Help Please with a monitor PSU.

[specmaster]

I'm trying to troubleshoot a Digimate monitor PSU, all caps check out OK as does the voltage ref chips TL431K. I have a 9 pin connector taking power away to the other parts and there are 2 pins that I'm not sure what they are doing. There is ref to the function of these pins printed on the PCB but 2 of which I'm struggling to work out their function is. Left right I have the following :-
2 x 12v+, 1 x Grnd, 2 x 5v+, 2 x Grnd, 1x BR and 1 x EN, what are the BR and EN referring to please, all the cables on the plug for the connector are white, so no clues there?

 

[kellys_eye]

Pictures?

The EN would likely be an enable signal.

 

[specmaster]

Your wish is my command
Yeah, I thought one of them would be the enable signal from the power button, so what about the BR, is the power to the button and the EN the return at all?

 

[kellys_eye]

Wider view! Where do the tracks from the EN and BR pins go? What about a pic of the whole board too?

 

[specmaster]

OK, here is a high res photo of the entire board.

 

[specmaster]

Strange, I think, but I have both 12V and 5V present all the time, but I switch on, the power LED lights up blue and after a few seconds, the LED flashes on and off in red. Measuring the backlight supply, both slowly build up to between 20 and 26V and drops to zero and the Power LED flashes red. Switch off and on again and the process repeats

 

[kellys_eye]

Power cycling is indicative of problems in the power supply circuitry and, in your case, obviously the backlight circuitry.

Find the backlight board power connector and disconnect it - see if the monitor starts up 'properly' (although you won't see any image) and if it does then check the backlight circuit for the fault. It's possibly (probably) the backlight itself. If it's CFL then you'll need new tubes but if the backlight is LED-based then, usually, ONE (possibly, but not always, more than one) LED has gone dud.

Modern LED backlights come in 'strips' - it's not easy to desolder individual LEDs although it's possible if you have the tools and skill - and replacement strips are relatively easy to source but actually putting them in means dismantling the screen - completely.

 

[specmaster]

Power cycling is indicative of problems in the power supply circuitry and, in your case, obviously the backlight circuitry.

Find the backlight board power connector and disconnect it - see if the monitor starts up 'properly' (although you won't see any image) and if it does then check the backlight circuit for the fault. It's possibly (probably) the backlight itself. If it's CFL then you'll need new tubes but if the backlight is LED-based then, usually, ONE (possibly, but not always, more than one) LED has gone dud.

Modern LED backlights come in 'strips' - it's not easy to desolder individual LEDs although it's possible if you have the tools and skill - and replacement strips are relatively easy to source but actually putting them in means dismantling the screen - completely.

The test was done with the screen removed so the backlights weren't in circuit and the voltage still collapsed to zero on both circuits. Both of the electrolytic caps in section have been taken out of circuit and tested and are well within the rated tolerances so I'm not expecting any issues from those, so now I'm wondering if it is the PWM chip, which is hard to locate in the format, most of them these days are SMD format.

 

[kellys_eye]

Both of the electrolytic caps in section have been taken out of circuit and tested and are well within the rated tolerances

In value maybe - did you check for ESR?

The SMPS secondary seems to have two separate outputs - one delivering the +5V and the other some 'higher' voltage for the backlight and +12V (via a regulator).

The supply to the inverters goes via F1 (beside T2) where F1 is actually a wire link. Disconnect that link to remove the DC to the inverter modules and check for correct operation. If normal operation is resumed then one of the inverters could be dud. Trace the copper track from the F1 link to the individual inverters and lift the pins (there are two pins on each inverter that take the +ve supply input, I think one of the inverters is powered through JP1) to each inverter and see which one stops the PSU firing up.

 

[specmaster]

In value maybe - did you check for ESR?

The SMPS secondary seems to have two separate outputs - one delivering the +5V and the other some 'higher' voltage for the backlight and +12V (via a regulator).

The supply to the inverters goes via F1 (beside T2) where F1 is actually a wire link. Disconnect that link to remove the DC to the inverter modules and check for correct operation. If normal operation is resumed then one of the inverters could be dud. Trace the copper track from the F1 link to the individual inverters and lift the pins (there are two pins on each inverter that take the +ve supply input, I think one of the inverters is powered through JP1) to each inverter and see which one stops the PSU firing up.

Yes, I did check the ESR, and they were around the .3 to .5 ohms apart from the 4.7uF which was just under 2 ohms which is what I would expect given it is the smallest electrolytic cap on the board. I'll give your suggestion a try later today to see if the PSU stays active when the F1 link is removed, and if not, I'll extract the jumpers JP1 and JP2 one at a time and see if the inverters are dodgy. This monitor has all appearances of not being used much in its life, as the insides and indeed the casing all appear to be like new.

 

[specmaster]

In value maybe - did you check for ESR?

The SMPS secondary seems to have two separate outputs - one delivering the +5V and the other some 'higher' voltage for the backlight and +12V (via a regulator).

