Hi all
There's two other threads about this monitor with seemingly the same problem. The problem was resolved in one case by replacing the entire PSU board and in the second one by replacing an unknown IC.
https://www.badcaps.net/forum/showthread.php?t=50657
https://www.badcaps.net/forum/showthread.php?t=40512
The first one has pictures of the pcbs and I noticed that there's one IC and assorted caps not populated on mine (but the PCB itself looks identical) so I thought that'd warrant a new thread.
It's my dad's old monitor. He brought it to me with the problem description that after 15-20 minutes it would start power cycling with each cycle lasting shorter that the last one.
When he brought it and we hooked it up we couldn't reproduce the problem, it ran fine for at least 2 hours after which we turned it off.
This was about half a year ago, the monitor's been gathering dust since.
I pulled it out to set up a new pc last week and well, the problem returned, only it started right away after turning it on, no 15-20 minute delay. To me this indicates it is not a heat problem - cold start, the problem is there.
The exact cycle is:
- Monitor turns on, power light is blue, image comes up, duration about 1-3 seconds
- Monitor turns off completely, no power light, duration 1-2 seconds
- Monitor goes into standby, power light is amber, duration about 1-2 seconds
- repeat indefinitely
This occurs with or without attaching it to a computer. Without computer there's no image but just the floating "Cable is not conected" message.
Disassembling it, there are no visually bloated caps. On a whim, and because I had the right values I replaced the 2 smallest (in value) caps with Panasonic FR series (top right in the first image, just above the opto-isolator, C807, C827). This brought no change.
I also replaced the large filter cap, 120u/450V, temporarily but that also didn't make a difference.
Finally I repaced all the caps on the main board with Panasonic FRs, again no change in behaviour.
Reading through the first thread mentioned above I started taking measurements at the 8-pin connector between PSU and Main PCB:
With the monitor off (no power light):
Pin 1: 5.09V (VCC5V)
Pin 2: 5.09V (VCC5V)
Pin 3: GND
Pin 4: GND
Pin 5: 124mV (ON/OFF)
Pin 6: 3.18V (Brightness)
Pin 7: 3.30V (Volume)
Pin 8: 3.30V (Mute)
However, when the monitor is manifesting the power cycling, all these voltages will drop toward 0V when the power light goes off (same as in the the first post mentioned above), never reaching 0V because the monitor turns into standby (amber power light) again rather quickly.
When the monitor is on (blue power light):
Pin 1: 5.07V (VCC5V)
Pin 2: 5.07V (VCC5V)
Pin 3: GND
Pin 4: GND
Pin 5: 3.21V (ON/OFF)
Pin 6: 2.7V +/- 10mV fluctuating (Brightness)
Pin 7: 3.3V (Volume)
Pin 8: 3.3V (Mute)
The outputs of the regulators on the main PCB are 3.33V and 1.84V.
Now while I was taking these measurements repeatedly during the power cycling I pressed the power button to see if any of the voltages changed when pressing the button.
What happened is, the monitor turned off (no power light) and stays off. The VCC5V voltage remains at a steady 5.09V, all the other voltages are steady as well (first set of measurements above).
This is a notable difference because while it was power cycling, the VCC5V (as well as the other voltages) would drop towards 0V.
Pressing the power button again, the monitor turns on (going through standby phase for a second a so), and stays on, no power cycling anymore. It's been rock solid ever since. I've had it disconnected from mains power for 36 hours even and this morning it still runs fine after plugging it back in again.
I cannot reproduce the power cycling anymore.
Now, I don't have a need for this monitor and was going to give it to someone who has a need but I would really like to understand what exactly happened when I interrupted the power cycling by pressing the power button, and of course if this "fix" is only temporary.
If anyone can shed some light on what is happening here I'd love to hear from you. I'd also be happy to take more measurements as I'm really intrigued by this.
