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Defective Graphic Equalizer LED Array Found In SONY Stereo Deck Receiver
This Sony HST-D40 Stereo Deck Receiver was picked up by a guy, whom we had an acquaintance with for the first time, when he entered into our house for doing painting job. Soon, because of his loving and humble nature, he became rather a family friend. He has been in touch with us often and my wife ensures that he does not go without having at-least a cup of tea and snacks (something only very few of my customers get the honour!).
This is how sometime human relationship begins, irrespective of who the other guy is. The humbleness in us allows the water of kindness in others to flow into us, building a long standing relationship. This is a peculiar guy, who is a crazy admirer and collector of antique sets, especially tuner amplifiers. Though he had brought a couple of sets in the past for service to me, it is after a very long gap, he brought this to me. He keeps hunting for antique sets lying in the attics of houses, where he goes for painting or in scrap dealers or in electronic technicians’ shops, where he makes frequent visits during his free time. He does this, in spite of the fact that his earnings do not allow such expenses. So, you can imagine how crazy he is! Anyhow, putting a full stop to this background story, let us continue on the set.
The complaint on the set was that all LEDs on the graphic equalizer were lighting up, but no output from any of the sources. When I switched on, I noticed that it was true and further that there was no display. The play knob of Tape B was found broken, and a cassette stuck inside. I had to release the door by pressing the release lever manually from behind to take out the cassette. As usual, the firs job is to open and clean the set, which had enough dirt and dust inside, for which he helped me. After that I brought the set inside and started probing into the possible reasons, which naturally could only be in the power supply. I thought of fixing the defective knob of the Tape B first, and therefore dismantled it. Saw that the holding bracket of the knob had broken. So, took a thick wire shaping it round to suit, and fixed it using soldering iron. A similar work had already been done on this mechanism by another technician, which I found to be an excellent professional job. Here, you can see a few pictures, which are self-explanatory. The first two pictures are of the Amplifier-cum-PS Board and Tuner board.
I downloaded its service manual from open sources in google search. Checked the power supply and noticed that there was a secondary power controller in the form of an IC LA5667. I found that this IC was getting hot, and the outputs were missing. So, I removed it, applied the input voltages to check whether the required outputs were present. This is a dual input/output multiple voltage regulator, as can be seen from the datasheet in the link https://pdf1.alldatasheet.com/datasheet-pdf/view/40189/SANYO/LA5667.html I am also giving an extract of its schematic from the service manual.
At 18V input, the outputs of 5V and 12V were perfect. It was not getting hot by itself. Having found that the IC was ok, I moved on to check the loads. I disconnected the loads of Tuner. No change. So, disconnected the load of front panel. Voltages were present then. So, removed the front control board, and dismantled the whole components from there, as I had a schematic and ensuring that what I removed and its value was matching with it. Started checking every one of them. When there were identical components, I marked the number on it to switch its places. Look at the pictures:
Naturally, the question would be why I resorted to this cumbersome method, instead of using the isolation method. This board was housing the uController of the Tuner switch board, as well as display. Something in it was loading. Whatever I could check by lifting one end of the component, I did off-line. I saw a lot of dry solders all over the board. So, it is better to remove the components and refit it, as there were not much. That’s what I did. Well, the result was that I did not find any component defective in the board, though I replaced the electrolytic capacitors and a few fixed capacitors. The resistance array and capacitance array were also showing correct reading.
The crystal also did not show any short. After replacing and re-fixing the components as they were, I applied the power and found that the outputs in the IC were perfect this time. So, something that I did had removed the load. It could be some passive component that failed when power was applied or dry solder for some strange reasons. But the display was not getting on, as the filament was not getting voltage. Then I traced its track to the PS Board and found one Zener diode leaky. Unfortunately, I forgot to click these, so reproducing the schematic here:
This is RD30ES 30V diode. When I replaced it, the display came on and on further checking, found that the uController was responding to the control switches of tuner and function selector. So, that portion was restored. Turned my attention to the Amp board and studied its controls. Also, as a part of routine, checked electrolytic capacitors at PS and in the control section. Changed a few that I suspected to be problem creators. After cleaning the head, pinch rollers and mechanism parts thoroughly, and applying power to the motor externally, I checked and ensured that all functions of the mechanisms were working perfectly, like play, FF, RW and pause. Fit the mechanism back to its place. Connected the heads and motor control connectors to the Main Board. Applied power and checked whether the Tuner was working. It did not, even though I had restored its power supply after checking the LA5667 IC. The controls were working, tuning indicator was showing up and down, but no stations at all.
