate. The used low noise transistors with its higher gain shows noticeable improvement on the sound signature. The electrical discharge problem do not comes from the head.
It exist even on shorted playback head input and disconnected head. The investigation show some spark pulse on the mechanism control motor.
In first look that suspect problems in the control processor. When the mechanism motor activates it generate high current consumption from the power supply and more noise and interference. In normal playback mode the motor is in static mode and do not generate noise and power supply peaks. The final conclusion confirm that the spark interference comes from bad capstan motor.
Lets look at the playback EQ amplifier. One have to be noted that it is playback phase compensated. That kind of approach is used in few cassette decks. TCDA use playback phase compensation and it helps the previously recorded tape without phase compensation in the recording to be played properly on TCDA.
Most of the prerecorded tapes are not phase compensated in the recording process. The phase compensation all-pass filter is done in amplifier with the following transistors: Q, Q, Q C and R determines the starting point of the phase rotation.
The switching time constant between uS aud 70uS is done by Q at the output of the playback amplifier. The head is connected directly to the base of the first amplification transistor and there is no any coupling capacitor between. Both are well polarized so it is not required to be BP caps. The Q changes the HF playback roll-off of the input amplifier in stop, fast forward and rewind mode also on program search modes.
It is required because of the cue function of the deck. When it is in stop mode and the stop button is hold on by pressing the fast forward or rewind the searching function is activated. The number of pressing the fast forward or rewind button simultaneously with the stop set the number of the searched track on the tape. This is a simple mod and the primary goal is to remove the large electrolyte capacitor in the sensitive parts of the amplifier C The same way will be removed the DC current flow to the playback head and this avoid any head magnetization in time.
The emitter have to be replaced with source terminal, the base with the gate and the collector with the drain. Short the C short circuit. Remove the R Replace the R from 8. Reducing the resistance from 8. The plus terminal should be connected to the upper side of the circuit and the minus to the emitter of the Q That is all — the following mods will reduce the interference and distortions comes from C and the second will decrease the self thermal noise of the R The signal of the playback amplifier goes to the Dolby playback expander circuit, which is placed in separate board.
The board will be review later in this publication. There exists supposition that the reference level specified in the service manual is wrong listed but we made test bench that confirms the listed value in the manual.
Detailed explanation can be seen in another article that can be found here. After the buffer it goes to the fixed out RCA connector and to the volume control resistor of the headphone amplifier which is the variable output amplifier. Lets look at the buffer circuit. As can be seen it is very close to the diamond buffer design. There are several confirmation that if the current passed through the capacitor is reduced then the harmonic distortion is also reduced. If a capacitor is connected to high impedance circuit and input then the current passed into the capacitor will be reduced.
The unity gain circuit is based on k input impedance and two parallel 0,47uF good poly caps. It is probably the best solution avoiding use of electrolyte capacitors. The DC offset at the output is set by biasing of the input transistor by R The output is directly connected to the RCA line-out plug using just one serial resistor of ohm R That approach avoid use of large electrolyte coupling capacitor at the output to reduce the harmonic distortion.
Removing the output coupling capacitor is possible because the zero offset voltage of the output, which can be trimmed by R Two major modifications of the circuit are made. Second the output serial resistance, which determine the output impedance of the RCA line out is replaced from to ohm.
The lower output impedance is better to drive the high capacitance interconnects. For example the capacitance of the line out cable is in the range of 75pF — pF per meter. If it is used long cables of 2 meters then the higher capacitance of the cable tends as low-pass filter and the output impedance determine the HF cut-off frequency.
We will continue this review with the variable output buffer and amplifier. The same time it acts as a headphone amplifier. The circuit implements diamond buffer structure in the front and reduced use of negative feedback.
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