Thanks for reading
All voltages measured with Keithley 195A benchtop multimeter
There's two other threads about this monitor with seemingly the same problem. The problem was resolved in one case by replacing the entire PSU board and in the second one by replacing an unknown IC.
https://www.badcaps.net/forum/showthread.php?t=50657
https://www.badcaps.net/forum/showthread.php?t=40512
The first one has pictures of the pcbs and I noticed that there's one IC and assorted caps not populated on mine (but the PCB itself looks identical) so I thought that'd warrant a new thread.
It's my dad's old monitor. He brought it to me with the problem description that after 15-20 minutes it would start power cycling with each cycle lasting shorter that the last one.
When he brought it and we hooked it up we couldn't reproduce the problem, it ran fine for at least 2 hours after which we turned it off.
This was about half a year ago, the monitor's been gathering dust since.
I pulled it out to set up a new pc last week and well, the problem returned, only it started right away after turning it on, no 15-20 minute delay. To me this indicates it is not a heat problem - cold start, the problem is there.
The exact cycle is:
- Monitor turns on, power light is blue, image comes up, duration about 1-3 seconds
- Monitor turns off completely, no power light, duration 1-2 seconds
- Monitor goes into standby, power light is amber, duration about 1-2 seconds
- repeat indefinitely
This occurs with or without attaching it to a computer. Without computer there's no image but just the floating "Cable is not conected" message.
Disassembling it, there are no visually bloated caps. On a whim, and because I had the right values I replaced the 2 smallest (in value) caps with Panasonic FR series (top right in the first image, just above the opto-isolator, C807, C827). This brought no change.
I also replaced the large filter cap, 120u/450V, temporarily but that also didn't make a difference.
Finally I repaced all the caps on the main board with Panasonic FRs, again no change in behaviour.
Reading through the first thread mentioned above I started taking measurements at the 8-pin connector between PSU and Main PCB:
With the monitor off (no power light):
Pin 1: 5.09V (VCC5V)
Pin 2: 5.09V (VCC5V)
Pin 3: GND
Pin 4: GND
Pin 5: 124mV (ON/OFF)
Pin 6: 3.18V (Brightness)
Pin 7: 3.30V (Volume)
Pin 8: 3.30V (Mute)
However, when the monitor is manifesting the power cycling, all these voltages will drop toward 0V when the power light goes off (same as in the the first post mentioned above), never reaching 0V because the monitor turns into standby (amber power light) again rather quickly.
When the monitor is on (blue power light):
Pin 1: 5.07V (VCC5V)
Pin 2: 5.07V (VCC5V)
Pin 3: GND
Pin 4: GND
Pin 5: 3.21V (ON/OFF)
Pin 6: 2.7V +/- 10mV fluctuating (Brightness)
Pin 7: 3.3V (Volume)
Pin 8: 3.3V (Mute)
The outputs of the regulators on the main PCB are 3.33V and 1.84V.
Now while I was taking these measurements repeatedly during the power cycling I pressed the power button to see if any of the voltages changed when pressing the button.
What happened is, the monitor turned off (no power light) and stays off. The VCC5V voltage remains at a steady 5.09V, all the other voltages are steady as well (first set of measurements above).
This is a notable difference because while it was power cycling, the VCC5V (as well as the other voltages) would drop towards 0V.
Pressing the power button again, the monitor turns on (going through standby phase for a second a so), and stays on, no power cycling anymore. It's been rock solid ever since. I've had it disconnected from mains power for 36 hours even and this morning it still runs fine after plugging it back in again.
I cannot reproduce the power cycling anymore.
Now, I don't have a need for this monitor and was going to give it to someone who has a need but I would really like to understand what exactly happened when I interrupted the power cycling by pressing the power button, and of course if this "fix" is only temporary.
If anyone can shed some light on what is happening here I'd love to hear from you. I'd also be happy to take more measurements as I'm really intrigued by this.
Thanks for reading
All voltages measured with Keithley 195A benchtop multimeter
Attached Files
Comment