So, turned my attention to the Tapes. Played cassette in Tape A. No output. Played it in Tape B. Output was coming and the Amp worked very well, but for crackling noise. The graphic equalizer controls made too much of irritating noise when moved up or down, in both the left and right controls. Loaded another cassette to Tape A and selected A. Then the output was dead. So, I traced the output signals from the head in my oscilloscope and found these to be present. It was present even in the input and output of the preamp IC. But the signals were not reaching the preamp of the Amplifier. I studied the circuit and understood that the signals from both the decks are controlled by a set of ICs which have AND, NAND and OR gates. The circuit was so arranged that the gates would open to let the signal pass through when it meets with certain voltages present at the allotted gates. This is that complex circuit you can see:
I do not know whether you will be able to download this and view it zooming. (I hope Jestine Yong would include a facility to zoom a picture and see it clearly) The numbers of the ICs used were M4073 (AND), M4011 (NAND) and M4030 (OR). Since I knew that one or more of the ICs were malfunctioning, I checked the voltages that should be present on each condition at all the points. Traced that the M4030 and M4011 were not having the required output. So, removed all these ICs from the board and checked it on a breadboard, applying power, tapped from the PS Board, and checking its voltage output that is specified, by giving a required trigger voltage at the respective inputs. Instead of showing almost a near supply voltage, some pins were showing almost two volts less. You can study the pictures and results shown below:
Noted down the voltages present and found that M4011 and M4030 were not alright and the IC 4073 also was erratic at times. So, replaced all the three ICs. I also replaced the electrolytic capacitors in that region. The tape A and B also started working fine, and outputs were coming from both, though the output from the Tape B, which is used for recording also, was comparatively less. It was due to slightly worn out head.
Having done this much, turned my attention to the Graphic Equalizer Board and started investigating what was causing the crackling noise and why there were hissing noise when the controls were moved up and down. At this point, let me be very honest with you that this was the first time that I worked on such complicated set having gate controls for working of tapes and built-in LEDs on sliding pots in graphic equalizer. So, I was very much charged with inquisitiveness to learn and add these to my knowledge. I fed signal to the input of IC M5226P of left channel and traced it in my Oscilloscope from the output. It was working fine. Then fed signal to the IC of right channel. It was distorted and noisy. Checked and compared the voltages in both ICs and found these to be different, except for the PS points. Moreover, the IC was getting hot. Here also I used the same method like before; i.e., used the tapped power supplies, applied it keeping the ICs in a breadboard, and feeding signal etc. So, replaced it and checked and found these to be ok. The output now was without any crackling noises, except for the noise while moving the sliding pots. In order to isolate, I removed all the pots one by one and checked in my Analogue Multimeter and also checked the LED points. The results were positive and all LEDs were getting lit when voltages were applied by the X1 Ohms range of the Multimeter. So, refit these back and disconnected the power supply to the LEDs and tried. Then the noise had gone in one channel. But it was there in the other. So, cut off the PS in that section also. Noise stopped. Now, you can see some of the pictures related to my statements:
Since replacement of the sliding pots were not available, with the consent of the customer, I disconnected the PS to the LEDs permanently. Checked and found that all inputs from as Tape A, Tape B and Audio In were working perfectly well and the output was also good.
Turned my attention to the Tuner Board. Checked for any bad components. Did not find any. I forgot to mention that I had retouched all the points on the Amp Board and Graphic Equalizer Board like what I did for the front control board. I did the same thing on Tuner Board also, which was housing the secondary power supply. Changed crystals of the IC CXA1269 and LC7217, as I noticed that voltages were ok at IC pins and crystals could cause such non-functioning of the ICs. The option that I had was to replace both the ICs. But the cost of these ICs were very, very high and the customer did not want to have it replaced. I also did not encourage him, as I was not sure whether any problems in the RF/Coil circuit existed, which cannot be easily detected, could cause such failure of outputs. So, abandoned this portion. Now, you can see a few pictures and also the schematic of Tuner Section.
You can see how the natural sun light is peeping in the morning hours to my workshop table! Finished fitting the set and tested it thoroughly and it worked very well except for the Tuner, which was decommissioned! Here is the set along with the snaps of components replaced, which includes the mains cord, and two pinch rollers of the Tape Mechanism (replacements of which, I forgot to mention earlier).
Thus the curtain fell on the story of this set; another Satisfaction Mouse sneaking into my Bag as usual!
This article was prepared for you by Parasuraman Subramanian from India. He is 69 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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