144MHz SSPA Project – BLF188XR MOSFET failed – Repaired…. but…..

Hi guys, I have an update regarding my mosfet troubles. To keep the story short……….:

Last weekend I got the replacement mosfet, I installed it, SUCCESS!!! According to its markings, it was a couple of years older than the other one, and it needed abt. 0.5 volt higher bias voltage, but this is fine once adjusted.

Then I started testing again, this time with a more mature swr protection system, this system tripped immediately. Hmmmmm. What could be wrong???

OK, fast forward, back to the time of breakdown event: It was a late weekend evening. The operator may have been a bit tired, eager to produce some results. There may have been a couple of beer cans in the way. 🙂 And the shack do not look good in daylight… this does not help at midnight, but it may appear better, visually.

I am now, pretty sure that the cause of the mosfet breakdown was a missing load, not spikes or over drive from the driver. Looking for the cause for the high swr situation mentions above, I found that the TX relay (narda 023 series) had a short due to internal welding!!!! Also, the coax connected to the transmitter port had a short. The RG-142 Teflon center insulator had developed a puncture, shorting it out!!!!! There have been some serious voltage on the loose here.

Well. I repaired the relay and exchanged the damaged coax. Up and running again, but not for long. A piece of advice; knowing about their internals, relays like the narda 023s, do not abuse them. They are high cost, wide band, low insertion loss, high power relays, but keep them below the rated power limit. I think these are rated to at abt. 1Kw @ 100 MHZ @ low vswrs conditions. Well, I tested it for key down, 2Kw at 3 minutes and it would not operate any more. Wye?? Because the internal plastic bits has melted and permanently deformed.

Looking for a better replacement I discovered that an electromechanically solution does not in practice exist at these power levels. Not what my wallet is going to accept any way.

Then I come to think of SM5BSZ Leifs page, about High Power PIN Diode Switch. Well, I do not have suitable PIN diodes, but I have many different relays. Let’s try that instead. …….And it works. Only trouble is that my Q is to high. RF-current melt the solder and things broke down……again. But this can easily be rectified.

……..to be continued……….

……here are some pix.