Wednesday, May 25, 2011

JCV Television Color problem

This Television came in for repair with the screen color purple, please note that this in not JVC and it’s not even a typing error. It is simply JCV, that’s why as a buyer you should always be keen when buying electronics gadget because some of them come with a name very close to a branded one.

A brief history from the owner was that the television works for a few minutes with normal color and after sometimes the screen color changes and he suspected the tube is faulty( I advice technicians to listen carefully to the customer history of the problem but when it come to suggestion where he think the problem is.. be wary)

I powered up the set to confirm what he has said and truly the set came up and after 5 minutes the color was messed up again. Looking at the screen I could see the problem pointed at the RGB circuit.







I opened the set and went straight to the suspect circuit and scanning around this circuit I could see the three RGB transistor (2SC2482) are all darkened on the board. This pointed to overheating and in the meantime I decided to take the voltage around this circuit.



I did the voltage testing and all the three cathodes were within the expected range. Then I remembered one clue the owner gave me was that the problem was coming up after sometimes and I reasoned most likely there could be a component which is getting overheated and hence start behaving funny.

So I used my finger to touch the three BIG resistor which take 180 volts to the collector of each transistor (Drops 180 Volts to around 145 volts at the collector of the RGB transistors)

Two of them were extremely hot for my finger and I concluded there could be a component failing under load and the most likely culprit could be the two transistors after these resistors. Luckily I had a junk board and I salvaged the two transistors and used them in this assignment.



I powered the set and it came up smiling again as you can see the picture below.



Thank you guys

Let meet in the next class

Humphrey Kimathi

Author CRT Television repair course









Monday, May 16, 2011

Panasonic CD STEREO SYSTEM. AM/FM RADIO Model no.SA-AK5 CD & Cassette player not working



Panasonic CD STEREO SYSTEM. AM/FM RADIO Model no.SA-AK5 CD & Cassette player not working.




This Panasonic radio/CD player with over 10 years of service was brought to me because the CD player and also cassette player was not working at all but radio more or less is good.

After opening the cover I found 10years old dust inside, i used the blower to clean off first.
I decided to started with CD section which has the problem of not able to READ the CD ” NO CD” display.

 Because of too much dust inside the machine I decided to clean the laser eye with white alcohol using Q-tip.

After cleaning the laser eye the CD stereo start working again.
   


Next I took a look at power supply which was linear(not SMPS)  The power supply consisted of a big transformer (step down AC in the entrance of AC VOLTAGES) and a relay, two big electrolytic capacitors, four big diodes as a bridge rectifier  and two ic regulators and some resistors. 



The machine works only with two voltages sources:- 5 volt for all parts except motor of the cassette compartment and the amplifiers that works with 14 volt. 

After a tour on power supply section i noticed there was a problem in cassette recorder section.

This section is dead ,no movement is seen here motor belts more or less are good but motor is not working at all I checked the voltages to motor.Yes the voltages were good but the motor is not working.

So i decided to change the motor as well as the belts. I found the motor in our market with same code Number but the vendor said this is 12 volt motor.

I have to mention that I changed the belts with smaller size because the belts after 10years of service and due to heat inside the machine are getting stretched and did not fit into the motor rail well and therefore needed replacement with new one.





After changing the motor and belts the Cassette section also started working.


Author of this article is Mr Beh.
A professional repairer based at Tehran - Iran.


Thursday, May 12, 2011

Relay found in switch mode power supply (s.m.p.s)


Relay found in switch mode power supply (s.m.p.s)

Sometimes in CRT Television s.m.p.s you will find relay in the power supply. In this lesson I would like to have an overview of this component.

First what is a relay? Basically a relay is a switch which is operated electrically.

 The internal structure of a relay is composed of:

1. Coil – work with low current
2. Contacts-Work with high current



Basic relay Circuit


A relay allows one circuit to switch ON a second circuit which is completely separate from the first.

In the example above a low voltage (12Vdc) is used to turn on a relay to switch on a 240V ac main circuit. Please note that there is no electrical connection inside the relay between the two circuits, the link is only magnetic and mechanical.

If there is Zero (0) volt at the Base (B) of transistor Q1. The transistor is open and therefore current is not able to flow from the 12V rail to the Ground and hence the relay contacts are open and the 240V ac Bulb is off.

Now if a Voltage is applied at the Base (B) of transistor Q1. The transistor will close(ON) and the current will start flowing from the 12V rail through the relay coil to the Collector of the Q1 down to the Ground.

When the current flow through the coil a magnetic field is created around the wire and this magnet (electro) will attract the contacts of the relay. Once the contacts closes then the other circuit is switched on and the Bulb light.

With that basic knowledge let us now move on to see how relay are used in switch mode power supply (S.M.P.S) to degauss the CRT tube.

Below is the actual circuit of relay in a JVC Television power supply 



When the Television is first switched on a signal is send from the TV micro processor to the base of Q973 just for a moment (2 seconds). When this signal reaches the base of transistor Q973 the transistor is turned ON and the 12 Volts DC moves to the ground via relay coil. When current flow through the relay coil it behaves like a magnet and relay switch (contacts) is closed. When the relay switch is closed the current flow through the relay on the 240V ac into the Degaussing Coil and the screen is cleaned.

The picture of the actual relay used in this supply


Testing a relay:

Coil: Use your meter set to diode test and you should get a reading if the coil is okay. If no reading then the coil is open and therefore the relay is bad.

Contacts: To test if the contacts are working you need to apply some Voltage (5V dc) to the two pins of the coil and if the contact is working then you should hear a click sound from the relay.

Thanks you guys
Let meet in our next class.