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Azur Electronics

February 2016

The HP 8405A Vector Voltmeter has several faults, which is not surprising after some 30+ years. This unit is S/N 2412A where as the Operation and Service Manual is for S/N's up to 946 so possibly significant differences.
With both Probes connected to terminated BNC Tees and a Power Splitter fed with 1MHz, Channel A is indicating fairly accurate voltmeter/decibel readings over the +10dBm to -50dBm range. Channel B has a fixed reading of 75mV. Tracing the circuit operation and the problem seems to be in the Probe.

Channel B Probe parts
Dismantling is fairly easy thanks to Richard Carey W6RLC for "Repair of Probes for HP 8405A Vector Voltmeter". Probe B is a type B 08405-6049. Probe A is the later type C 08405-6055. One of the Probes was probably replaced at some time. These are susceptible to burn-out as the maximum ac input is 4Vpp and the maximum dc input is ±50V. The Schottky diodes test ok. Transistor Q1 (the red dot) is a SMT FET and the part is not listed.

August 2019

One of the few repairs that have not been completed! The 8405A has been sold along with the rest of my Network Analyser System.