HP 5386A 3 GHz Frequency Counter

[mojoe]

I bought this counter used about 15 years ago. When I got it, it didn't power on. I found the DC fuse blown (it also has an AC fuse). I replaced the fuse and it worked just fine.

Over the years, I've used the counter infrequently. Once, long ago, the DC fuse blew again, but I was busy with other things, so replaced the fuse and continued using it. I thought that perhaps there was a power surge, or the caps were getting old.

Fast forward to yesterday, and I needed the counter again. Now it was blowing the DC fuse often (sometimes on power up, sometimes after hours of use). Time to download the service manual. Don't you love old HP service manuals?

I checked all the voltages, and they were well within spec. I even let it run all day while the DMM was logging. Again, well within spec. Next, I checked the obvious, like caps, diodes and the SCR in the crowbar circuit. Eveything looked good. Digging a little further, I was checking voltages in various parts of the board - all looked good. I then checked U15, the 2.5 V reference chip. The 5 V input and 2.5 V output looked fine, but while probing, the DC fuse blew again. Hmmm, curious.

Taking a closer look, I found what you see in the picture. It has been like this since manufacture. The entire top side of the board is a ground plane, so it took some heat to solder that ground pin.

I'm letting it cook for a while, to make sure, but I think I fixed it. Next, I'll have to adjust the internal TCXO. It's 1.37 Hz high    I usually don't use the internal reference, but my GPSDO.