Project TouCans and the Squizzled PA FETs

Or, how I learned to miss hard resetting RFI.

Up until about a week and a half ago, Project TouCans and the Flying Rockmite before it had never blown a final amplifier transistor. Our good fortune with transistors was caused in large part not by any particular genius in the construction or handling of the radio, but almost entirely to radio frequency interference. When too much RF energy was reflected back into the radio on key down, the Rockmite's picokeyer would reset chirping out an only slightly irritated 73 in Morse code. Hence, we couldn't use the rig, hence we changed the antenna or repaired the rig, or turned down the bias voltage on the Tuna Topper final until we could use the rig without resetting that little keyer.

Then! Then we did something that was both cool and, (we'd later find out), somewhat daunting. We removed the last of the wires that attached Project TouCans to the Earth and our RFI just went away. Which has been great for signal quality, and as it tuns out it's occasionally been hell on the amplifier.

At present count, we're three transistors down.

After the rig dropped from about 15 feet up to the ground recently, I did a quick inspection and then, (why!!!!??? why did I do that???), decided I'd measure the rig's output power since I hadn't done that in a while. The power meter reported 3 Watts. The rig nominally outputs 5 Watts. I adjusted the bias voltage on the Tuna Topper and.. still 3 Watts. I increased the voltage two more times, and? The final transistor blew out. (Eventually taking the rechargeable battery with it.) KO6BTY and I completed our debug yesterday, reattached the power meter, (which apparently no longer works since it reported 0 Watts at all bias voltages), and fairly quickly blew out the amp transistor again as I blithely upped the bias voltage.

Turns out I can't do that anymore. our keyer RFI safeguard is gone.

Interestingly, we did discover that when the bias is set to low on the amp, there's a whistling feedback tone that appears in the audio output. When the amp is biased correctly, and before it's biased too high and takes out the transistor, (i.e. in nominal operating mode), the whistling tone goes away. Audio debug lives on to a certain extent.

We'll be dropping in transistor number four tomorrow. I'll keep you posted.

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