Thursday, October 15, 2009
Thursday, May 21, 2009
Understanding VPNs and the VPN Gateway
Virtual private networks (VPNs) enable users to connect to a remote private network through the Internet. Virtual private networks therefore span the Internet because the user connects over the Internet to the remote VPN server. With a VPN, data is first encrypted and encapsulated before it is sent to the remote VPN server. When the VPN server obtains the data, it decrypts the packet so that is can be interpreted.
VPNs are usually implemented to provide for the following scenarios:
Enable remote access users to connect to and access the network.
Provide connectivity between two or multiple private networks or LANs.
A VPN gateway, also called a VPN router, is a connection point that connects two LANs which are connected by a nonsecure network such as the Internet. A VPN gateway therefore connects to either a single VPN gateway, or to multiple VPN gateways to extend the LAN. This scenario is typically referred to as a router-to-router VPN. The corporate networks are connected through the VPN servers running Routing And Remote Access (RRAS). The actual medium that connects the LANs is usually the Internet. This means that the VPN gateway or router will be configured with the address on the LAN that it is connected to, and a public IP address.
VPNs are usually implemented to provide for the following scenarios:
Enable remote access users to connect to and access the network.
Provide connectivity between two or multiple private networks or LANs.
A VPN gateway, also called a VPN router, is a connection point that connects two LANs which are connected by a nonsecure network such as the Internet. A VPN gateway therefore connects to either a single VPN gateway, or to multiple VPN gateways to extend the LAN. This scenario is typically referred to as a router-to-router VPN. The corporate networks are connected through the VPN servers running Routing And Remote Access (RRAS). The actual medium that connects the LANs is usually the Internet. This means that the VPN gateway or router will be configured with the address on the LAN that it is connected to, and a public IP address.
Tuesday, May 12, 2009
Friday, February 20, 2009
MONITOR REPAIR LESSON
Troubleshooting a dead monitor
Checking the primary side
If the monitor is dead the first thing to do is to verify if there is voltage on the drain of the switching FET of the main power supply
In case there is not voltage, you have to seeking for some open thing like blown fuse, resistors or broken traces.
The normal voltage on its drain is about 150V when the monitor is plugged to a 120V AC outlet (USA users) or 300V when the monitor is hooked to a 220V AC outlet (UK users)
But if you check it and find voltage on its drain, it means that the rectification is good and the power supply unit is blocked for some reason
In this case, you must verify if there is start-up voltage on the main power supply
If the unit uses the IC 3842 or 3882 as shown below, the start-up voltage must be about 12V
For other ICs check its datasheet to find out which is its start-up pin
If the start-up voltage is not present, you must seek for blown start-up resistors
The start-up resistors are high resistance resistors such as 220K or 470K resistors
In many cases you find the start-up voltage, but the power supply does not work
In this case the problem can be:
1 - Short on the secondary side
2 - Short on the optical coupler or on some part hooked to it
3 - Short on some part in the primary side itself
Step for analysis of short on the secondary side
Remove or suck up the collector of the HOT.
If the power supply works, then it means that the power supply has no problem
In this case the problem is on the horizontal output: Fly-back, HOT or yoke.
In some monitors if you remove the HOT and do not apply a load resistor on its place, some capacitors will explode because the power supply may increase beyond its normal value. In other lessons you will see how to apply load resistors
If you don´t want apply a load resistor you must turn the monitor on only for a brief moment
If the main power supply won't work yet, maybe there's a short in any of the secondary lines
You can check that with the multimeter in the scale of ohms (100R)
For checking shorts, unplug the monitor from the AC outlet and check the resistance of each one of the sources of the secondary.
First put the red probe of the multimeter on the source and the black tip on the ground, then invert and put the black tip in the source
Check all of the sources of the secondary.
When you put the red tip in the cathode of a diode, it is normal to find low resistance.
However when inverting the tip you should find a higher resistance.
If you find the same resistance on both sides, there's a short on that source.
In the illustration above when the black tip is on the cathode and red tip is on the ground, the resistance should be higher.
Important: To measure the resistance of the source of the heater you must remove CRT PCB, because the CRT heater resistance is very low and it will show a false short. The heater is the lowest secondary power supply line
Never measure on the anode of the diode , measure on the cathode because the anode is hooked the a coil of the chopper which has low resistance and this coil has a terminal hooked to the GND. Therefore there will always be low resistance on the anodes that are hooked to the chopper.
If the short is in a medium power supply (between 20 and 35v), probably the cause can be a short in the vertical output.
In this case, if you don't know which pin is +B of the vertical IC, to remove the IC from the PCB and the short will disappear.
Replace the IC and the problem will be solved
If the short is on a higher source as 53 to 150v, probably this is the source of the horizontal output.
Then you must remove the HOT or to suck up the pin of the fly-back +B.
If the short is on the highest source (usually 150v) then it is probable that the short is in the RGB output.
There are monitors which use three RGB output transistors, others uses a power RGB IC.
A shorted RGB IC can also cause the main power supply to be dead because it's hooked directly to the power supply
If you found no problem on the secondary side, then you must check the primary side
In monitors that use 3842 IC on its power supply, check capacitors hooked to its pins # 3 and #4
Also replace small electrolytic capacitors as shown below:
It's recommended to replace the capacitors because many of them can pass the multimeter test and still be bad
At last, if the problem is not found, the optical coupler must be replaced
Below you can see two types of optical couplers
In lesson 5 we show how you can test an optical coupler and how does it work
You can choose between testing or replacing it without testing
That's all for a while
Troubleshooting a dead monitor
Checking the primary side
If the monitor is dead the first thing to do is to verify if there is voltage on the drain of the switching FET of the main power supply
In case there is not voltage, you have to seeking for some open thing like blown fuse, resistors or broken traces.
