Symptom: Loss of raster
Set ID: Philco, chassis E25-6
Photofact: 1882-2
This Philco color set was brought in with a complaint of intermittent loss of raster. My first thought was that it might be loss of high voltage. Closer observation showed that the picture dropped to a horizontal line, which then instantly jumped up above the viewing area of the screen. That symptom eliminated the HV as a possible cause of the problem.
I started my probing at the yoke, where I found that there was no vertical sweep at pin 4. Going back to the complementary vertical oscillator/driver, IC302, I discovered that the bottom vertical output at pin 10 distorted slightly whenever the raster disappeared, but stayed fairly constant in level. I noticed the same distortion of the signal at pin 2.
This symptom didn't appear at pin 6 and pin 3, which both dropped to an un readable level. The dc voltage at pin 3 was normal. From the chip manufacturer's block diagram of the chip, I found that pin 3 is tied internally to the side of the chip feeding pin 6. I used coolant spray to check the solder connections and components (such as L350 and C350) tied to pin 3, but it had no effect.
All of these symptoms seemed to point to IC302, so I changed it out.
When I restored power to the set, every thing seemed normal for a few minutes, but then the symptom returned.
In my haste to correct what seemed to be a positive indication of IC302 as the problem source, I had failed to check voltages at Q300 and Q302. If IC302 wasn't the problem, that left these two components as the prime suspects.
Checking voltages at the leads of these two transistors revealed that the collector voltage of Q300 was only 9V, indicating an open collector. Apparently this condition caused enough of a loss at pin 6 to drop the signal. I replaced Q300, which restored the set to normal operation.
This was a lesson to me: No matter how definitely the readings seem to point to one component as the cause of the problem, something else may be the cause of the problem. When you're absolutely sure which component is faulty, especially when it's an IC, check surrounding components to confirm your diagnosis.
-- Michael H. Whitco Wellsburg, WV
Figure 1. The collector voltage at the lead of transistor Q300 was only 9V,
indicating an open collector. This condition caused enough of a loss at pin
6 to drop the signal.