k4600 vertical size jumping

delroy666

Well-known member

Donor 2011
Joined
Jul 30, 2009
Messages
1,189
Reaction score
26
Location
Ottawa, Ontario, Canada
The WG 19K4600 on my test bench just started acting up last night. It was recapped about 2 months ago and has been working perfectly until now. About a minute after powered on, the vertical size starts changing rapidly. The image gets stretched out off the screen, then almost goes back to normal for an instant, and so on. Here's a video:



The B+ is measuring 127V and the vertical size pot VR302 tests okay with a DMM. Horizontally the picture appears stable, and there are no unusual arcing/snapping sounds when it jumps. If I put the Vert/Hor board from this monitor into another working K4600 I have, it does the same thing, so I'm pretty sure the problem is on the Vert/Hor board. I reflowed all the connections on this board, plus the daughter board headers on the main board. I'm not quite sure what to check next. Any suggestions would be appreciated.
 
Thanks, Ken. I've already reflowed all connections on that board to take care of any possible cold solder joints. I suppose the DMM wouldn't show me an intermittent connection on the vert size pot necessarily, so I'll replace VR302 tonight and see what happens.
 
I replaced the vertical size pot, but it didn't change anything. I checked the resistors in the vertical section of the vert/hor board with a DMM and they all appear to be fine. I guess I'll check out the transistors in that section next. :confused:
 
Success! :headbang:

The transistors all tested good, so I fired up the oscilloscope and looked at the test points shown in the manual. Everything up to and including Point 'D' (collector of TR302) was fine, but the signal at point 'E' (base of TR303) looked crappy and its voltage was way too high - about 7Vp-p instead of the 0.84Vp-p listed. After double checking the resistors in that area, I replaced the diode at X302 and it fixed the problem.

Interestingly, that diode tests good with my multimeter so it must be breaking down with the higher voltage in-circuit.
 
Back
Top Bottom