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PA-1000 biasing current changes with time

PostPosted: 12 Dec 2020 23:52
Hello forum members,

My wonderful PA-1000 developed a fault which manifested itself in "flat-topping" (audio) distortion and output power variation during operation in SSB. It has been in operation for just under 2 years. Mostly SSB and some CW at Pmax = 400W (VK legal limit).

I then checked the idling current by setting my transceiver RF output and mic gain to zero. When pressing TX, the idling current starts from about 1.0 A and increases within about 10 sec to 6.0 A. It then slowly decreases to about 1.0 A. Supply voltage remains constant at 49.4 V +/- 0.2V. PA output power was zero during the test.
A fault in the LDMOS module is currently suspected but the biasing circuit is also suspect.

Any comment or advice would be most welcome.

thanks & 73

Dieter, VK3FFB