In this episode Shahriar takes a close look at a defective Agilent E4440A spectrum analyzer. Initial analysis reveals that the startup errors are caused by a defective pre-amplifier module. During initial alignment and calibration, the defective microwave relays inside the pre-amplifier module fail insertion loss specifications and cause alignment failures.
Unfortunately, the relays are not repairable and must be replaced. Two new relays are purchased and replacement clears all alignment issues. The main motherboard battery has also failed; a replacement battery corrects any time and date keeping errors. The repaired instrument is cycled through several alignment procedures without any problems. The performance of the unit is verified by examining the DANL of the instrument with and without the pre-amplifier activated.
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The 3458A unit fails startup conditions with several errors. After close inspection of the boards, the initial problem points to a poorly plugged in fiber-optic cable. After this fix, the unit still fails ACAL on the AC Board. The unit’s NVRAM modules all have to be replaced. During this process the memory of the unit is also upgraded and the firmware is updated to the latest available version. After analysis of the schematics the problem is traced to a damaged JFET input operational amplifier on the attenuator flatness calibrator unit. The replacement allows the unit to pass all self-test and ACAL calibration. The instrument is then NIST calibrated by AllTest Instruments and shows excellent performance when compared with various multimeters in the lab.
In this episode Shahriar repairs a BK Precision 9185 600V linear power supply. The instrument is missing a power connector and therefore cannot be powered on. To be cautious, the unit is disassembled first before being retrofitted with a new connector. The teardown reveals damaged resistors at the back of the main power supply PCB. The damage points to a defective high-voltage MOSFET. This transistor is one of six devices in parallel. The impacted components are all replaced before the unit is retrofitted with a new power connector.
After power on the unit is verified for proper operation. Using a Keithley DMM7510, the power supply is calibrated and is ready for future experiments.
In this episode Shahriar repairs an Agilent 8562E 13.2GHz spectrum analyzer which does not power on. Upon pressing the power button the power LED briefly flashes and turns off. Teardown of the unit reveals a compact instrument where the power supply is rather difficult to reach.
The power supply block diagram shows various internal block functions which include startup, DC-DC converter and output regulation stages. In order to diagnose the instrument, the CMOS driver ICs were removed and swamped. An error during the installation of the gate drivers caused a cascade failure of the supply components! After replacing all the affected components as well as the DC-DC converter PWM controller IC, the power supply is repaired.
After powering on the unit it becomes clear that there are several internal self-test and alignment errors. The errors point to YIG oscillator failure. The YIG oscillator no longer has the full range of frequency tuning. The core of the YIG block is replaced with a spare unit from a different Agilent instrument. After re-alignment of the YIG the unit functions correctly. The principle operation of the YIG oscillator is explained and the internals of the defective unit are examined under the microscope.
In this episode Shahriar repairs an Agilent PSA Series Spectrum Analyzer. The instrument generates many errors during self-alignment and produces no measurements below 3.2GHz. The block diagram of the unit is thoroughly presented and various possible failure points are considered. Based on the observation of the noise floor, the most likely cause is the second LO module. The measurement of the LO power indicates that the second LO power is fall below nominal.
The second LO signal (at 3.6GHz) is generated by a DRO PLL module which is locked to 600MHz. The PLL is functional, however the output power is below -20dBm. Teardown of the module reveals a simple design with a doubler and filter. The filter (which is discolored) shows a huge loss likely caused by RF losses due to trace degradation. Thinning the traces provides some improvement and the remaining loss is compensated with a two stage RF amplifier. With this modifications, the instrument’s alignment errors are partially resolved.
The next problem is with the input attenuator at the 20dB range. This problem is simply resolved by providing lubrication on the attenuator solenoids after disassembly. The instrument is then used for various measurements to verify its correct operation.
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.