IBM 655743N Service Manual - Page 19

SECTION 3 SAFETY RELATED ADJUSTMENT 6557-03N/03S/43N/43S When replacing or repairing the shown below table, the following operational checks must be performed as a safety precaution against X-rays emissions from the unit. HV ADJ Part Replaced ([) RV901 Part Replaced (]) HV Regulator Circuit D board IC901, R903, R922, T901, RV901 • Mounted D board HV HOLD DOWN Circuit D board D913, D916, C923, R920, R927, R935, R936, T901 • Mounted D board Beam Current Protector Circuit D board IC901, D901, D902, D918, R053, R918, R923, R928, R932, R933, R934, R939, T901 • Mounted D board * Confirm one minute later turning on the power. • HV Protector Circuit Check (1) Confirm that the voltage between cathode of D913 and GND is more than 17.5 V DC. (2) Using external DC Power Supply, apply the voltage shown below between cathode of D913 and GND, and confirm that the HV HOLD DOWN circuite works. (TV Raster disappears) Standard voltage: Less than 23.40 V DC • Beam Current Protector Circuite Check (1) Hardware logic circuit • Turn the POWER SW on, and check that the RASTER fades in. • Connect a variable resistor (20 kΩ or more) and an ammeter in series between FBT pin !¡ and - 15 V line (- side of C637). Decrease gradually the resistance of the variable resistor from maximum to minimum, and confirm that the Beam Current Protector Circuite works (TV Raster disappears). The current must be within the range shown below. Standard current: Less than 1.09 mA (2) Software logic circuit • Turn the POWER SW on, and check that the RAS- TER fades in. • Short between + of C639 and GND. • Connect a variable resistor (20 kΩ or more) and an ammeter in series between FBT pin !¡ and - 15 V line (- side of C637). Decrease gradually the resistance of the variable resistor form maximum to minimum, and confirm that the Beam Current Protector Circuite works (TV Raster disappears). The current must be within the range shown below. Standard current: Less than 1.14 mA 3-1