How to Test a Horizontal Output Transistor Using a Multimeter

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As an avid TV user, did you know that there’s a very important component in any TV set? That component is a horizontal output transistor (HOT). In this post, we will evaluate some of the processes involved in measuring and testing its output. 

Have you ever tried to repair a TV with faulty signals throughout? Maybe you have ever tried to replace a horizontal output transistor only to have it malfunction once more. Lucky for you because we have a strategy that can help you save a huge amount of time and effort of work and needless replacement of parts.

All you should do is check the HOT pulse or the signal at the horizontal output transistor’s collector. But before we move on into that process, let’s dig deeper first and answer the question, “what is a horizontal output transistor?”

What is a Horizontal Output Transistor, by the way? 

Horizontal output transistors (HOT) are fast switches and not linear. They are created to drive the flyback transformer for hi-voltage to the picture tube anode. HOT normally energizes the major windings of a line-output transformer (LOPT). Keep in mind that it’s a high voltage and high-frequency power transistor that drives the transformer. 

In simple terms, a horizontal output transistor is a switch offering a path for current to flow through the primary winding of the flyback and horizontal yoke. It’s switched on and off through a signal distributed to the base. 

Given that there’s a power transistor, a current drive signal is required. That drive current is provided by the driver transformer and horizontal driver. Apart from the current setup, the driver transformer offers impedance matching. 

Beware of these Problems with Horizontal Output Transistor

You will find many breakdowns due to enhanced TV chassis reliability. However, more issues still happen in the high-voltage and horizontal circuits than in any other area of a TV set. 

Here are some of the common HOT problems you need to consider,

Remember that a leaky HOY might blow a fuse and cause the chassis to shut down. A leaking damper diode might also cause the same signs. Further, an intermittent output transistor or surrounding parts might generate that TV tough dog system. An open output circuit may create a dead chassis. 

A recurrent cause of failure in the early flybacks or horizontal output transformer was firing between and arcing over windings. Frequent damage to the horizontal output transistor might be triggered by a leaky, arcing flyback and overloaded secondary circuits. 

In the earliest solid-state TV sets, the damper diode was connected to the collector terminal of the HOT, next to the safety capacitor. Eventually, the damper diode was mounted within the same case along with the output transistor.

Do you suspect a leaky HOT? Take resistance and diode tests at its terminals. Assess the schematic diagram to identify if the damper diode is inside the output transistor case prior to making any tests or changing HOT itself. 

We recommend that you check the transistor number in the semiconductor replacement manual to identify if the damper diode is inside. In other TV chassis, you might notice a flat plastic horizontal output transistor attached to the big heat sink. 

How Do You Test a Horizontal Output Transistor?

In the past, we have heard from different beginner technicians and even a few more experienced technicians who have encountered meter damage by taking voltage measurements upon the HOT’s metal body. Don’t take any measurements upon the collector output terminal, especially if the output circuits are working. As an alternative, use an oscilloscope to observe waveforms at that terminal. 

One simple way to test your horizontal output transistor is with the help of a digital multimeter. Follow the steps below.

  1. Get your digital multimeter and set it in the resistance setting. 
  1. Attach the red test probe to the base and the black test probe to the emitter. 
  1. Its measurement should be at least 40 ohms to 60 ohms. 
  1. Reverse it to 40 ohms and 60 ohms again. Remember that there’s a resistor inside the HOT between the base and emitter. That’s why you see 45 ohms. That’s quite normal, though. 
  1. For the next step, attach the red test probe to the collector and the black test probe to the emitter. You will not see any number showing on your digital multimeter. That’s very normal as it has a high resistance. 
  1. Reverse the red test probe to the emitter and the black test probe to the collector of the horizontal output transistor. If you see a reading of 1.3 megaohms, it means it’s good. 

A low voltage measurement might suggest a leaky output transistor or damper, overloaded horizontal circuits, or wrong drive voltage. Higher than average voltage might suggest an open output transistor or emitter transistor if there is an emitter resistor in the circuit. 

In the horizontal output circuits in some imported sets, a small resistor is seen in the emitter circuit. As a word of caution: do not ever use a digital multimeter to measure the high voltage at the anode terminal of the picture tube either. 

You will get a nasty shock, and the multimeter will be damaged that is even beyond repair. Always utilize a high voltage probe meter to measure the picture tube anode voltage, preferably one that measures at least 40K VDC. 

Keep in mind that a high voltage probe connected to a VTVM does a good job. Just make sure that you connect the meter ground cable into the TV chassis. 

Final Thoughts

There you have it! With this guide, we have tried to show you how to make a fast and proper test in servicing your horizontal output transistors. We also liked to point out that you can also learn from other technicians. Continuous learning could help keep you on the right track. 

What about you? Are you ready to test your horizontal output transistor? We hope you find this post informative and useful. Share your thoughts with us by leaving your comments below!

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