0:10 Initial Problem: The Agilent E4448A spectrum analyzer (up to 50GHz) exhibited intermittent power-on issues, often shutting down during boot.
1:12 Internal Inspection: The analyzer uses a triple conversion superheterodyne architecture. Initial suspicion fell on the power supply or the power switch.
2:17 Power Switch Verification: The service manual revealed a straightforward power-on sequence involving MOSFETs. The front panel switch was confirmed to be functioning correctly.
3:38 Power Sequencing Investigation: Further probing indicated that the power supply was likely being overloaded, causing the shutdown.
5:52 Isolating the Fault: Disconnecting assemblies one by one revealed that the dual YIG pre-selector assembly (A19) was the likely culprit.
6:57 Pinpointing the Issue: Further isolation pointed to a potential issue with the YIG pre-selectors themselves or their driver circuitry.
7:55 YIG Pre-selector Details: The A19 assembly contains two YIG pre-selectors covering different frequency bands (millimeter wave and high band). High currents through the YIG coils were a potential concern.
8:17 Evidence of Prior Repair: The analyzer showed signs of a previous repair attempt, with improperly tightened coaxial cables and a concerning proximity of coil wires to the chassis.
9:27 Short Circuit Found: A short circuit was discovered between one of the YIG coils and the chassis, likely explaining the intermittent power issues.
10:06 Reassembly and Testing: After insulating the shorted coil, the analyzer successfully booted and completed its internal alignment procedures.
11:12 Attenuator Step Error: An "attenuation steps error" was encountered, a common issue in older spectrum analyzers due to failing electromechanical attenuators.
11:39 Attenuator Testing: Using a Keysight FieldFox, the attenuator was tested across its frequency range (DC to 50 GHz), revealing that the 20dB and 30dB attenuation paths were faulty at lower frequencies due to poor contact.
13:25 Attenuator Repair: The attenuator was disassembled, and the individual attenuation pads were carefully cleaned to improve contact.
18:12 Lid Details: The attenuator lid features RF absorbers, gaskets for RF sealing, and adjustable screws that likely serve as fine-tuning elements for the frequency response.
19:16 Second Test and Result: After reassembly, the attenuator functioned correctly across its entire range. The alignment errors were also resolved.
20:19 Remaining Issues: Despite the fixes, certain frequency bands still failed to measure correctly, suggesting problems with the front-end YIG switching or mixers.
20:41 Conclusion of Part 1: The video concludes with a preview of Part 2, where the remaining frequency band issues will be investigated.