Is the HOT(Horizontal Output Transistor) connected to the ....

[Seabass]

Hey everyone, i was wondering if the HOT was the biggest transistor that is connected to the BIGGEST heatsink in the monitor, there are like 3 or 4 transistors connected to that big heatsink, but the HOT is the biggest one right....? And the voltage from the HOT should be around 60-150V DC right...1 more question, there are 3 wires that come out of the transistor...which ones do i use to check the voltage, middle and a side one or not even touch the middle..?

[BernieDD]

Most monitors connect the horiz output transistor to the flyback transformer to develop the high voltage that goes to your picture tube. Yes, the HOT is usually the biggest transistor in the circuitry and is also connected to the biggest heatsink since it is the hardest working transistor in the whole set.

With regards to identifying the wires from it, the one that connects to its metal case is the collector, and the one that's connected to ground (or is at ground potential) is the emitter. The last remaining wire is the base. And with regards to measuring the voltages on it, I strongly advise against it. The collector especially has very large and fast rising spike voltages on it, and these nasties could very well damage any multimeter or oscilloscope that is connected to the collector circuit. I think you want to test the transistor for damage, hence you want to see what voltages are on it. The more prudent and safer approach is to unhook the whole transistor from the circuit (with the power off) and then test the transistor with your multimeter's ohmeter or transistor tester. Be warned though that large voltages (upwards of 12,000 volts) can remain in the flyback circuit for a long time after the power has been turned off, so unless you know exactly what you are doing, don't mess with your monitor's innards.

[Seabass]

Originally posted by BernieDD

With regards to identifying the wires from it, the one that connects to its metal case is the collector, and the one that's connected to ground (or is at ground potential) is the emitter. The last remaining wire is the base. And with regards to measuring the voltages on it, I strongly advise against it. The collector especially has very large and fast rising spike voltages on it, and these nasties could very well damage any multimeter or oscilloscope that is connected to the collector circuit. I think you want to test the transistor for damage, hence you want to see what voltages are on it. The more prudent and safer approach is to unhook the whole transistor from the circuit (with the power off) and then test the transistor with your multimeter's ohmeter or transistor tester. Be warned though that large voltages (upwards of 12,000 volts) can remain in the flyback circuit for a long time after the power has been turned off, so unless you know exactly what you are doing, don't mess with your monitor's innards.

Thanks for the response...thing is is that the transistor is connected to the heatsink with a goop right...ok well theres 3 wires, 1 in the middle and then one on each side of it...so has 3 in total...which ones which....and i dont mind if i blow this multimeter...better then zapping myself, and how do i take off that transistor..would i have to unsolder it, and im going to test it while its on...how many V should it read?

[Seabass]

just tested it right....i think, and got 475V DC....sound kinda right or something....?

[Seabass]

bump...anyone?

[BernieDD]

Yup, you'd have to unsolder at least 2 of the leads to test the transistor properly with your tester. I guess the lead with your 475 volts is the collector. The voltage looks just about right. The collector is the one with the B+ voltage and it usually has the highest voltage applied among all the 3 leads. You've been lucky so far, your meter survived the test. If you like to push your luck, try measuring the voltage on the other 2 leads. The emitter usually is closest to ground potential, and has the lowest voltage (with respect to ground). The DC voltage on the base is usually 0.6 volts more positive (higher) than that on the emitter. Or to put it in another way, the base has a DC voltage of 0.6 volts with respect to the emitter.

I'm just curious, why are you testing the voltages on your HOT?

[Seabass]

Originally posted by BernieDD
Yup, you'd have to unsolder at least 2 of the leads to test the transistor properly with your tester. I guess the lead with your 475 volts is the collector. The voltage looks just about right. The collector is the one with the B+ voltage and it usually has the highest voltage applied among all the 3 leads. You've been lucky so far, your meter survived the test. If you like to push your luck, try measuring the voltage on the other 2 leads. The emitter usually is closest to ground potential, and has the lowest voltage (with respect to ground). The DC voltage on the base is usually 0.6 volts more positive (higher) than that on the emitter. Or to put it in another way, the base has a DC voltage of 0.6 volts with respect to the emitter.

