I've got a Kenwood TS-590SG that's putting out full 100 watts on 6 meters but only about 60-70 watts on the HF bands. The finals appear to be getting proper bias and the ALC circuit isn't limiting. Before I start pulling boards, what's the logical diagnostic sequence here? I suspect power supply voltage drop under load, but want to confirm the methodology before diving deeper into component-level troubleshooting.

Classic TS-590 issue! Voltage drop under TX load from poor power cables, corroded fuse holders, or loose connections causes reduced HF output while 6m often draws less current. Measure DC voltage at the rig's power jack during full-power CW transmission into a dummy load first.

I'd also check the PA bias voltages on the HF driver stages. The TS-590 has separate bias circuits for different band groups, and if one's drifting low, you'll see exactly this symptom pattern. Service manual has the test points clearly marked.

Don't overlook the obvious - verify your dummy load and power meter are accurate across all bands. I've seen "defective" rigs that were just measurement errors. Also, thermal issues can cause band-specific power rollback if the PA temperature sensors are acting up.