NotTaR of Television Sets : Jagged or uneven vertical sweep         
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Jagged or uneven vertical sweep

(From: Matthias Meerwein (Matthias.Meerwein@rt.bosch.de).)

I recently fixed two CRT display devices that both developed a very similar problem: The vertical deflection was severely "jagged" with uneven line spacing and partial vertical foldover. One patient was a nameless el-cheapo 28-inch TV (1988 made), the other one a 14 inch ADI SVGA monitor (1991 vintage).

My first suspicions were bad contacts on the PCB or yoke connectors or isolation / connectivity problems inside the yoke. However, as the picture didn't change with warmup or tapping, those causes could be ruled out. Examining the vertical deflection waveform with the scope showed the problem being a parasitic high frequency oscillation around the vertical output IC. On the TV, the oscillation extended over the entire scan period, while the monitor exhibited the problem only near the vertical current zero cross.

In both cases I found the capacitor of the RC damping network on the amp output to be at fault. Replacing it fixed the problem in both sets. This is not the well-known dried-up-electrolytic problem described in the FAQ. The culprits were mylar caps (.1 and .47 uF) looking completely unsuspicious. They were probably a bit underrated voltage-wise (40 volts) so I replaced them with 100 volts rated ones. The 2.2 ohms resistor in series with the cap was fine in both cases.