PVM-20M2U vertical jitter... sync? vhold ic? v output transistors? - Page 1
Forum section: electronics / repair

author: deezdrama
author_rank: Contributor
timestamp: December 03, 2022, 12:15:08 am
content: So I have a 20m2u I picked up recently.It was stuck in protection mode with the red fault/tally light on and no high voltage to the tube.Power board was outputting correct voltages.A hand full of capacitors were reading high or borderline high esr readings so went ahead and recapped the entire deflection side of the board with 105c temp rated nichicon capacitors.The pvm came back to life but the front panel board was dead and the tube was discharging a loud arc on startup.Replaced the front panel board and soldered in a custom ground lead directly to the tube grounding fingers that contact the aquadag. No more arcing and the front panel worked so could now access the menu and select input sources.I fired up 240p test suite to callibrate the geometry and image.... Had it looking great!Started playing a game and after about 30 minutes the lower portion of the image started to stretch horizontaly and then after a few minutes the image started to bounce vertically , then after another minute or so the vertical bouncing turned into a full vertical rolling like I lost vertical sync. Vertical hold adjustment would not get rid of the rolling.https://youtube.com/shorts/qzyXlMsnjnw?feature=shareI double and triple checked all the caps I replaced, ended up replacing the sync seperator IC chip..... Same vertical rolling.Swapped in a different power board.... Same issue.Happens with composite, component, and rgb inputs.The menu itself jumps around and is unstable now.Couldnt remember if I tried swapping the rear signal input board so tonight I tried that and noticed the vertical rolling had slowed down. I then tried V hold adjustment and was actually able to stop the rolling this time but the raster still jumps and shakes vertically.It seems like a bad cap but I recapped the whole deflection side of the board.Im not very experienced in troubleshooting live electronics and still need to get a scope but ive recapped a dozen or so crt's and consoles without lifting a pad so pretty possitive its not my cap work thats the issue.I guess its possible I got a bad capacitor from the factory.I noticed one of the V output transistors was way hotter than the others. The others were reading 25-30c on a cheap temp gun and that one V out transistor was reading around 55c. Ive never had a horizontal or vertical transistor go bad in a crt before so not sure if they will always just fail or if they can go bad causing issues like im having?Im now planning to replace the vertical out transistors and check caps esr while the board is out.I know this shotgunning approach is not the best way to go but Im still learning about doing live testing and yet to get a scope. Trying to save for a 50mHz rigol.Just wondered if anyone had any similar issues or if anyone had guidance, opinions or suggestions?Any thoughts are appreciated!Heres videos of the saga if anyone cared to watch lolhttps://youtu.be/cUFdCbEcRZQhttps://youtu.be/0EEp3wBwiochttps://youtube.com/shorts/E0V1uV9gBVE?feature=share

author: deezdrama
author_rank: Contributor
timestamp: December 06, 2022, 07:16:16 am
content: Well I took a break from the 20m2u and worked on the 14m2u that ive been stealing parts from.Got it up and running fine and noticed its vertical ic's are just as hot so I dont think thats the issue with the 20m2u...https://youtu.be/43_7_rWHju8Back to the 20m2u, i stayed home today and spent (wasted) the whole day on this 20m2u...Swapped rear input board ..... Same issue but now vhold would stopthe vertical rolling but image still horizontally stretched a bit like bad pin amp adjustment and image was still vertically jumping all around.Probed voltages coming into chassis.... All seemed good. 15v was spot on, 115v spot on, -15v is like -25v which is weird, 5v is 4.8v which im not sure if thats within spec or not.Spent entire day looking over board with magnifying lense.... Checked all caps with 2 esr meters.Found sync seperator ic pinout in serv manual.... Live probed each pin.... Voltages look good.Reflowed ENTIRE deflection half of the board, every damn tiny surface component and everything...... Still has same issue.Checked that all resistors on deflection side werent failed open. Checked all diodes for typical diode voltage drop.Studied serv manual looking for clues.... Seen ic107 and that it controls vhold and other vertical functions....Found it on the board opposite of the deflection side..... It had a few electrolytic caps nearby that tested high esr..... Replaced those caps..... Same damn problem with stretched image like the pin amp is off and vertically jumping.After the initial recap and repair the geometry and image were spot on. It wasnt untill the pvm ran for 30min that a cap or something failed over a minute or two timeframe which is why I think its a cap.....I just wish I knew where to look, im sure its a simple fix.Grasping for straws..... Anyone have some straws? Lol

author: deezdrama
author_rank: Contributor
timestamp: December 18, 2022, 06:55:27 am
content: Well, pulled the board for the umpteenth time, used my new nerd 3.5x magnified visor to double check all polarity of caps I installed, checked esr, looked for cracked solder joints...... This is probably the 4th time ive done this and whole deflection half of the board has been reflowed.Looked for waveform points to check in the deflection schematic and that sync IC chip that ive already replaced seems like last spot to check waveforms against the ones in the service manual before signal goes to V drive, and Voutput transistors.Im a total oscilloscope noob but to my untrained eyes all the waveforms looked good.Bought the scope to help troubleshoot this monitor but im just more confused than ever and discouraged.Starting to feel insane with all the time ive scratched my head over this.No clue what it could be or what to check at this point.Thought maybe a bad cap in power supply board but I tested another board before and the issue didnt change.Wondering if vertical out transistors could partially fail like this.Probably something stupid simple but out of ideas how to find it.EDIT....I see some weird spikes in the signal of one of the pins of sync seperator ichttps://youtu.be/7fuTLJ6yqhc