On Jan 19, 2025, at 10:27, Ashhar Farhan via groups.io <farhanbox@...> wrote:

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Gordon,
The zeners dont work. I measured this, the zeners cant switch that fast. You need 1n4148s two in series to the ground and two in series to the 12v. A classic clampiny circuit.
-? f

On Sun, Jan 19, 2025, 7:24 PM Gordon Gibby KX4Z via <docvacuumtubes=[email protected]> wrote:

Thanks, Ashhar

I guess that’s possible.? It would mean the Zener’s that I had on the gate at that point failed. I’ve previously demonstrated (with oscilloscope) that they clip and at very high frequencies their enormous capacitance should reduce gain. ?

The failure I had was with the band set to 80 m and Unknown load because I hadn’t properly tuned the auto tuner.? So I don’t know what load impedance the system saw.

I’ll have to go back and look and see if I had reduced the delay times before the failure— if so, that would be powerful data to support your thesis!!! (and I would add back in some of the delay times!)

Gordon?

On Jan 19, 2025, at 08:28, Ashhar Farhan via <farhanbox=[email protected]> wrote:

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I have concluded that the irfz24n is blowing up due to over voltage at the gate.
The high voltage was due to self oscillations when the rx bypass of pa was in effect providing a feedback path from the antenna to the PA input.
We also had too much gain at the lower frequencies that allowed very high RF levels. This was due to the unfortunate use of 2N2219s in the driver chain that runs out of steam at higher frequencies and had to be run at high current to get better gain.

On Sun, Jan 19, 2025, 6:12 PM Gordon Gibby KX4Z via <docvacuumtubes=[email protected]> wrote:

The MOUSER order for?

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942-IRF520NPBF 12 12 0 0.592 7.10
?MFG Part No: IRF520NPBF
?Infineon Technologies MOSFT 100V 9.7A 2

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IRF520's from "infineon" arrived a few days ago.? ?They are marked "IR"? (International Rectifier) on the body.? ?I managed to get them installed over Aluminum nitride insulators and got the idling current set for about 0.25-0.30 A earlier today.

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1.? Getting the tab to easily go FLAT on the heatsink proved to be more difficult than I had expected.? ?I think I didn't allow enough "slack" in the device leads when I soldered it and that caused it to be pushed into an odd angle that meant a lot of force had to be supplied by the "bar" to hold them down.? ?I didn't like that and slept on it to think it through better.

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2.? I ended up re-heating the solder connections while the devices were held down by the "bar" to allow some movement of the leads? ?I think that helped.? ?I should have used a tiny bit more lead length.? ?

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3.? (I'm still using the staked-in female screw jacks -- I haven't drilled through to add a back nut.? ?If they were to fail, I think I woud do that next time)

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4.? Starting from FULL COUNTER CLOCKWISE, and using a small "wall wart" 12-VDC supply, I was able to set the idling current.? ?I try to always do this with a WALL WART with limited current capacity since I don't have a laboratory current limited power supply.? These are only few dollars and are WELL WORTH HAVING for non-destructive re-setup of one of these radios.? ? (For me, I also have a polarity protection circuit inline -- these are valuable radios and I hate the thought of destroying one with reverse polary.? ?Once I get the polar protection inserted, it never comes out, STAYS with the radio from then on.? ?)

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5.? Based on the rotation of the potentiometer, a slightly higher (more positive) bias gate voltage was needed on the IRF520 than with the previous IRFZ24N.? ? That's just differences in the devices.? ??

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6.? I have Ashhars recommended bi-directional TVS diodes (with miniscule 3pf capacitance) in place.? ?Soldering those with my tools was quite a feat!!? ? They have astonishingly low junction capacitance compared to what I see for typical axial leaded devices.? ?

Later today I should be able to start making some power measurements and resetting the hardware coefficients for the various bands.? ? I bet I'll see more power on the higher frequencies with the significantly lower junction capacitances.? ??

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Gordon KX4Z

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