Jump to content
Due to a large amount of spamers, accounts will now have to be approved by the Admins so please be patient. ×
IGNORED

Nanao MS2931 stuck in protection


soyl

Recommended Posts

Hello,

 

I'm trying to fix this monitor that doesn't display any picture and was supposed to have shorted components in the deflection area due to potting compound leakage from the linearity coil:

 

IMG_7018.JPG

 

I've removed the coil, the HOT and the flyback to clean up that mess but the HOT turns out to be OK:

 

http://img12.imagevenue.com/loc453/th_343454500_008_122_453lo.JPG

 

http://img188.imagevenue.com/loc456/th_834232041_004_122_456lo.JPG

 

http://img21.imagevenue.com/loc422/th_840638573_003_122_422lo.JPG

 

So I did a light bulb test to see if the problem is in the deflection or in the power supply and the bulb lights up for a couple of seconds and the voltage is the expected 180V but then it shuts down and goes in protection:

 

http://img12.imagevenue.com/loc543/th_345021819_0112_122_543lo.JPG

 

According to the manual, if voltage at pin 7 of the control IC U901 (M62281) goes above 4.0V the power circuit operation stops and in fact I have 5.3V there:

 

http://img154.imagevenue.com/loc156/th_540817947_015_122_156lo.JPG

 

I've tested pretty much everything in the primary side: the mosfet Q901 is good, the diodes D901, D902, D903, D904, ZD901, ZD902 are all OK. C910 is OK, photocoupler PC901 is OK and I didn't find any bad resistors including the SMD ones. The diodes on the outputs of the power transformer (D921, D922, D923, D924) are also good.

 

What could be the culprit? Bad control IC (U901)? Bad b+ detection IC (U923)? Or could it still be something in the deflection?

Link to comment
Share on other sites

  • 4 weeks later...

Hi benchman, so far no luck with this (it's my chassis and Soyl has been so very kind to help me out with it!!)

It's just very frustrating as there is almost no reference anywhere to a same (resolved) issue.

Should you solve yours before ours I would gladly hear what did the trick :-)

 

Thanks!

Link to comment
Share on other sites

  • 2 months later...

Hi,

my 2933 also broke and gave it to our "jomac" in germany. Problem is, that he fixes a lot of chassis, but with this hes stressed and problem for me: i cant get any spare chassis (Rodotron said no, arcadespareparts: no, still waiting for an answer from Wei-Ya).

 

Here his last tries:

----

The fault is located in the HV-producing-circuit.

In the meantime i got the manual from the 2931, it seems to be similar to the 2933.

First i separated the HV-circuit from the power-supply and inserted a 300 Ohm / 30W as current-limiter.

Now the sparks were gone and the normal sound charging the tube with HV was to hear.

The waveform at the drain of the HV-switch looks normal and clean, a few hundred Vpp.

But after two or three seconds the protect-line shuts down the supply. Everytime.

The manual says the HV regulation depends on the voltage-feedback at the bottom of the HV-windings.

Similar like the beam-current-limitation at the cold end, there is nothing bad to see with the scope.

A third wiring is used to deliver a voltage for the X-protect, after rectifiing and smoothing 24V.

This is exactly the same as in the manual described and leads to the x-protect input from HV-reg.

Interestingly this 24V should be the reference voltage for correct setting the HV!

All secondary voltages from the switch-mode-supply are fine without ripple and in the correct range.

Looking with the scope at the vertical and horizontal deflection pulses showed no unusual things.

The H-defl. pulse only was ca. 700Vpp, in the manual it should be aprox. 1000Vpp.

Also i changed the yoke from 320µH to 200µH, as suggested few days ago and then i got the 1000Vpp.

This action didnt stop the protect-line from shutting down every time, unfortunately Dead

 

Here comes the special:

When i lower the current limitation for the HV-regulator and take instead the resistor only a 60W bulb:

After powering up there comes little sparkling noise and HV immediatly shuts down.

Interrupting the x-prot-line will give little longer sparking from top of the tube, but nothing is to see.

The Lopt works, i think. I cannot imagine that it is defective. Flyback condensor is also in spec.....

Without the limitation i do not trust myself to power up the set......until yet nothing is damaged!

 

Or is this a significant note, that the Lopt is dead...........i cannot believe

 

---------

The whole thread: http://www.jammaplus.co.uk/forum/forum_posts.asp?TID=81624&title=nanao-ms2933

 

So any Ideas you guys could have?

Link to comment
Share on other sites

  • 1 year later...
  • 4 weeks later...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...