JUMA Amateur Radio Forum

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

Hello Dieter,

I didn’t see any reply to your post about this issue but I suspect that you are correct in figuring that there is a fault in the LDMOS chip. I had a failure in the chip while still in warranty and it was sent in and repaired promptly. The bias voltage fluctuation occurred and then failed completely due to a failure to one side of the LDMOS transistors which resulted in distorted audio reports. Because there aren’t any schematics available it is very difficult to troubleshoot and repair any problem occurring in the amplifier. It would be nice if even the basic voltage and bias settings were available in the documents.
I have since had a second failure of the LDMOS chip and replaced it myself. I normally run mine at 600watts out and use with a Elecraft KX3 so overdriving it is not an issue here. My amplifier went into fault when I turned it on one day which was a complete surprise. It could be a power surge from the switcher supply when first turned on.

73,
Skip NC9O