The majority of Ludlum instruments utilize a high voltage (HV) power supply circuit similar to the one illustrated below to produce various GM, scintillator, and proportional detector operating voltages. The primary sections of the HV power supply circuit are a blocking oscillator, a "step-up" transformer to amplify the oscillator pulses, a voltage multiplier to rectify and multiply the voltage output, resistor divider network to lower the HV providing a low voltage feedback signal to the regulator, and an op-amp configured as a comparator which provides voltage regulation.
The most common problem reported is the loss of HV at the detector input. The probably cause is a large transient pulse has caused U1 to be defective (either an inadvertent short at the detector input or loss of R1 causing the HV to drive to maximum). This problem can be identified by determining if the blocking oscillator and voltage multiplier circuits are functioning by momentarily shorting TP1 to chassis ground and observing the HV at the detector connector. If the HV comes up, then the problem should be between TP2 and TP1. R1 and the related resistor divider network can be isolated by measuring the "-" input of U1 as TP1 is shorted; the voltage should range from 0.2 to 4 Vdc depending upon the HV output and resistor divider ratio (typically 1000 volts at HV out should equal 0.5 volts at U1 "-" for a 1.2 Vdc reference). The output of U1 should go positive or "high" when the "+" goes above the "-" input, saturating Q1. When the circuit is functioning properly, the "-" and "+" inputs should equal each other.
Another problem commonly confronted with the older designs is with R1. R1 is typically a 1 gigohm chip type resistor network (covered in heat shrink) which is still in some of the older designs. It is common to find 1 or 2 of the resistors in those networks to open or begin to open when encountering HV output problems with this design. These resistors should be replaced, when faulty, with the 1G chip resistor. This type of fault will produce the same type symptoms described in the above paragraph and can be isolated with the same fault finding procedure. Please contact the LMI Repair Department for additional technical support in isolating and repairing LMI instruments.
By David Wyatt, December 1995 Newsletter