IC-28A transmit section carnage

"David Brodbeck" "Phil Allison"

Didn't find a can,

** Funny how you used that word earlier.

so almost certainly it's a tantalum. Good catch, i probably would have replaced it with an electrolytic if you hadn't said anything.

** Did you read the warning above about tants I just gave you ???

There is no reason to use them if space permits a normal electro.

.... Phil

OK, fair enough. I wasn't sure if they had different leakage characteristics or something like that.

For those curious how this went:

I replaced Q17 and C54.

I now have correct voltages on all four pins of the PA module in transmit mode, but still almost no power output -- I suspect a bad PA module, at this point. Reinforcing this is the fact that I appear to have more signal on the input of the PA module than I have on the output.

I think I'm going to set this one aside and just use it as a receiver; at this point it's not worth spending much more time on, unless I run across another IC-28A with a different problem.

Your assumption is most likely correct... I can give you an idea how ever.. If you can remove the cover of the module, and inspect the board, you may find a crack. You'll need a high temp soldering iron to put it back together.. This is common with RF modules or various types.

I've done this 3 times already on different rigs and it worked. Most of the time a crack develops, mostly from abuse. Other times, you have a bad module..

It doesn't require abuse. See:

That's the guts of the VHF power module from my Kenwood TM-732A radio. The circled areas are where I had to solder the trace back together. The two failures were seperated by about 2 years and will probably continue at the same interval. My guess(tm) is that it was caused by either incomplete baking of the hybrid, or a bad solder paste mix when the hybrid was silk screened.

The gaps were almost invisible and very difficult to see. They would have been far more visible if I could have shined a light from the back, but there's a big copper heat spreader in the way. Instead, I found them with an ohmmeter. I started with the probes at the end of the conductor trace, which showed no continuity. I slowly moved the probes closer to each other and eventually closed in on the break.

Soldering was also rather difficult. The ceramic hybrid is a wonderful heatsink and literally sucks all the heat out of the soldering iron. I had to use a rather large 850C screwdriver solder tip and still managed to do a lousy job of soldering.

Also note on the photograph the two big power xsistors. The goo covering them is rather soft and flexible silicon rubber. Please resist the temptation to push on it, or you'll break the wire bonds underneath.

If desperate, you can get replacement PA modules from RF Parts.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

On Mon, 31 May 2010 18:15:27 -0700, David Brodbeck ??o??:

No new PA available or is it too costly?

I use a mini gas torch that has nice tips for soldering, no flames get in there but the tip gets hot as hell. The heat sink has no chance of holding back. :)