Service manual

Wells-Garnder Color Vector Monitor Guide
Page 32 of 73
Symptom: Blooming/"Weak" Brightness/Low HV
Since most of you do not have a HV probe, the most common symptom of low HV is that the
screen looks as though you are looking at the center through a magnifying glass. This visual
symptom is known as "blooming". I have seen several HV boards where ZD902 (150 volt Zener
diode) goes bad and the HV drops from 19.5 kilovolts to around 10 kV. It is kind of like the
electron beam moves slower with less HV giving the deflection magnets on the yoke more time
to deflect the beam (but what is really happening is that there is not enough HV to strip all of the
electrons off of the phosphor coating which causes the screen to develop a negative charge which
then deflects new electrons which are expected to be hitting a screen with no charge on it). A
new ZD902 and everything is better. NTE5100A is a common modern day replacement for this
part. If ZD902 is OK, check the following:
• R915 (open)
• R922 (open or wrong value)
• Q900 (emitter pin open)
• Q902 (bad)
• Q906; white and black leads reversed (scope pattern is wrong)
• ZD901 (shorted)
• IC901; if scope output is a little high, replace IC901
• P900 (ribbon pin #7 broken)
Symptom: HV Range Wrong, What Causes It?
Normal HV range is between 16-24 kV
HV Voltage Range Look at
??.?-12.0 kV Q902 (reversed)
7.0-10.0 kV ZD902 (bad)
7.0-17.0 kV ZD902 (bad)
7.0-22.0 kV Q903 (bad)
9.0-18.0 kV ZD902 (bad)
11.0-22.0 kV ZD902 (bad)
15.0-18.0 kV ZD901 (wrong value)
16.0-19.0 kV ZD902 (bad)
18.0-27.0 kV ZD902 (bad)
19.0-27.0 kV R912 (wrong value)
21.7-30.5 kV C916 (open)
28.0-20.0 kV C916 (bad)