The supply to the inverters goes via F1 (beside T2) where F1 is actually a wire link. Disconnect that link to remove the DC to the inverter modules and check for correct operation. If normal operation is resumed then one of the inverters could be dud. Trace the copper track from the F1 link to the individual inverters and lift the pins (there are two pins on each inverter that take the +ve supply input, I think one of the inverters is powered through JP1) to each inverter and see which one stops the PSU firing up.

Ok, just disconnected the link F1 and the PSU still shows a flashing red LED in place of a steady blue one. The 5v rail is still present as is the 12V rail and the voltage on the EN terminal which is I think as you said the enable terminal is just .5v. But when pressing the start button this reads 5.1V and drops out when the power LED starts flashing.

 

[kellys_eye]

The 5V supply comes from its own transformer secondary but the 12V comes from a HIGHER voltage supply (the other secondary winding) via a regulator. This 'higher' (unregulated) voltage is what? 24V? What does it measure as you first power up or are turning the power on? Does it fall?

With F1 removed you have disconnected the load on this 'higher voltage' supply so if the equipment is still 'tripping' then the problem is in the main SMPS side (secondary) and related to the over-current measuring circuitry that reports a fault and shuts it down. Measure the voltage at F1 (with it NOT powering the inverters).

The EN/BK signal lines look to go to Q1/Q2 which looks to be a simple latch circuit(?). Can't make it out at the resolution shown but I don't think it's related to the problem.

 

[specmaster]

The 5V supply comes from its own transformer secondary but the 12V comes from a HIGHER voltage supply (the other secondary winding) via a regulator. This 'higher' (unregulated) voltage is what? 24V? What does it measure as you first power up or are turning the power on? Does it fall?

With F1 removed you have disconnected the load on this 'higher voltage' supply so if the equipment is still 'tripping' then the problem is in the main SMPS side (secondary) and related to the over-current measuring circuitry that reports a fault and shuts it down. Measure the voltage at F1 (with it NOT powering the inverters).

The EN/BK signal lines look to go to Q1/Q2 which looks to be a simple latch circuit(?). Can't make it out at the resolution shown but I don't think it's related to the problem.

I was under the impression that only the 5V supply was constantly on in order to allow the "soft" power button to enable the PSU coming to life and thus thought that the 12V rail was only active once the enable signal was given and acted upon. Is that not the case then, as the 12V rail is currently always ON as well. The power LED is only lit once the power button has been pressed and pressing the power button again switches it off, but leaves both 5 and 12V rails alive.

Does this forum allow linking to a photo hosting site? If so, I can take some better quality photos and provide links to them on a hosting platform? I'll do the voltage measurement at F1 point tomorrow (Monday), but currently the only load on the PSU is the input and display PCB along with the push buttons on the monitor. The backlights are unplugged physically as well F1 link being broken, the LCD panel and speakers are not connected either.

 

[specmaster]

I was under the impression that only the 5V supply was constantly on in order to allow the "soft" power button to enable the PSU coming to life and thus thought that the 12V rail was only active once the enable signal was given and acted upon. Is that not the case then, as the 12V rail is currently always ON as well. The power LED is only lit once the power button has been pressed and pressing the power button again switches it off, but leaves both 5 and 12V rails alive.

Does this forum allow linking to a photo hosting site? If so, I can take some better quality photos and provide links to them on a hosting platform? I'll do the voltage measurement at F1 point tomorrow (Monday), but currently the only load on the PSU is the input and display PCB along with the push buttons on the monitor. The backlights are unplugged physically as well F1 link being broken, the LCD panel and speakers are not connected either.

The 5V supply comes from its own transformer secondary but the 12V comes from a HIGHER voltage supply (the other secondary winding) via a regulator. This 'higher' (unregulated) voltage is what? 24V? What does it measure as you first power up or are turning the power on? Does it fall?

With F1 removed you have disconnected the load on this 'higher voltage' supply so if the equipment is still 'tripping' then the problem is in the main SMPS side (secondary) and related to the over-current measuring circuitry that reports a fault and shuts it down. Measure the voltage at F1 (with it NOT powering the inverters).

The EN/BK signal lines look to go to Q1/Q2 which looks to be a simple latch circuit(?). Can't make it out at the resolution shown but I don't think it's related to the problem.

I checked the voltage at F1 location, with the link pulled to isolate the inverters and the voltage is 12.8V including when the PSU seems to shut down (Led turns red and flashes), is there a FET or something should be switching this?

I'll try and host some higher quality photos shortly.

 

[specmaster]

Finally I found a few minutes to try and post some better photos to give you a better view of the boards.

 

[specmaster]

Finally I found a few minutes to try and post some better photos to give you a better view of the boards. Try againhttps://imgur.com/L8X2CwT

 

[specmaster]

That worked, here are a few more better photos.

pcbs connected togetherTrack side

 

[specmaster]

I hope this come out OK for you, if you click on the three dots in the top right corner of the photo, there is an option to download the photo in a far bigger and clearer format. I do appreciate your kind as stance in trying to resolve this problem.