I'm just curious, why are you testing the voltages on your HOT?

Hey, ok....i kinda dont understand what your talking about...theres 3 wires coming from the bottom of the transistor...1 in the middle that goes straight down, and then there is 1 on each side of that straight wire, making a total of 3. When i test the transistor i cant just put 2 of my testing leads to the same wire...so how can i test each wire seperatly...and i hooked up one of the ends of the lead to one end of the transitor, and the other end to the other end of the transistor...so that left the middle transistor empty...thats how i did it, and i got 475 volts...can you please try to explain what you mean again?

[BernieDD]

Ah, you got the 475 volts as you measured the voltage between two of the 3 wires. What you can do is to connect the negative lead of your voltmeter to ground (chassis, or B-). Now measure the voltage (one at a time) on each of the 3 wires from the transistor with your positive lead. The wire which has the lowest voltage is almost always the emitter. The wire with the highest (it will be +475 volts) is the collector. The remaining wire would then be the base - the DC voltage you will measure on it is about 0.6 volt higher than the voltage off the emitter.So if the emitter reads, say, 5 volts, the base should read about 5.6 volts. This is because with silicon bipolar transistors, the base operates at 0.6 volts higher than the emitter.

Im curious, again, why do you want to measure the voltages?

[Colossus]

Maybe he is bored? You cannot truely tell if the transistor is shot from the voltage output. To test the transister full, you would need to isolate it from the circuit.

I can be wrong, it was quite a few years since my electronics days.

[Seabass]

Originally posted by Colossus
[BTo test the transister full, you would need to isolate it from the circuit.
[/B]

How do i isolate it from the circuit...unsolder it? And if i do need to unsolder then i need to remove the crt so i can access the board better....how can i remove the crt...discharge the cr and then just remove it...?

[Seabass]

Readings are 223, 8, and 475......shorted HOT? And if it did short do you think a lot more stuff got messed up...and where can i pick up a Q503 2sc5587 transistor

[BernieDD]

Nah, it's not shorted. If it were, the emitter and the collector voltages would be almost equal and most likely you'd have a blown fuse in the set. Consider the flyback transformer as a likely culprit if you have horizontal or high voltage circuit problems. Or could be that the HOT is open, not shorted.

[Seabass]

Originally posted by BernieDD
Nah, it's not shorted. If it were, the emitter and the collector voltages would be almost equal and most likely you'd have a blown fuse in the set. Consider the flyback transformer as a likely culprit if you have horizontal or high voltage circuit problems. Or could be that the HOT is open, not shorted.

What do you mean that it can be open...and how can i check if it is...also how can i test the flyback...i dont know what is wrong with the monitor but i have plenty of time to test it so i will do it nice and slowly just for something fun and new to learn...

[BernieDD]

Originally posted by Seabass


What do you mean that it can be open...and how can i check if it is...also how can i test the flyback...i dont know what is wrong with the monitor but i have plenty of time to test it so i will do it nice and slowly just for something fun and new to learn...

I suggest you leave the testing to the experts/professionals. It is clear to me now that you have very little knowledge of electronic circuits, and without that knowledge you'd not likely find the set's defective parts. Poking around the circuits in your monitor may result in getting a nasty shock or causing more damage to it. And testing the flyback for one requires special equipment even I do not have.

If you want to continue working on your set, I also suggest you first buy and read a book that explains the fundamentals of monitor circuitry. Then you will be in a better position to understand what you're doing.

[Mechanic]

I have been following this thread for a while and I feel that you are treading on dangerous ground. Flyback voltages can easily exceed 20,000 volts. Please don't take offense but you do not seem to be familure with the operation of a tv/monitor and you could really get yourself badly hurt. I suggest that you forget repairing the monitor or that you take it to a qualified repair person.