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Oscilloscope VP-5403A ( National )

[Outline] [Inside view] [Plug in units] [Trouble shoots]
[Inability to control intensity] [Disappearing of the trace]

Fig.1 Disappearing of the trace was caused by unstable of +75V line.	Fig.2 Sometimes the contact of D611 ( ZH135 35V zener diode ) was open.

Fig.3 Back side of the chassis.	Fig.4 +75V AVR unit.

Fig.5 After replace D611 ( series of 20V and 15V zener diode ).	Fig.6 Original D611 ( Stanley ZH135 ).