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Bluesky BST 2104, CRT TV, not working repair
This set belongs to a relative of my good friend Makis. He left it to me some days ago when he visited me asking me to take a look on it and if possible to repair it. He explained to me that before it stopped working there was smoke coming out of it along with a strong bad smell. This is always bad news…
After the advent of the digital transmission in my country this set kept working with the aid of a terrestrial digital transmission receiver connected at its scart inputs and is used since then as a video monitor rather than as a regular TV set. The set’s model is shown below:
As soon as I removed the back cover of it, I didn’t need to take any measurements in order to locate the problem. My nose led me directly to the culprit component. You can see below what happened.
Look as well what was the condition on the main board when I removed the transformer…
The type of the transformer is shown below:
I asked all the component suppliers I knew about that. No one of them had it as a spare part. Then I looked for equivalent types. I found one equivalent of HR. I kept with phone calls about that. The type was HR 8902. Finally I found a replacement in a distant shop, far away from the place I live. Anyway I went there and bought it. When I was back at home, after a thorough cleaning of the PCB with cleaner spray, I installed it in the set.
You can see it below:
When I tested the set after the replacement, it worked at once. I adjusted the focus and screen G2 voltages with the potentiometers located on the body of the line output transformer and practically I had finished with it.
Inspecting afterwards carefully the solder side of the main board, I noticed plenty of dry joints around the pins of the two “scart” connectors. Also many components’ ends were too long and bent here and there, which on one hand is a spectacle I hate to see and on the other hand they could cause a serious problem while moving the main board in and out for the tests…So I started a “hair cut” on them…
When finished, I checked the PCB and cleaned it (using an old tooth brush) from the tiny metal remainders of the components’ terminals which were dispersed all over it. Then, trying to restart the set, I was surprised (once again)! The set refused to start.
Not believing that something serious had happened in between (what could really that be, by only cutting the long terminal ends of the soldered components?) I started to verify the existence of the basic operating voltages. Everything was OK. I had the 5V standby there, the 117V for the horizontal output, the 30V for the sound IC and the vertical oscillator after further processing. My problem was, as usually, that I had no schematic available. Working in blind, just following the foil tracks to see what goes where, is both time consuming and frustrating…
I started checking the production of the starting command on the keyboard and its destination on the respective CPU pin. After some time spent on this I saw that the starting command was there and I could detect it on a pin of the CPU chip. Now I was in front of a new enigma. What could be wrong with this set? I was, above all, very curious to find out the cause of it, but under the circumstances I could not enter into detailed troubleshooting.
Because the feeling I had with the soldering of the components was indeed disappointing, I suspected that I had more similar problems. Soon I decided to refresh all the joints of the main board and started this work. In many joints, as soon as the hot soldering iron’s tip touched the joint to be refreshed, I could see the pin hole empty. When I finished this rework I cleaned the entire PCB with flux removing spray. The main board became like a new one. Fresh and shiny…
Trying afterwards to start the set, there was no response again. Everything I checked was OK, except a voltage of 8V. This was absent. I felt I was losing my patience with it. On the top of that, the lack of a drawing in order to simplify the work further was worsening the case. This common and frustrating feeling every technician experiences from time to time was present again…
Following the copper tracks backwards in reference to the 8V indication printed on the PCB, I located a small signal transistor switch in the power supply section, which was responsible for controlling this voltage. This transistor should have a signal (forward bias) on its base when I was pressing the programs button in order to start the set.
I connected my multimeter on its base against the ground, expecting to see there 0,7V dc of its forward bias. Pressing the programs button no signal was appearing at the base. This transistor was not receiving the “power on” command.
Since no sign of major trouble was apparent, I decided to apply manual control on this transistor to see how the set reacts. So I bridged momentarily its collector with its emitter, simulating the effect of the missing “power on” command signal sent by the CPU in its base, while watching what happens. The set started normally for a while and switched off upon removal of the short I put on the transistor’s C-E terminals. It was now clear to me that something was wrong with the CPU. This was very strange, but nevertheless a reality in front of my eyes.
I directed all my attention on the CPU. A small piece of paper was stuck on the upper side of its body, with a serial number along with a bar code printed on it. I removed it gently using a “label off” spray. Given that I had no drawing, I wanted to see the type of the CPU and then find it in the web, in order to get some basic information for further troubleshooting. The IC was a ST92195B of ST. I found the data sheet of it in the web. It was not very explanatory for my case, but certainly better than nothing!
I already knew that the starting command was coming at an input pin of the CPU and that there was no output command from the CPU to the base of that transistor to control the 8V supply. So it would make sense now to check if there was active oscillator signal on the CPU terminals, where the crystal was connected. This proved to be OK as well. This was a good sign of the health of the CPU.
The next step I had in mind was to check the status of the reset pin. According to the data sheet, the reset pin (No 2) should always have the 5V STBY signal on it for normal operation. “Power on reset” of the CPU happens whenever this 5V STBY signal appears the first time the set is switched on with the power switch (i.e. not by the remote control).
This means that as long the 5V STBY signal is present, I should always have these 5 volts present on pin 2 of the CPU. When I checked this condition the result was null volts. The CPU was stuck on permanent reset status. That’s why I had a “not starting” symptom. Following the track of pin 2, I located a SMD resistor of 100 KΩ connecting pin 2 with the 5V STBY supply. The relevant test revealed that this resistor was open, causing the problem.
Now if you ask me “how could such a thing happen”, I really cannot answer that. I am a technician and this question should rather be answered by someone like Mr Hudini the magician! Fortunately I had a spare resistor like this one available and replaced it without being obliged to go shopping again!
You can see this resistor below, exactly at the center of the photo, in line with pin 2 of the CPU.
I also located a small electrolytic capacitor of 1μF connected to this pin against the ground. This results in a time constant of 100ms for the reset circuit. So the first time 5V STBY appears, the CPU is supplied with its operating voltage but its reset terminal is held at ground level for about 100ms delay time, after the appearance of the operating voltage, in order to reset it. After this delay time the (resetting) capacitor is fully charged and the voltage at its positive terminal stays permanently at the of the 5V STBY level (until the next reset).
Below you can see the final result after the replacement of the open 100K resistor.
After confirming the normal operation of the set and leaving it to play for some hours, I informed my friend Makis to come by and pick the set up. The repair was successfully completed.
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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Note: Please check his previous repair article in the below link: