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29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 16th, 2019, 2:30 pm
by Samelak
This is actually the 2nd tube that has done this. My replacement tube for my Blast City 2931 recently got a heater short. All I did was power up the cabinet and I heard loud crackling and I shut it down. I thought it might be a dirty anode cup that was arcing because this was an issue with the monitor when I first got it. The grease that the factory used was dirty and I saw arcing. Cleaned it off and it was good to go. That was about 6 months ago. Now I hear crackling again. Pulled the monitor and I see arcing in the neck tube. I put my 467 on it and I am getting a heater short. Unfortunately I wasn't able to clean the short and decided the tube was a loss...again. Anyone know what causes heater shorts? Has anyone ever found a 29" TV (seems like an odd size) to get a donor tube from? I pulled the yoke, degauss, rings and chassis before tossing the tube.

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 16th, 2019, 4:13 pm
by nem
No idea why it happens, but unfortunately it's somewhat common.

viewtopic.php?p=511870

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 16th, 2019, 5:20 pm
by mufunyo
For me it was an SMD capacitor on a sub board that was causing HV to spike, which arced the neck and shorted out the tube as well as cracking the glass by sheer force of the arcs. I'd recommend taking off the tube, doing a full recap (including all sub boards), and checking HV before attaching a new tube.

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 16th, 2019, 5:47 pm
by nem
That's interesting. How the heck did you identify the faulty SMD cap? Which sub-board was it?

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 16th, 2019, 6:46 pm
by mufunyo
nem wrote:
December 16th, 2019, 5:47 MP
That's interesting. How the heck did you identify the faulty SMD cap? Which sub-board was it?
It wasn't me who performed the repair. Just going off what I remember the repair technician told me on the phone. But I did burn through something like 3 or 4 tubes before identifying the exact cause. The short also burned out one of the RGB amps on the neck board, btw.

It was this little sub board:
Image

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 17th, 2019, 5:31 am
by Samelak
My first tube that went also had the glass crack in the neck. What is the SMD cap? Plus, my chassis was recapped.

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 17th, 2019, 6:17 am
by grantspain
failing flybacks are a major issue as well, I have not seen it myself but have been reliably informed that when they fail the HT spikes massively - its possible this causes an internal short to the focus or screen which in turn damages the gun, but its just a guess

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 17th, 2019, 9:26 am
by PrincessPrinPrin
If the anode voltage rises high enough to damage the CRT it means that the x-ray protection is not doing its job so the mosfet Q701 keeps driving the flyback when it shouldn't. The small module (U701) receives the x-ray protection voltage from the flyback (pin 10) and if this is detected as excessive the IC that outputs the drive signal to Q701 should shut down. A failed component on the module would explain the failure of the high voltage protection but what causes the flyback to output excessive EHT must be something else. In standard res monitors (where the horizontal circuit has a single transistor driving both flyback and yoke) the cause is usually the failure of the flyback capacitor in parallel with the HOT. In these hi res monitor it could be the EHT capacitor inside the flyback (across EHT and pin 14) which 15KHz flybacks don't have.

As a side note, when I googled the part number for the IC on the module (MMC PCS02) I found this page that shows the entire module is present on a PC monitor made by NEC:

http://www.abcelectronique.com/forum/sh ... php?t=5635

Re: 29" NANAO 2931 Tube Heater Short - What Happened?

Posted: December 21st, 2019, 10:37 pm
by Samelak
I'm not gonna lie, I understood very little of that. Can someone point me to a specific component to test?