I'm a newbie here and a trying to get back into repairing things rather than binning them. So please bear with my first attempt at posting...
This is a board from a Dell E173FPf 17" monitor. I've not been able to find a service manual or schematic online.
It's original fault was simply completely dead. There were no obvious burned-out components or bulging caps so I decided to try the heavier silicon stuff! I removed Q850 marked '27611' a MOSFET I believe ('A' in my component side image) and D854 marked 'SRF10-10' - a Schottky Barrier Rectifier (B).
With a DMM the MOSFET seemed to be ok (I need to find a more thorough guide to testing MOSFET's), but the Schottky diode pair read O/C across both legs. In my "spares" collection I found another Schottky diode pair - an S16C40C, somewhat under-specced compared to the original but I thought I would try it just to see if I got anywhere. I resoldered the MOSFET and my alternative Schottky diodes.
Much to my surprise, the power LED on the monitor now flashed green/yellow - apparently classic Dell for PSU problem - but at least there was *some* power now. (I also discovered the original SRF10-10 seems to have a pretty poor reputation, so I'll look for a more compatible alternative.)
So, the situation now:
Voltage across C854 (F), main capacitor hot side - 333V stable (UK mains, so 240V in)
Voltage across C862, C864 and C869 (C, D and E), what I take to be the main output caps - all around 4V - unstable, varying by +/- 1-2V
I dug out my old scope and luckily it still worked. Wasn't sure where to start so tried across the secondary (or could be two secondaries in parallel?) of the main transformer T850 (G).
The scope showed regular short bursts of voltage, maybe 2-3 bursts per second, each burst a short decaying roughly sinusoidal voltage. I have a very short avi of the trace, if I can't post that here, I'll try and get a still of the relevant frame.
I'm wondering if it might just be my poorly matched Schottky diodes but my instinct says look elsewhere, I'm just not sure where next.
Fascinating though this new-fangled stuff is, linear PSU's where a helluva-lot easier to troubleshoot! But my aim is to learn...
Any suggestions would be most welcome.
- jarsudsco
This is a board from a Dell E173FPf 17" monitor. I've not been able to find a service manual or schematic online.
It's original fault was simply completely dead. There were no obvious burned-out components or bulging caps so I decided to try the heavier silicon stuff! I removed Q850 marked '27611' a MOSFET I believe ('A' in my component side image) and D854 marked 'SRF10-10' - a Schottky Barrier Rectifier (B).
With a DMM the MOSFET seemed to be ok (I need to find a more thorough guide to testing MOSFET's), but the Schottky diode pair read O/C across both legs. In my "spares" collection I found another Schottky diode pair - an S16C40C, somewhat under-specced compared to the original but I thought I would try it just to see if I got anywhere. I resoldered the MOSFET and my alternative Schottky diodes.
Much to my surprise, the power LED on the monitor now flashed green/yellow - apparently classic Dell for PSU problem - but at least there was *some* power now. (I also discovered the original SRF10-10 seems to have a pretty poor reputation, so I'll look for a more compatible alternative.)
So, the situation now:
Voltage across C854 (F), main capacitor hot side - 333V stable (UK mains, so 240V in)
Voltage across C862, C864 and C869 (C, D and E), what I take to be the main output caps - all around 4V - unstable, varying by +/- 1-2V
I dug out my old scope and luckily it still worked. Wasn't sure where to start so tried across the secondary (or could be two secondaries in parallel?) of the main transformer T850 (G).
The scope showed regular short bursts of voltage, maybe 2-3 bursts per second, each burst a short decaying roughly sinusoidal voltage. I have a very short avi of the trace, if I can't post that here, I'll try and get a still of the relevant frame.
I'm wondering if it might just be my poorly matched Schottky diodes but my instinct says look elsewhere, I'm just not sure where next.
Fascinating though this new-fangled stuff is, linear PSU's where a helluva-lot easier to troubleshoot! But my aim is to learn...
Any suggestions would be most welcome.
- jarsudsco
Attached Files
I'm from just down the road in Jarrow.
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