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REPAIR HP 8405A
VECTOR VOLTMETER
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.

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