Pop Goes The Weasel

For the past two months I've been slowly working my way through the HardRock50 amplifier kit designed and manufactured by Jim Veatch at HobbyPCB. This is the second of these kits I've put together. The first one I bought off of QRZ.com last year. It was used, but new-in-the-box. The owner bought it from a friend, who had bought it from HobbyPCB. Neither of the two had the time to put the kit together so the last owner sold it to me at a significant discount. Everything was still in its factory fresh packaging. However, it was only after I got this kit that I realized it was an early 2014 vintage unit. HobbyPCB still supports them, but getting the amp to work with modern QRP rigs like the the IC-705 was going to be difficult. Lots of jumpering, some cutting of traces, etc. This unit works well with my Yaesu FT-817, but I still wanted something I could use with the IC-705. So, I bought a new kit from HobbyPCB and figured I could sell the old amp to help cover the cost of this new one.

HardRock50 paired with the Elecraft KX3

Construction of the HardRock50 is pretty straightforward, and Jim has tweaked the build instructions over the years to the point that everything is clearly described and diagrammed. Having already built one of these kits, this second build went quick and relatively easy. One of the upgrades Jim has introduced over the years is that a lot more of the components are now pre-installed at the factory. With the 2014 kit you got a bag of relays and instructions on how to stuff them on the board. With the current kit, the relays are already on the board. This cuts down on a lot of the tedious soldering. If you've soldered one relay, you've soldered them all.

Everything went pretty straight forward until I got to the step where you have to set the DC bias. There's a Rube Goldberg-esque setup in the manual that has you interconnect your power supply with your ammeter and the amplifier. I've done this a few times with my old amp, with no problems. Just take your time and follow the test lead and power connection instructions in the diagram.

OK, all cables connected and checked and re-checked. I reach over and hit the ON switch on my power supply and POP! Lots and lots of smoke coming out from under the board. Damn, damn, damn, damn, damn. I pull everything apart to get a look at the underside of the board and find that capacitor C36 - a pre-installed component, had popped. What the hell had I done wrong? I checked and re-checked my test lead setup, and everything looked good. But SOMETHING went wrong. The evidence was inescapable. Clearly I screwed up.

I contacted Jim and asked him if he had any ideas as to what could have caused this - something upstream in the build I had put in wrong? Maybe a solder bridge I didn't catch? Jim wrote back almost matter-of-factly,

"Tantalum caps have a habit of blowing the first time voltage is applied"

Oh really?! I'd heard 'tant caps' could be problematic. As the comedian Bill Engvall says, "Here's your sign".

So Jim is mailing me a new cap, and I'll report back on how well this one survives. At least this experience gives me the excuse I've been looking for to buy a hot air solder rework station.

W8BYH out