The high-voltage metering resistors in the 8410 are responsible for the accuracy of the plate voltage reported via telemetry and displayed on the front-panel LEDs. As reported on the Plate Voltage Issue page, the original metering resistors were not up to the task. When I got replacement metering resistors from RF Concepts (under warranty) Molly sent instructions for their replacement. Those directions were for replacement in a 9500. While the overall process is similar I thought I'd document what I did to replace the metering resistors in my 8410. - Disconnect and remove the power transformer.
- Remove the screws holding the HV and mains board stack (the stack between the power transformer and the front of the chassis).
- Cut the small cable ties that hold the white wire bundle in the U-channel between the stack and the power transformer cavity.
- Disconnect the front panel ribbon connector.
- Disconnect the grounding ring terminal from the center partition.
- Swing the entire stack 90 degrees counter-clockwise (looking down into the chassis). Basically the stack will go into the power transformer cavity.
- Confirm the resistors are replacement candidates (I don't know if there's a certain manufacture date and/or serial number beyond which the new resistors became factory installed).
Original resistors close up:
New resistors close up:
- De-solder the existing resistors. RF Concepts recommended just clipping the resistor leads near the resistor bodies and soldering the replacement resistors to those stubs. I preferred de-soldering because I have a great temperature-controlled iron and I didn't want it to look like a retrofit. Choose the method best for you.
- Once the resistors are soldered in place be sure to remove all flux with isopropyl alcohol. At 3 kV it's possible flux residue will break down, ionize and allow an arc-over to ground or a low-voltage trace. According to Glenn at RKR, this has happened and it took out all four PCBs.
- Swing the stack back into place.
- Re-attach the front-panel ribbon connector and the center partition ground lead.
- Dress the lower white wire bundle back into the U-channel between the stack and the power transformer cavity and secure with small cable ties.
- Replace the stack mounting screws
- Replace the power transformer and reconnect the transformer leads.
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