Old Emac CRT Vertical Hold

[Razbith]

Hi

For awhile now I've been trying to repair an old eMac G4 700, it's an education edition cousin to the iMac.
Specs https://www.macintoshrepository.org/...52-emac-g4-700

I've replaced drives, memory etc and now ready to do a fresh install. But at some point I think I've bumped something and the screen now shows a picture but rolls vertically. I've tried the service manual but it's go to for all screen problems seems to be replace the cable.
Manual

I doubt thats it, the picture is perfect, it just rolls.

I can't see anything I've obviously bumped and the only adjustment options I can see are 2 pots on the side labled "V" and "Focus" then a set on the tube under "YHC", "YV1", "YH", and "(delta symbol) V". None of these seem to be an obvious 'vertical hold' to me.

Does anybody have any familiarity with these old CRT macs or just enough with CRTs to tell me where I might find the culprit? I spent a month waiting on parts and was finally ready to reinstall the OS when this happened. Frustration... yeah a little.

[R_J]

Re: Old Emac CRT Vertical Hold

You are likely missing the vert sync pulse going to the monitor Check all the cables that connect the main board to the monitors deflection board.
I doubt there is an actual vertical hold control on this monitor

[redwire]

Re: Old Emac CRT Vertical Hold

Despite the rolling, is the picture linear or squashed/compressed etc? It's more likely a bad connection to VSYNC.

I find a critical vertical deflection capacitor in CRT monitors that fails often is the yoke output capacitor, but low value the deflection is gone or squished.
Sometimes it is part of the vertical oscillator feedback circuit. Not likely your problem, but if you think the vertical osc is too far off frequency to lock I would look at whatever circuit is doing the vertical oscillator/driving the yoke.

[Razbith]

Re: Old Emac CRT Vertical Hold

To my eye I'm seeing a section of the screen repeating vertically with ghosted copies and the whole thing rolls. I tried a camera to get a decent shot without the persistence of vision effect and if i catch it at the right moment in the roll it shows the entire picture is there with no distortion. Though I think it's moved up slightly.

Apparently the IVAD cable is a pretty common problem with these so I think your right and whichever wire controls sync has a problem. I'll take it apart again and test the cable with a multimeter.

Thanks