This morning I had one of those "Oh sh!t" moments.

I was working Grant VK2MAX on 144.2MHz with OK signals and we QSY'ed to 432.2MHz. Turned on the 432MHz amplifier and called. Nothing heard. Called again, and this time when I released the TX, the noise floor on 432.2MHz was up significantly. At the same time I noticed that the amplifier PSU was still 'cycling' high, as though it was still in TX... and a quick glance to the amplifiers display panel revealed that it was continuing to generate approx 150w of carrier while enabled in TX mode.

I turned the mains power off... RX audio returned to normal. Mains back on, enable TX and the same occurred.WTF!

A nanosecond later I realise that I had not reduced the drive levels from the ICOM IC-910H and had TX ~70w into the 150w driver stage of the amplifier... it only requires 30w for full output ;(

Luckily I had a spare driver board. Replaced it and the amp is back at full output, without the 'self oscillations'!

I must be very careful until the attenuator pad arrives and I can install it!