Neotec 500DX high pitch regular chirping - no power

[Nexus13]

Hey all!

I've got a doozy of a neotec that is giving me grief.  This is my first time working on a monitor like this one with what I think is an SMPS power supply.  Neotecs i've read are famous for having their HOTs go but after pulling mine out and checking it with my Atlas DCA75 I don't think its bad.  This type of power supply is making it really hard to trouble shoot as its constantly pulsing make it hard to measure voltages anywhere.   Does anyone have any advice?

I'm currently following lilshawn's advice from an older thread here to see if any of those caps near the power regulators have gone.  C525, C527, C519, C501, C502.  will also check Q505 and Q511

Thanks so much in advance!

[Nexus13]

Further update.  I couldn't find a short anywhere but did find both C502 and C503 at the very beginning of the power supply circuit seem low.  They are the blue (propylitic - not sure what they're called) caps just before L502 and the bridge rectifier.   They are both supposed to be 4700P.  Mine are registering 2360P and 2120P.   Gonna try scoring some replacements tomorrow to see if that fixes things.....


EDIT-  just looked at original under scope and they actually look like ther are supposed to be 2200P     
So i guess these are different to what's in the manual.  Hrm. Not sure what next...

[Nexus13]

I'm not sure if I"m looking in the right spot but I've checked almost everything in the mosfet/IC area of this D500 and so far everything looks good!  Checked all my diodes, and electrolytics, did a tone of resistors, and now I"m onto non-electrolytic caps.... still everything is checkingi out good. 

I'm questioning IC501 right now.  I've got two monitors in the exact same position with this chirping sound.  I've transplanted the 501 IC between the two and no change.

Does anyone know enough about SMTP power supplies to let me know if the chirping means there's a problem after the main transformer or it the problem is before it. 

THanks in advance!

[lilshawn]

hi, don't know if you got this sorted, i just happened to be perusing the monitor forum and saw this.

the NT500 has an SMPS power supply that has an overcurrent protection built into the main switching FET. if the output goes too high it will shut down (hence the tweet or chirp)

if the fault was with the SMPS part of the circuit, it would likely not make any noise at all.

in this case, tweeting/clicking,chirping... is all good, it means the SMPS IS working as it should.

the best way to start troubleshooting is to hit up the output diodes on the low voltage side of the SMPS.  lift out the cathode (banded) side away from the SMPS transformer (T501)

D508 runs the 140v B+ system (HV)
D509 runs the 51 volt system
D510 (C551) runs the 15/12 volt systems
D502 runs the 5 volt logic systems

likely your issue is going to be with the 15/12 volt system... if lifting out D510 (and remove or lift out one side of C551) allows the SMPS to get up and run (just run it a second don't run it long at all or you may damage other systems. basically just long enough to verify the SMPS is not ticking) your issue is in that system (likely IC301, R623, R623A, C323 etc.) if it's another system go from there.
you can lift out each individual voltage out of the circuit and find out which one causes the SMPS to stop.

[Nexus13]

Hey man,

Thanks for the very detailed response!  Yah you're totally right, I found a short on the 140V B+ system.  It led me all the way up to the neckboard where I located a shorted out IC203.  After replacing this component, I'm getting the monitor to power up and stay on but I don't get the "Disconnect" message flashing around the screen I thought all of these monitors do.  Should I be able to hook up any VGA signal to test it?  I.e. hook up a laptop and get an image on it? 

Thanks !

[lilshawn]

I don't recall if the nt500 does a no signal message or not.  it has an I2C driver IC so it might. it's been a while since I've had to work on one.

the OSD graphics are supplied by IC202 on the neckboard. there is a weird blanking thing that has to go on in order for it to display the OSD.

double check you have at P202 on the neckboard ... 7 volts in at at pin 11 and 12 volts at pin 7.

these feed voltage to the IC's on the neck board. if one or both is low, they won't operate properly.


in any case you should be able to feed a 640x480@60hz signal to the monitor and get some kind of picture. i normally set the resolution on the computer...verify the resolution setting (are you sure you want to change to this resolution?) then disconnect my computer monitor and plug in the unit. that way i know for sure i'm running the correct resolution and windows won't try and switch back to the old resolution. some monitors really don't like trying to run odd resolutions they aren't designed for.