Monitor C1770NSL horizontal problem

[alessioblaupunkt]

Dear Badcaps staff,
this Hp MX70 crt monitor has the chassis Lite-On C1770NSL. In this video you can see the problem.

Here's the schematich.

There are 53V at pin 2 of Flyback Transformer (FBT) when the monitor is off.
There are pulses up to 250V at pin 2 of FBT when the monitor is on. In this situation there is no voltage coming out from pin 4 of FBT.

Transistor and diode are not shorted or opened.
There are 4 N-ch mosfets, all ok.
I've checked with an LCR meter some polyester capacitor near the FBT, they are good.

The voltages 80V, 59V, 11.6V and 7V are ok.
There are horizontal drive pulses.
The one coming from pin 8 of IC401 TDA4841 is not stable in amplitude, but don't know if this is the cause or an effect.
The other two pulses coming from pin 15 and 29 are stable (TDA4841).
All the electrolytic capacitor have been tested.
The entire board and crt board have been resolded.

I think I'll do the lamp test removing the 424 resistor (base of Q402) and connecting the lamp to C and E.
because I'm too lazy to remove Q402 again.

Can't find the solution.
Thank you for reading.
Feel free to suggest some test to do.

[alessioblaupunkt]

Re: Monitor C1770NSL horizontal problem

X-ray protection is on that's why TDA4841 floats Hdrive (pin8)
then all deflection circuit stop working.

This chassis randomly switch from the problem you can see in the video
and the X-ray protection = black screen.

Lamp test was useless because there was no feedback from flyback so no deflection.

Still no chance to solve.