## Friday, February 19, 2016

Sizwe Sizwe from South Africa writes.
Hi Humphrey.
As a new student in this course please be patient as most of my questions will be on things that seem simple to someone else but not so to me.

I have these questions:
1.The difference between drive transistor and horizontal output transistor

When explaining something to my student I believe a photo speaks a thousand words. On the picture below we have the Drive transistor and horizontal output transistor (H.O.T).

Drive transistor is basically on/off switch, can be either bipolar or mosfet transistor which together with the drive transformer to drive the H.O.T (drive transformer is used as impedance matching)
Horizontal output transistor (H.O.T) - also act as an on/off switch, if there is horizontal pulses at the base of the HOT and B+ is present and everything is okay the transistor will switch very fast and by doing so current will flow through the primary of the fbt to the ground and this will cause voltage to appear on the secondary side of the fbt like the HV, screen, focus voltages will be generated.

Below you can their relationship on the circuit diagram.

2.Difference between neck circuit/RGB circuit

RGB circuit and neck board circuit is one and the same thing but RGB is a better name.

Circuit diagram of RGB circuit

Bottom side of the same circuit

3.Vertical circuit.
This circuit is used to open the screen upwards (vertically), therefore when you see a TV screen with one horizontal line means the vertical circuit is not working and therefore when solving this problem concentrate on the vertical circuit.

Here is the circuit diagram of the same.

circuit diagram of the same circuit(vertical)

Mphora Nkomo from South Africa
Hi Humphrey, Please explain to me the B+ P.O.T.

B+ P.O.T is used to set the B+ voltage in CRT television, usually they are preset at the factory and therefore should not be adjusted by the technician unless you found it faulty and need replacement, then only you can set the B+ voltage using the circuit diagram specification for the model and size.

Here is the exact reference on the circuit diagram.

Eng.Chandana  Ekanayake from United Kingdom
I am reading your CRT TV book, these days. What is the surge limiter indicated in your photos?? Is that high watt resistor or some Thermistor?   How to test it?
Thanks.
Eng.Chandana  Ekanayake  B.sc,MIET(UK),AMIE(SL)

Surge limiter is just a high watt/ low ohm resistor.
Test it like a fuse and expect low ohm...it usually go faulty by reading open on your meter.

Thanks you guys.
let meet in the next class

1. sir

i need ur kind help i'm facing an issue with my LG tv

(which is a Chinese one, discovered later)

the problem is

at first 2-3 channels became black & white & soundless

bu after a few days

all the channels got B/W n SOUNDLESS

i re-soldered all the dry joints of the jungle ic N channel box

but unfortunately no result still the very same problem

plz help by showing me the right path to the culprit

thanx....

2. Dear Humphrey,
Thank you for your educative ebooks that I have been reading on repair of CRT televisions. Please I would like you to help me on:
1. how I can know the presence of base voltage on SOT base lead. Your book 'CRT Television Repair Course' states that this voltage could be about -1.38v. The question is - Is this DC or AC and which digital meter range can I use to measure this negative voltage. I have been trying to determine it with my digital meter but have failed to come up with the answer.
2. My CRT television can't give me any voltage on the secondary side, not even the standby light is on. However, when subjected to the current limiter, it indicates that there is no short as the light grows briefly and then goes off. This TV uses a transistor to switch the chopper transformer on and off. Upon testing about 297v is present at collector of SOT but it is difficult to know if -1.38v base voltage is also reaching its base for I do not know how to test it. I checked the SOT drive transistor with analogue multimeter out of circuit board and it seemed ok. The HOT also seems to be ok upon testing.