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.
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