Predicta Predicament

[init4fun]

Originally Posted by Down Under

Hi guys,

I'm currently working on a Preficta Holiday. Although I now have an image of sorts on the screen, it's far from perfect. The really strange thing is that the brightness and associated voltages around the chassis seem to be directly related to the horizontal oscillator frequency.

For example, if I turn both the horizontal hold and the horizontal range controls fully clockwise, I get a reasonably bright display. Not as bright as it could be, but ok. Of course, the sync is way off, as the oscillator is now running at around 40kHz. With the tv running like this, B+ is at 260V, and boost is way up at 620V.

As I adjust the controls back to where they should be, the width of the pic shrinks, the brightness almost fades to nothing and B+ is now down to 230V, boost at 310V. Overall though, the raster is still too small, not at full height or width, even with both of the associated controls set at maximum, and this applies regardless of what frequency the horizontal osc is running at.

I've rebuilt all the K networks except the vertical integrator and checked all the tubes. Replacing the networks has made virtually no difference. Of course, I've also replaced all resistors and caps on the board and most of the resistors on the chassis as well. I've replaced all chassis caps except the ceramics.

Currently, I'm running the board on jumper leads, so I can test my networks properly.

Anything I should be looking for? There's not much left to replace! The resistances on the flyback all read ok as well.

Cheers,

Adam

How is your picture tube ?

I ask this because raising the voltages to get "normal" brightness on the screen could indicate a CRT with weak emission . Course , it could be other things as well , but since I didn't notice mention of the CRT's condition I figured I'd ask .