A legal limit am transmitter for 75/160 meters from WO1U

[JimIO]

I see no need to measure voltage in a circuit like that. It's going to be about 1.4 X the RMS line voltage. I can see wanting to know the current when first testing it. After that temperature, forward and reverse power would be all I care about. Search for w1209 on ebay. Those are cheap enough to have one for every FET. youtube w1209 to.
 

[redhat]

When playing with solid state, good metering is essential.  PA Volts and PA Amps measurement will allow you to do indirect power calculations, and when compared to output metering provides a troubleshooting aid.

If you want to stay in the dark thats fine, but your going to have a lot of trouble trying to fix it when something isn't right.

+-RH

[JimIO]

Not Class D, that's like Switch Mode Power Supplys. You don't troubleshoot SMPSs with the power on. You just replace all the bad components and power it up. If you miss any then you do it again. All you need is a Huntron Tracker.

[redhat]

Some measurements you have to do with the power on.  Gate drive waveforms can tell you a lot about the health of a mosfet and the load downstream.  Lousy drive makes for lousy efficiency, and eventually burnout.  Drain waveforms will tell you about parasitics and whether the output network is properly tuned.

Better information makes for better troubleshooting.  Case in point, I just got my prototype transmitter running last night.  The thing had survived a 3 hour excursion to 1200 watts carrier on 4070 KHz in the back yard a few months back.  Following that I retuned it to 43M but never tested it at power.  The next day I drove to a site and setup.  When I flicked the HV switch, the PA failed, as indicated by a driver overcurrent condition.  I brought it home and replaced all the silicon, brought it up to 250W with modulation, and it ran for about two minutes and failed again.  Over the last few months of armchair troubleshooting, I theorized that some of the passive components must have been overstressed and developed partial breakdown problems that was taking out my fets.  I again replaced all the silicon, and all the passive parts in the RF power path (DC blocking caps and resonator caps).  In the process I decided to change out the output balun core and go back to material 61.  I quickly discovered that I could not unwind the balun because the supposed teflon jacket on the coax had melted and several of the adjacent turns had punched through, as evidenced by soot marks here and there.

I still don't know if the passive components failed, but I changed them anyway.  In light of the smoking gun, it seems they were fine.  At this point its not worth the trouble to find out, especially since all Cree SiC devices across the board are out of stock until October and I have a limited stock of replacement devices.

BTW in any commercial transmitter, PAV, PAI, and FWD/REV power at a minimum are measured.  This monitors not necessarily the peak voltage across the output devices, but the average DC input to the PA stage following the modulator.  PAV and PAI are  multiplied together, then multiplied against an assumed efficiency factor to determine power output.  This is called indirect power measurement.  Direct power measurement is carried out with a calibrated watt meter, or using calorimetry.

+-RH

[JimIO]

I know all about how the commercial guys do it. Worked for a place that made and installed UHF TV transmitters. Is it me or is management all right-wing dick-heads in those places? Anyway, I'm only talking about hobby/pirate stuff.

BTW When I first heard about Cree I thought it would be a good place to invest.
 

[redhat]

I'm only talking about hobby/pirate stuff.

Why should we expect any less?  I guess it boils down to operator philosophy,  but I would rather carry something into the field that was designed with commercial 24/7 operation in mind.  There is a better chance it will work when I flip the switch.

+-RH