Last week my TS-590S has been at the official Kenwood Service Center Sarikaya Funktechnik in Bochum/Germany for the ALC modification provided by Kenwood Japan. This modification eliminates the ALC overshoot, also called power spike, which may cause power amplifiers to fail or break. Please read this article and this article for further information. The modification itself was free of charge, but shipping had to be organized and sold by me.
After testing the modified TS-590S with my Acom 2000A first, which was totally okay, I made some measurements with a power attenuator and an oscilloscope in my employer’s RF lab. To cut a long story short: The huge power spike has gone! There is an extremely slight overshoot for CW rise times greater than 1 millisecond, but that is really nothing to worry or complain about. Well done, Kenwood engineers!
Here are the shapes of the first dit with different CW rise times. The output power was set to 100 watts and the frequency was 14.1 MHz. CW speed was set to 28 WPM. Please note: I checked all others bands too, including 50 MHz and there is no overshoot anywhere, neither in CW nor in SSB.