Electronics > Repair
Fix vertical hold on a monochrome monitor
<< < (4/7) > >>
Ground_Loop:
I had the same problem with an old computer and monitor. It turned out to be the sync signal in the computer. I had a bad CMOS logic chip. Have you tried connecting a different device to the monitor?
james_s:
I'm not sure why you replaced the pot, if adjusting the pot affects the rolling then the pot is obviously not the problem. I do not advocate shotgun replacement of electrolytic capacitors either, much too often I've seen people replace good quality parts with the cheapest no-name Chinese junk around and then mess something else up in the process.

Are you sure the video card is producing a valid sync signal? Is the signal getting all the way to the board in the monitor? The symptom you describe is exactly what you will see if there's a bad connection somewhere and the sync signal is not getting through.
vk6zgo:

--- Quote from: BrokenYugo on June 12, 2021, 08:07:17 pm ---Without looking up the standard my assumption (a dangerous word) would be there should be a 5V TTL signal on the port and it shouldn't be distorted much when the monitor is plugged in.

--- End quote ---

If you have a scope, have a look at the back of the sync input socket, or the equivalent pin of a multiway socket.
If there is a signal, & it looks correct, the connector, cable, & display adaptor is ok.
You should be able to trace either a cable, or if it directly connects to the PCB, the track which runs from that point.(often, if a cable is used, it connects to the PCB, so you will end up following a track, anyway).
There may be a dry joint, or other disconnection--------if so, the signal will either disappear, or become reduced in amplitude.

If that hasn't happened, you can then probe around the device it ends up connected to.
If this is an unidentified IC, take note of the type number, then Google for it, as these often show typical circuits using the device.

Many of these monitors are fairly generic, so it also may be possible to find the schematic of a similar one.



rea5245:
I don't have another monitor or another PC to try it with.

On my scope, the V Sync signal seems fairly steady, but there are two odd things, and I don't know if they're significant. First, the scope (a Rigol DS1054Z) calculates the V Sync signal alternately as 50Hz and 49.5Hz. Second, if I watch the trace for several seconds, I can notice that it's slowly crawling the left. But is that really a problem with the signal, or just with how the scope is calculating and drawing it?

At the same time the scope's trace is slowly crawling, the monitor's V Hold is stable enough that the screen is not rolling.

As I mentioned before, if I turn the monitor off and on, it loses the vertical hold. If the problem is in the signal from the PC, would you expect that I could adjust the V Hold knob and stabilize the screen, then lose it by turning the monitor off and on?
BrokenYugo:
Did you happen to get a picture of the scope trace? Probably a monitor side problem if it looked something like this.

http://minuszerodegrees.net/mda_cga_ega/mda_sync_ver.bmp
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod