MS-2933 issue ! Picture disapears after 10 mins

[Barth0lin0]

Hey !
I thinks my lovely Windy II is possessed by the devil .... lol
Managed to replace the OG Toshiba chassis by a nanao 2933 ! Everything worked fine for more than a year and then then chassis died !

The CRT works for 10 mins then the image shuts down. It need to cool down to reboot.

Here are my observations :

• B+ remains at 180 V
• Horizontal whine continues
• Neck heater stays on
• Picture disappears completely
• OSD disappears
• No raster after that the picture disapears, Increasing the Screen pot on the flyback does nothing.
• HV static gradually bleeds off
• After a few minutes of cooling, it works again
• Bright raster at power-up if no video source is connected, lowering the Screen pot on the flyback darkens the raster ( of course !) but the issue remains

My friend Chat (GPT) ! is telling be that is either a flyback or a HOT thermal failure.

Any thoughts about this ? The chassis have been recapped in the past , it's probably the original flyback

thx !

 

[trashedcabs]

It's likely going into xray protection due to HV overvoltage. Try turning the HV adjust pot down a bit.

 

[Barth0lin0]

It's likely going into xray protection due to HV overvoltage. Try turning the HV adjust pot down a bit.

i did turned down the HV-ADJ down a bit ( and a lot) and it did nothing.
I tested voltage at TP701 ( X-ray protection circuit) and i get around 16V until the picture disapear and I then get around 0V ( only a few mV) . (the service manual says the normal voltage is around 21V). the x-ray pot is still sealed with silicone so it's still factory set.
this means the X-ray protection circuit is not in cause i guess??

My guess would be a thermally failling flyback....i dont have an HV probe to test the anode HV voltage unfortunately.

Any thoughts ? Any other test i should do . ?

thx

 

[trashedcabs]

Took a quick look at the manual and it says that the xray protection circuit should activate when TP701 hits 24v, assuming the protection pot is adjusted properly. It gets a feedback voltage off one of the secondary windings of the flyback, and since that voltage is staying low I wouldn't suspect flyback. If the protection is activating lower than that, either the protection pot is dirty/bad (this happens on older monitors sometimes despite being glued in place), or something else in the circuit is failing.