Sony KV-2782R and 2785R, power supply problems.
I already posted this message under the name "John Wright". Forgive me
if this is a double post, but the newsgroup software or server used on the other account seems to not be functioning, in that I posted the message over an hour and a half ago and it has yet to reach the rest of the world. Hi there. This television has it out for me. I recently began work on a Sony Trinitron television, model KV-2782R. Initial symptoms were that the audio amp would come on (producing audible hiss) but the rest of the set was dead. 130V fuse F501 was blown. Removing all aux supplies from the flyback (by way of desoldering one end of components) did not appear to remove the short. Removing the HOT did. Replacing the HOT did not. During my attempts to locate the short, power supply "GA" board made a sizzling noise, then my 130V disappeared altogether, along with the audio amp power.. Testing revealed one of the two TO-220 2SC transistors on said board was shorted. I replaced this transistor with a substitute identified via a website. Upon reapplying power (with the 130V output plug to the mainboard disconnected), the new transistor shot out a large spark, the resistor located between the two transistors (electronically - between one's collector and the other's emitter) burned open, and the second transistor shorted. At this point, I replaced both transistors and the resistor, then tested all of the other diodes and resistors. I was presented with a power supply that would turn on then immediately turn itself back off. I must admit, I next did something I shouldn't have - disabled the protection circuit (by isolating one pin of mainboard D-27). The power supply remained on, the amp came to life, and I had 130V. Very shortly thereafter, board GB emitted a large quantity of smoke. I have not done anything with it since. Today, I picked up a KV-2785R, presumed to be working with a minor tuner problem, expecting to measure the voltages in the power supply section in order to repair the 2782. However, I was disappointed to find that the 2785 has input from the rectifier to the GA board, and some voltage (apparently -300V? I'm not using my own meter and can't hardly read the one I am using!) on its output to the GB board. However, GB has no output to power amp or 130V... What is likely to be fried on the 2782's GB board? I'm hoping to repair it with parts from the 2785, then attach it to the 2785's GA board. Yes? No? What else might I be looking for that could be causing the 130V short on the 2782 mainboard? What voltages am I looking for at the various points throughout the power supply block? Specifically, what should be the output voltage of GA, before GB. Second, what should be the output voltages of GB? Lastly, what is the purpose of these two boards? They are obviously some form of SMPS, yet I certainly cannot see any isolation on GA, and haven't tried to follow GB yet. GA APPEARS to invert the polarity of its input. No? I have no service manual or valid information to go on - so any informative input is appreciated. The eventual goal is to have both sets working - but having one working is the first step. |
Sony KV-2782R and 2785R, power supply problems.
These sets are a challenge for any of the service techs, because of their
complexity. The first thing you should invest in is the service manual. Without the proper Sony training, these sets are even a greater challenge. Over an email, it will not be possible to give detailed help. From what you describe, the flyback assy is most likely defective, and there is most likely missing drive from the scan gen to the horiz output transistor. The driver transformer also can give problems in this series of set. If the drive pulses to the horiz output trans are missing, this device can go short. More than likely there are some capacitors that have become high in ESR, in the horiz output section, and in the power supply section. These will cause failures. You will have to first fix the power supply. You can work on the main supply with the horiz output trans pulled out. You then must find all the defective parts in the horiz output stage before re-applying the power to the set. If the set does not come on, and there is a short, the new parts will most likely be failed again. This is instant when there is no proper start-up, and there is a short in these areas. I have very rarely seen non trained technicians be able to have success with these sets when there are complex faults with them. This is the same for most of this category of TV set, and or monitors. -- Greetings, Jerry Greenberg GLG Technologies GLG ========================================= WebPage http://www.zoom-one.com Electronics http://www.zoom-one.com/electron.htm ========================================= "Michael Parker" wrote in message om... I already posted this message under the name "John Wright". Forgive me if this is a double post, but the newsgroup software or server used on the other account seems to not be functioning, in that I posted the message over an hour and a half ago and it has yet to reach the rest of the world. Hi there. This television has it out for me. I recently began work on a Sony Trinitron television, model KV-2782R. Initial symptoms were that the audio amp would come on (producing audible hiss) but the rest of the set was dead. 130V fuse F501 was blown. Removing all aux supplies from the flyback (by way of desoldering one end of components) did not appear to remove the short. Removing the HOT did. Replacing the HOT did not. During my attempts to locate the short, power supply "GA" board made a sizzling noise, then my 130V disappeared altogether, along with the audio amp power.. Testing revealed one of the two TO-220 2SC transistors on said board was shorted. I replaced this transistor with a substitute identified via a website. Upon reapplying power (with the 130V output plug to the mainboard disconnected), the new transistor shot out a large spark, the resistor located between the two transistors (electronically - between one's collector and the other's emitter) burned open, and the second transistor shorted. At this point, I replaced both transistors and the resistor, then tested all of the other diodes and resistors. I was presented with a power supply that would turn on then immediately turn itself back off. I must admit, I next did something I shouldn't have - disabled the protection circuit (by isolating one pin of mainboard D-27). The power supply remained on, the amp came to life, and I had 130V. Very shortly thereafter, board GB emitted a large quantity of smoke. I have not done anything with it since. Today, I picked up a KV-2785R, presumed to be working with a minor tuner problem, expecting to measure the voltages in the power supply section in order to repair the 2782. However, I was disappointed to find that the 2785 has input from the rectifier to the GA board, and some voltage (apparently -300V? I'm not using my own meter and can't hardly read the one I am using!) on its output to the GB board. However, GB has no output to power amp or 130V... What is likely to be fried on the 2782's GB board? I'm hoping to repair it with parts from the 2785, then attach it to the 2785's GA board. Yes? No? What else might I be looking for that could be causing the 130V short on the 2782 mainboard? What voltages am I looking for at the various points throughout the power supply block? Specifically, what should be the output voltage of GA, before GB. Second, what should be the output voltages of GB? Lastly, what is the purpose of these two boards? They are obviously some form of SMPS, yet I certainly cannot see any isolation on GA, and haven't tried to follow GB yet. GA APPEARS to invert the polarity of its input. No? I have no service manual or valid information to go on - so any informative input is appreciated. The eventual goal is to have both sets working - but having one working is the first step. |
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