Shaun,
Before replacing the IRF510s, do a voltage check on Q90.? That one is the most common culprit when both CW and SSB power output drops to less than a watt.? The values for the v4 power sections are the same for all versions.? It can be found by searching the files section for vu2zap in the Troubleshooting Folder.? Raj did the measurements and posted the spreadsheet.? Be sure to follow the instructions on the sheet: The power amp sections are measured by pressing the PTT and supplying no audio to the mic.
You can also do the bias test on the tuneup page of HFSignals:
If you can set the bias, the finals should be OK.
73
Evan
AC9TU
