Dell U2413 turning off

"After some time of trouble shooting i found that the shut down issue was related to a crow bar circuit in a voltage control circuit, this circuit is used to set the output voltage of the 5.3v supply. With the out put at pins 3 and 4 using the red ground wire a pin one. There is a sensitive gate SCR Q971 PN. MCR101L http://www.unisonic.com.tw/datasheet/MCR... ( near the top left corner of the pcb) that was turning on when heat was applied to it, some place around 150 deg. F. There is also another SCR Q972 that is used for the 24 / 11.9 supply that may cause the same issue, Q972 is near the lower center of the pcb, near a heat sink. i found a similar part at digi-key PN BT169-L, there is an issue and the -parts leads are not in the correct order, with some care one can rearrange them to work.

As also ways i will not take any responsibility for the repair, or any damage to you or the equipment, (KMA)

I have am having success with my fix , happy fixing

FBG

Hope this helps,"

"A number of my colleagues have dual U2413f monitors on their CAD workstations. One of them has developed the same fault as described in this thread, it shuts down when it heats up.

I saw the solution of changing the thyristors to ones with less sensitive gates. I couldn’t easily get suitable replacement parts locally, so I looked for a different solution.

I drew out the schematic of the crowbar circuit which has a standard configuration. I decided that a simpler method of fixing the problem was to stiffen the Zener bias resistor to keep the thyristor gate voltage lower during normal operation.

The simplest method of achieving this is the put 1k in parallel with the existing 1k resistor resulting in halving the value.

The best part of this fix is that it can be done from the back of the PCB without unscrewing it and removing it from the metal frame.

For the benefit of others here is the process:

1.       Using a plastic guitar pick type tool along the seam between the front bezel and the back of the case, press in the clips and release the bezel.

2.       Place the monitor on its front and carefully lift off the back.

3.       Lift the metal shield and carefully peel the ribbon cable adhesive pad off the LCD.

4.       Press-in the clips and release the ribbon from the LCD.

5.       Fold over the PCAs to access the back of the PCBs.

6.       If necessary, the PSU PCA can be removed by squeezing the release clip on the inside of the PSU SIL connector on the signal PCA.

7.       The thyristors in question, Q971 & Q972 have 1k between their gate and cathode.

8.       Reduce the Zener bias resistance to 500R.

9.       Solder a 1k 0805 on the gate and scrape some resist off the ground plane and solder.

Since carrying out this modification the monitor has not had any further problems."

So the two thyristors are the answer for this problem. Q971 and Q972. Unforunately the original MCR part had a non-standard arrangement of Gate, Anode, Cathode (K) leads, and there are none I could find with the correct order, so you MUST bend the pins to have the right one go to the correct hole on the board, changing the Littlefuse Thyristor from KGA pin order to GAK order to match the original part - you CANNOT use the default arrangement.

It has been running for two days without turning off. I put new thermal paste on the K15A60U and I attached a radiator to the LD7904jgp7

Hi. This is not exactly the same problem. My monitor doesn’t turn on anymore. I opened it and checked the boards. It seems there has been something going on in power input circuit and couple of components has blown. Is it possible someone could provide me original values for C909 and FB901. The rest I can still read. If one of you wonderful persons could help me out with these components, I could start fixing the monitor.