Before removing the processor board, check the following areas:
- Tray transformer: 24 VAC – 16 VAC center tapped secondary
- Center tap is the source for logic ground
- three important logic ground voltages are the sequentially developed: Vun (10.5-11.5 VDC), and 5.6 VDC (RAM)
- VB is developed from the 7 VAC secondary tap of the main transformer
- VB becomes a half wave rectified, unregulated DC voltage for I/O drive
Before removing the processor board, check the following areas:
- Check to see if the rectifier diode is passing VAC or not passing VDC
- Check the filter capacitor to see if there is too much VAC (ripple voltage)
- Check the continuity of Vb and +5V
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