Wednesday, April 23, 2014

Fixing the K2/100, Again

D16 and D17, plugged into machined socket pins.
About a year and a half ago, I wrote about repairing my K2/100. Part of the fix involved replacing diodes D16 and D17 on the KPA100.

The same week I bricked my K3/100, I found the K2/100 was acting up. I was going to grab it in case I needed to send the bricked K3 back to Elecraft.

But, before I did that, I spent part of an evening trying to work some of the W1AW-portable operations. I didn't have a lot of luck. The power output seemed erratic on some bands, and on others, the automatic antenna tuner couldn't seem to find a match.

Seems to me I'd seen this behavior before -- when the SWR bridge diodes, D16 and D17, had undergone a soft failure. I figured that if this was going to be a regular occurrence, I ought to make the job easier.

I found an old machined-pin IC socket and carefully clipped away the plastic to reveal four individual pins. Actually, it was more like six pins, since I dropped one and couldn't find it under my desk. And another one I hopelessly bent. Fortunately, these things come packaged with fourteen to sixteen pins at a time, so I had plenty of spares.

A bit of work with a screw and nut driver released the KPA100 board from the heatsink. Pulling the old diodes from the board is a bit of a delicate operation, since you want to be carefully not to mess up the board traces. However, with the socket pins, this is the last time you'll do this.

Soldering the socket pins in place is one of those operations that would be easier with four hands. Even holding the board in a small Panavise, I needed one hand to hold the machine pin with pliers, one hand to hold the soldering iron, and one more hand to hold the solder. That's one more hand than God gave me, but somehow I managed.

Once the pins are in place, it is a simple matter to trim the diode leads and bend them down to fit the socket pins. I had ample spares from my earlier purchase.

I checked the diodes with my DVM. (Oh, I should mention that I discovered my 20 year old Radio Shack DVM bit the dust. Apparently one of the AA batteries inside managed to get enough corrosive goo on the circuit board to render it irreparable. So, I had to go out and buy another DVM. It would have been nice to know this before the last hamfest, as these types of devices can easily be had for a few dollars. As I was, I bought a cheap unit at the local Home Depot for about $20. More than I wanted to spend, but I had it in my hands immediately.) These diodes didn't show any obvious odd behavior. The forward voltage was a little off, and the reverse voltage was at the limit, just like you'd expect.

Nevertheless, I put new diodes in and ran the calibration procedure for the SWR bridge. Not having a dummy load handy (something else I left at my other QTH), I used a 40m antenna that was a pretty good match. After calibration, the K2/100 seemed to work as expected.

I'm surprised how sensitive these diodes appear to be to static discharges. I'm going to have to make sure I keep the rig antenna grounded when I'm not around.

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