Sony BVM-9044D flyback shorting out/ no power - Page 1
Forum section: electronics / repair

author: belzrebuth
author_rank: Frequent Contributor
timestamp: February 26, 2023, 09:43:33 pm
content: Hi all,I'm trying to repair a Sony BVM-9044D which can't stay on; a screeching sound (like a zap) is heard at power up and then nothing happens.The monitor stays pretty much dead no static on the screen and no power led.Right after pressing the power switch I can see the tube arcing briefly and some kind of heatshrinked neon lamp(?) (NL801) briefly lights up as well (arc limiter of some sort?!)Only voltages I have is the one from the main power supply (40V) and I also have 40V where the 120V should be which is kind of weird.I don't have a shorted HOT because it tests fine on my chinese ttester that gives me a bit of hope that the flyback might still be alive.I don't have any shorted diodes because I checked them all with continuity no dead shorts but my measurements were in circuit.Q803 and Q801 are also okay I've taken them out and tested them.No shorted electrolytics but I replaced C801 and C802 with fresh ones because they seemed a bit scorched.I tried powered up without the tube connected in case there is a short in the tube but nothing changes.The fault must be in the P-board because I've exchanged the D-board with another monitor and it works there.The colour board and input are pretty irelevant with this kind of behavior so I would assume they're fine.I'm a bit burned up at the moment and I can only blame the flyback so I would appreciate any fresh ideas..!I attached the schematic of the P-board for anyone willing to take a look:)

author: fzabkar
author_rank: Super Contributor
timestamp: February 26, 2023, 10:21:01 pm
content: If the horizontal output transistor has been switched off, then the 120V supply would be sitting at the same level as the B+ supply. Therefore, it appears that after the initial zap, some kind of protection has kicked in.Does NL801 still light up after disconnecting the electron gun?Is there a resistive path to ground via ABL (pin #8 of CN801)?ABL = automatic brightness or beam current limiterIf you open one of the heater pins (H1 or H2), does the zap go away, and does the FBT develop the correct secondary voltages?Could the internal HV capacitor be shorted to pin #11 of the FBT?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: February 26, 2023, 10:54:31 pm
content: Is there a resistive path to ground via ABL (pin #8 of CN801)?Νο!If you open one of the heater pins (H1 or H2), does the zap go away, and does the FBT develop the correct secondary voltages?How would I do that?You mean desoldering the tranformer pins?With the CN802 and CN803 disconnected at power up I still get the zap.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: February 26, 2023, 11:04:26 pm
content: Quote from: fzabkar on February 26, 2023, 10:21:01 pmNo it doesn't. I can only see arcing inside the tube neck no light on the NL801.Pin 11 and 12 (HV) of the transformer give me 470k when measured..if there's a capacitor between those pins shouldn't it be infinite?

author: fzabkar
author_rank: Super Contributor
timestamp: February 26, 2023, 11:20:14 pm
content: Is it possible that air has leaked into the tube via a crack?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: February 26, 2023, 11:25:44 pm
content: Quote from: fzabkar on February 26, 2023, 11:20:14 pmΙ've no idea..It's not mine..I do have a working second so I could try measuring the flyback pins to make sure the other one has pins 11 and 12 isolated as they should be.I think this would confirm a dead flyback.edit: Measured the same pins on my working monitor and it's indeed 470k. Hmm... So tha't not it. Good thing is I now have two monitors open so I can A/B:)

author: belzrebuth
author_rank: Frequent Contributor
timestamp: February 27, 2023, 03:47:21 pm
content: Quote from: fzabkar on February 26, 2023, 11:20:14 pmYou were right!There was a crack in the glass nipple at the end of the tube. I didn't notice it when removing the neck board but now the glass is cleanly broken there.I think my repair is pretty much done..BTW when I said it's not mine I meant that I'm not the owner so I don't know if it had ever been dropped or improperly handled which is exactly the case with this monitor since breaking that thing could only have happened by some rough handling.