Soldering MSF5000 RF Transistor

[screech67]

I am replacing one of the RF transistors in a 125w MSF5000 PA and would like some advice. It seems because of the heat sinking it is taking a fair amount of heat to solder, not having much luck with my weller 100/140w gun. I am worried about damaging the transistor, advice appreciated.

[jackhackett]

You mean the ones on the ceramic substrate modules? Good luck.. you're supposed to replace the whole module. I'm not sure it's even possible to replace just the transistor, or if it would work properly if you did manage to do it. Those are bonded to a copper heat spreader, probably after the transistor is soldered to the substrate.

The modules themselves are pretty easy, they use things called omega jumpers (because they look like an omega symbol) soldered across the gaps between them and the main board.

You should probably look at a service manual, there are some other issues involved in MSF5000 PA repair you need to know about. And don't mess with that ceramic too much... I believe it's got beryllium oxide in it, dust from it is toxic.

[screech67]

Thanks for the reply. It was my soldering gun, it was pretty much dead. Got a new one and all is well. Now for the live fire test.

Mark
W1ACP

[Will]

OH! OUCH! Do not use a Soldering gun on any transistor because the electrical current thru the tip will destroy the transistor.
You must have a isolated tip type soldering iron and most will not heat the transistor tabs let alone the ceramic substrate.

The wrong heat level will damage the plating on the substrate.

Most of the MSF5000 pa modules are soldered in a controlled oven.

[kato56]

I have to agree with jackhackett on replacing the module. One problem with replacing one transistor is that on amplifiers usually the output transistors are matched and balanced so that the power out is close you can get into other problems in achieving a stable output, efficiency, etc.

[RADIOMAN2002]

I have replace them by butchering the old transistor, and removing it, so as to not damage the substrate, and then using a 300+w pencil iron (more like a bat) my dad had, that was used back in the day for resoldering lead shielded phone cable.

[com501]

Doesn't anyone own a hot air rework station anymore?

[screech67]

Thanks again for the reply's. Well it still only puts out half power and according to the DMP it's still in the first PA module where I replaced the transistor. Either I fried it or it wasn't the problem.

[bona]

Do not use a Soldering gun on any transistor because the electrical current thru the tip will destroy the transistor.
You must have a isolated tip type soldering iron and most will not heat the transistor tabs let alone the ceramic substrate.

[jnezbeth]

I've repaired MANY(50+) of these PA's, change the module, chip caps, and use silver solder!
forget trying to change the transistor - you'll never get it to tune up...