FLUKE 5500A REPAIR

[gwideman]

In case anyone else stumbles in here with a question about Z8 and what it does: Definitely check the schematic in post #4.

The relays K6,7,8, select which voltage will be delivered to the VMID output, which ultimately is connected (via backplane board and relay) to the front panel "Normal High" output terminal.  Depending on which of K6, K7 and K8 are enabled, VMID either sees

• a 3.3V-range voltage direct from U42 and R67
• or a divide-by-10 version of that provided by Z8 pin 1 voltage divider (hence 330mV range)
• or a divide-by-100 version provided by Z8 pin 5, (hence 33mV range)

Nothing very sophisticated here, just plain old resistive dividers with output impedance 50 ohms (and of course sophisticated accuracy, temperature matching etc).

But of some note is that since this Z8 resistor network gets directly connected to the output terminal, it's relatively easy to damage it. For example, while setting the calibrator to output say 300mV, an accidental connection to external 5V supply would deliver 0.5W into Z8, which it's probably not designed to handle.