Video Enhancer & Stabiliser (November 1997):
A small number of constructors have still encountered difficulty in achieving correct operation of the stabilising circuitry, apparently due to either faulty triggering of either U4 or U5 (one-shots forming the blanking pulses), or insufficient adjustment range for presets RV1 or RV2, even if R8 is increased to 47k. Presumably these problems are still due to component tolerance variations.<BR>
Reader and constructor Chris Nelson of Morphett Vale in SA suggests that where U5 is not triggering reliably, C12 may be increased in value to 470pF. To increase the adjustment range of RV1 and RV2 he has also increased these to 20k and 5k respectively, and reduced R10 to 1.5k. However these changes should only be made if necessary.<BR>
In at least some cases, the stabilising circuitry actually seems to be working correctly -- the only problem is that our suggested method of setup (using a visible black bar on the video screen) cannot be used because of insufficient range in the vertical delay preset pot RV1. Our suggestion in such cases is therefore to adopt the following approach: First set RV1 fully clockwise; if no black bar is visible at the top of the picture, leave it in this position. Now turn RV2 clockwise until the picture begins to tear at the top. Then slowly turn it anticlockwise until the picture stabilises again. Your stabiliser should now be adjusted to its correct setting, or very close to it.<BR>