In this episode Shahriar & Rosanah investigate an Agilent power supply which does not appear to power on. It can be quickly observed that the fuse has failed on the unit. Using an isolation transformer a small amount of AC voltage is applied to the unit after the fuse replacement. It is clear that a short is present somewhere in the instrument since even at 10V AC the instrument consumes more than 1A.
After separating the internal transformer from the main PCB, the high current consumption is eliminated. The short is traced to a damaged main rectifier in the instrument. A replacement rectifier is used which allows the instrument to power on.
The VFD display is however extremely dim. After a failed attempt at restoring the VFD brightness, a new display is purchased from eBay. The replacement produces a bright vibrant VFD display. The performance of the power supply is verified under load.
In this episode Shahriar repair a Fluke PM6685R Rubidium Frequency Counter. The instrument shows highly unstable results and produces high-pitch noise when powered on. The high-frequency noises point to a problem with the DC-DC converter circuit in the power supply module. A close examination points to degraded capacitors. After all capacitors have been replaced, the noise issue is resolved. The instrument’s performance is then verified and calibrated against an external Rubidium reference. The agreement between the units is better than 10ppb.
In this episode Shahriar investigates a peculiar problem with a SRS DS345 function generator. The instrument does not appear to produce the correct output signal frequency. After some investigation it becomes clear that a few of the display digits are not active which hide the actual frequency settings. The problem is traced to a broken resistor on the display driver. The schematic of the instrument shows that the resistor is responsible for strobing the affected seven-segment digit. After the repair, the instrument’s performance is verified including the OCXO accuracy.
In this episode Shahriar investigates a faulty Agilent 53152A 46GHz frequency counter. The instrument does not power on and shows no sign of internal voltage presence. Teardown of the instrument reveals a large PCB where all analog and digital circuity is contained. The power supply module is a module components and upon measurements shows no activity.
The power supply is a simple switching architecture with functioning input rectifier and capacitor filter. By using an oscilloscope it is clear that the power supply PWM controller attempts to start. However, the main power supply pin shows unstable voltages indicating inadequate charge retention on the rectifying capacitor. Replacing the capacitor revives the startup condition and the power supply function returns. The PWM controller and main switching transistors are also replaced with new ones. After this repair the unit powers on and passes all self-tests. The unit can successfully measure signal frequencies and power.
In this short episode Shahriar takes a close look at an HP 83475B Lightwave Communication Analyzer / Oscilloscope which offers a 500MHz bandwidth on both the electrical and optical inputs. The sub-sampling scope has a minor issue where the CRT brightness knob has been disconnected internally.
The teardown of the scope reveals the sub-sampling ADCs, O/E front-end as well as CRT driver circuitry. The operation of the scope is demonstrated using various electrical and optical NRZ and multi-level signals.
The Signal Path (TSP) is an electrical engineering video blog for industry professionals, students and hobbyists. TSP is a non-for-profit website dedicated to provide free education spanning a wide range of electrical engineering topics. Equipment reviews, tutorials and repair videos are posted regularly.