k4900 vertical collapse? could it be a flyback issue?

Bullwinkle

Member
Joined
Sep 10, 2009
Messages
859
Reaction score
8
Location
Rochester, New Hampshire
k4900 vertical collapse? could it be a flyback issue?

I'm working on a K4956 that I'd stored away a few years back. pulled it from the shelf and essentially got what's in the vid clip below. I capped it, reflowed solder on all the connectors/pin headers and on the neckboard pins. The colors look brighter..but still same issue. Changed out Q303, no help. I didn't swap out Q302 as I thought it was part of the horizontal circuit.

If you listen to the video, you can hear a "snapping" as it refreshes the screen. Is this all possibly being caused by a faulty flyback? You can't actually hear the "snapping" when standing next to it, but I suspect it might be causing interference with my iPhone's camera.

Any suggestions on what to check next? would R313 cause something like this?

 
i would check both vertical transistors as it seems you have cold solder joints.

also turn down the screen nob on the fly.

and check for broken traces on the hole chassis.

Peace
Buffett
 
i would check both vertical transistors as it seems you have cold solder joints.

also turn down the screen nob on the fly.

and check for broken traces on the hole chassis.

Peace
Buffett

OK..so Q302 is in fact a vert transistor as well.. will swap it out and see what it gets me. Will continue to reflow solder.

Thanks

Tom
 
if it continues to tick. turn all the lites out and see if you can see where the arc is coming from.


Peace
Buffett
 
if it continues to tick. turn all the lites out and see if you can see where the arc is coming from.


Peace
Buffett

OK, reflowed every solder point on the chassis and neckboard..still no difference. swapped out Q302 as well with no change. Shut off all lights and there are no arcs happening. That snapping I think is just electrical interference with the camera while taking video as you can't hear any snapping/clicking sound at all ..only on the video.

I checked R313 as well and it's at 470ohms...

trying to follow the schematic (definitely not an expert on that) .. would IC501 or Q352 cause this (bottle cap transistor and I think 501 is the hot)
 
if IC501 was bad your fuse would be blown.
if Q352 is bad you should be in HV shutdown.

i would check to see if D302/303 is faulty or bad and R315/310 as they control vertical as well.

also re-flow the neck board they get cold solder joints as well.

Peace
Buffett
 
did you remember to hook up the dag wire? That can cause a snapping noise.

I know I have the last few times I've tested, but as for the test with the video, I can't be sure... I'll take another video to see if the snapping is still there.

if IC501 was bad your fuse would be blown.
if Q352 is bad you should be in HV shutdown.

i would check to see if D302/303 is faulty or bad and R315/310 as they control vertical as well.

also re-flow the neck board they get cold solder joints as well.

Peace
Buffett

I will check those components. I had already reflowed the entire neckboard.

Thanks ... will check those components and report back
 
do this

Check this. 4900s are notorious for this issue. There is a Grey molex plug which contains
the RGB signals off the neck windings that plugs into the center of the board. It connects
to the pins in the middle of the chassis. On several boards I saw that the pins would break
connection to the traces on the bottom of the chassis. Flip over the chassis. Look at the
pins closely and with a meter make sure you have continuity to all your traces. I bet one
trace leading to the pins is broken and needs to be re-connected.
 
Just wanted to make sure you've done this, you're gonna burn the shit out of that tube if you haven't.

ah yes... I should have mentioned that I turned it way down after buffet mentioned it.

One other curious noise though. If I turn the horz hold clockwise, I get a noticeable high pitched squeal, presumably from the flyback area. but there is no noise when it's where it should be. Is this normal?
 
I just fixed this issue on a 4906. There were cracks in the chassis all over the back right corner (vertical section) that required jumpers and whatever stressed that corner also caused both deflection transistors to have the center leg snapped...
 
I just fixed this issue on a 4906. There were cracks in the chassis all over the back right corner (vertical section) that required jumpers and whatever stressed that corner also caused both deflection transistors to have the center leg snapped...

Wow.. I reflowed solder, but will obviously have to go back and reverify all traces. There was nothing obvious and the ones that looked iffy I used the DMM to verify continuity and then reflowed solder, but I know they can have hairline cracks that I won't see. Will check it out tonight. Thanks.
 
I just fixed this issue on a 4906. There were cracks in the chassis all over the back right corner (vertical section) that required jumpers and whatever stressed that corner also caused both deflection transistors to have the center leg snapped...


i fixed one last week that had the same problem.
both of the center legs were broke on the transistors. and one jumper was needed.
some how that heat sink likes to kill those transistors and breaks traces when struck. a design flaw for sure when being mailed;)

Peace
Buffett
 
Fixed a 4900 similar to that this past weekend, although mine just had a continuous solid line without the jumping around. Randy Fromm's flow chart lists 3 or 4 caps that can cause that collapse, and one of them was obviously bad. After all the problems I have had trying to find collapse problems on those damn 7000 series monitors, I couldn't believe it was that simple.
 
OK, this one is getting frustrating, though it seems as though they test out fine, I'm going to replace D302 & D303 which calls for a 1S2076. I have some NTE519 fast switching diodes and some 1N4148 switching diodes. Will either of these two work as a suitable replacement?

Thanks
 
those are zenier diodes not regular standard use/fast switching diodes.
they are current limiting diodes.

lift one leg and see what reading you get.
test them like a normal diode.

also if Q302-303 are shorted they will make those diodes test bad in circuit

if you have a parts chassis rob them from it.

i dont know the values of them sorry. i have several parts chassis here i get them from.

Peace
Buffett
 
I do have a parts chassis. Q302 & 303 are new and I don't see any shorting... The Thermistor's outter casing (TH501) has broken off, but that wouldnt cause this would it? I'm assuming I should get values anywhere from 40 to 70 on the DMM's diode test w/ black lead to cathode, and open the other way? I will go ahead and transplant D302 & 303 and see what that does. the other chassis I have does not have collapse but has a mess of stuff across the screen... so between the two you'd think I should be able to fix this!

Thanks
 
Back
Top Bottom