I've got a TS-590S that's putting out full 100W on 6m but dropping to about 60W on all HF bands. This is a common issue in the TS-590 series, often caused by voltage drop under TX load from poor power cables, corroded fuse holders, or loose connections. The rig needs ~13.8V at the finals; even a 1-2V drop limits HF output (6m often draws less current). I've verified SWR is good on all bands with my dummy load, so it's definitely not an antenna issue.

Has anyone experienced this specific pattern where 6m works fine but HF doesn't? I plan to measure voltage AT THE RADIO while transmitting on high power, using terminal strips on the power leads so I can monitor voltage during TX. Any other diagnostic steps I should consider?

Low voltage, especially when transmitting, can cause radios to exhibit all sorts of strange behavior. The microprocessor may not function correctly, leading to bizarre displays, loss of external control, and incorrect response to controls. Low voltage can also result in low power output or poor RF stability. Check your DC cables first - they're usually the culprit.

If you suspect a poor connection, measure voltage at the load (such as the radio) and work your way back to the supply. Poor connections in a cable or connector cause the voltage to drop under load. I had the exact same issue and it turned out to be corroded Anderson Powerpoles.

This happened to my club's TS-590SG. We found a loose connection in the fuse holder that only showed up under high current draw. Check for wire sizing using the ABYC 3% Table and look for corroded fittings - clean and replace as indicated, then retest. Fixed it completely after replacing the fuse holder with a proper marine-grade one.