The normal voltage on its drain is about 150V when the monitor is plugged to a 120V AC outlet (USA users) or 300V when the monitor is hooked to a 220V AC outlet (UK users)
But if you check it and find voltage on its drain, it means that the rectification is good and the power supply unit is blocked for some reason
In this case, you must verify if there is start-up voltage on the main power supply
If the unit uses the IC 3842 or 3882 as shown below, the start-up voltage must be about 12V
For other ICs check its datasheet to find out which is its start-up pin
If the start-up voltage is not present, you must seek for blown start-up resistors
The start-up resistors are high resistance resistors such as 220K or 470K resistors
In many cases you find the start-up voltage, but the power supply does not work
In this case the problem can be:
1 - Short on the secondary side
2 - Short on the optical coupler or on some part hooked to it
3 - Short on some part in the primary side itself
Step for analysis of short on the secondary side
Remove or suck up the collector of the HOT.
If the power supply works, then it means that the power supply has no problem
In this case the problem is on the horizontal output: Fly-back, HOT or yoke.
In some monitors if you remove the HOT and do not apply a load resistor on its place, some capacitors will explode because the power supply may increase beyond its normal value. In other lessons you will see how to apply load resistors
If you don´t want apply a load resistor you must turn the monitor on only for a brief moment
If the main power supply won't work yet, maybe there's a short in any of the secondary lines
You can check that with the multimeter in the scale of ohms (100R)
For checking shorts, unplug the monitor from the AC outlet and check the resistance of each one of the sources of the secondary.
First put the red probe of the multimeter on the source and the black tip on the ground, then invert and put the black tip in the source
Check all of the sources of the secondary.
When you put the red tip in the cathode of a diode, it is normal to find low resistance.
However when inverting the tip you should find a higher resistance.
If you find the same resistance on both sides, there's a short on that source.
In the illustration above when the black tip is on the cathode and red tip is on the ground, the resistance should be higher.
Important: To measure the resistance of the source of the heater you must remove CRT PCB, because the CRT heater resistance is very low and it will show a false short. The heater is the lowest secondary power supply line
Never measure on the anode of the diode , measure on the cathode because the anode is hooked the a coil of the chopper which has low resistance and this coil has a terminal hooked to the GND. Therefore there will always be low resistance on the anodes that are hooked to the chopper.
If the short is in a medium power supply (between 20 and 35v), probably the cause can be a short in the vertical output.
In this case, if you don't know which pin is +B of the vertical IC, to remove the IC from the PCB and the short will disappear.
Replace the IC and the problem will be solved
If the short is on a higher source as 53 to 150v, probably this is the source of the horizontal output.
Then you must remove the HOT or to suck up the pin of the fly-back +B.
If the short is on the highest source (usually 150v) then it is probable that the short is in the RGB output.
There are monitors which use three RGB output transistors, others uses a power RGB IC.
A shorted RGB IC can also cause the main power supply to be dead because it's hooked directly to the power supply
If you found no problem on the secondary side, then you must check the primary side
In monitors that use 3842 IC on its power supply, check capacitors hooked to its pins # 3 and #4
Also replace small electrolytic capacitors as shown below:
It's recommended to replace the capacitors because many of them can pass the multimeter test and still be bad
At last, if the problem is not found, the optical coupler must be replaced
Below you can see two types of optical couplers
In lesson 5 we show how you can test an optical coupler and how does it work
You can choose between testing or replacing it without testing
That's all for a while
Wednesday, December 3, 2008
Friday, October 3, 2008
Wednesday, October 1, 2008
Monday, June 2, 2008
How To Make Xp Bootable Disc
The following items are required:
A copy of the original Windows CD/DVD or ISO.
A copy of the Boot Files (Downloaded Below).
A minimum of 1GB available hard disk space for CD's.
A minimum of 5GB available hard disk space for DVD's.
A copy of the original Windows CD/DVD or ISO.
A copy of the Boot Files (Downloaded Below).
A minimum of 1GB available hard disk space for CD's.
A minimum of 5GB available hard disk space for DVD's.
Wednesday, May 28, 2008
How to unlock the password protected memory card of your mobile phone.
have seen such a case when one my friend forgot the password of his memory card of his mobile phone and he have some very important data in the memory card which he want to recover, he tried almost everything then he lost hope and then asked the mobile phone company to find the way to recover the data from the memory card.
But you don’t worry if you are having the same problem…as now there is a much simple and easy way out.
Requirement: You need a file explorer like FExplorer which you can download from here.
How to Unlock MMC card:
Method 1:
Insert card into your phone but don’t access it through phone.
Run FExplorer and Open the path C:\system.
You will find a file called mmcstore ,rename the file mmcstore.txt
Copy that file(mmcstore.txt) to your pc and open that file in notepad.
You will find your password in that file.
But you don’t worry if you are having the same problem…as now there is a much simple and easy way out.
Requirement: You need a file explorer like FExplorer which you can download from here.
How to Unlock MMC card:
Method 1:
Insert card into your phone but don’t access it through phone.
Run FExplorer and Open the path C:\system.
You will find a file called mmcstore ,rename the file mmcstore.txt
Copy that file(mmcstore.txt) to your pc and open that file in notepad.
You will find your password in that file.
Saturday, April 26, 2008
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