Lets Talk Sync and Sync Separation!

[reichsrundfu]

Hi Guys,

What observations, experiences or suggestions may you offer on sync (mainly vertical) with our vintage TV's when connected to a cable TV input source?

OK, the basics: my cable company (RCN) supplies fully digital cable service and we have about 325 channels. Yes, the vintage TVs are all tuned to the standard definition (not the HD) channels. Now, the broadcast channels such as CBS, NBC, etc with 3/4 aspect ratios are all pretty stable, and with some of my early TV's the standard definition cable channels which come thru is other than 3/4 aspect (such as CNN for example) are stable as well. However a couple of my sets (the Philco Predicta BarberPole and a Zenith Mayflower porthole) do a couple weird things:

PHILCO PREDICTA BARBERPOLE:
On some cable channels such as AHC, or Destination America, etc. the picture will occasionally roll upwards when a scene changes or if a very dark scene such as a nighttime shot is on, and I find that adjusting the vertical sync is very critical (hard to lock it in). Yet when viewing a broadcast station such as the local CBS channel the sync is just fine and no problem locking in. Once in awhile however, if the cable company runs a local set of ads the vertical may become a bit unstable and again roll upwards once or twice with critical lock in of the vertical sync. As soon as the main broadcast program resumes the sync is just fine and locks right in. When watching a DVD I never have any vertical sync issues whatsoever, so its only on some channels or certain commercials from my cable system.

ZENITH MAYFLOWER:
The vertical is very stable excepting when watching CNN or MSNBC or another cable channel with an other-than-3/4 aspect ratio picture (which results in a blank area at the very top and the very bottom of the picture) Sometimes when going to a commercial or switching to another picture, the vertical picture will tend to drop so that the blank bottom of the picture meets the bottom of the picture tube, but the picture doesn't roll. As soon as another picture comes on the screen, or if I switch to a broadcast channel like CBS the picture locks in dead center and is very stable.

I know that vertical sweep and sync circuits are different in the Zenith vs the Predicta. But do any of you guys know of possible common threads that may affect these two sets?

The horizontal on both are always perfect and well locked in! Also, this issues does not appear on my other two Predicta sets, which is a Tandem (which shares the same chassis as the BarberPole) and my 17" tabletop set.

Thanks!

-George-