Honey, I blew up the amplifier

Actually, let’s start on a bright note, and then we’ll get to the part involving smoke.

Inside view of the rockmite 40 installed in a mity box
The Rockmite 40

My main reason to build the amplifier was to get a bit more power out of the rockmite. Part of the problem in getting the amp working was also likely low driving power. The basic design of the rockmite uses the prototypical bipolar NPN for the “final” amplification — a 2N2222.  There are variants that use other transistors and get the power above a watt, and there are also some tricks to increase the drive, but the stock rockmite should yield a nominal half watt or so with no mods. So, I took off the cover of the rockmite and poked around, checking all the part values. I had already made one substitution: C8 was decreased in value from 0.1 uF to 0.01 uF to knock the side tone volume level down to something tolerable.

Sure enough, I saw the problem — I had used 47 pF (marked 47J) instead of 470 pF (marked 471) for C15 and C17, which are on the output side. Yes, the “1” and the “J” looked similar. I stuck the right value capacitors in, and power output using a 13.2V supply was 550 m. Not too bad.

Next, I plugged the RM into the Texas Topper. I didn’t power it right away, though, because I was interested to see what sort of insertional loss the Texas Topper introduced when it was not active. Power output was about 400 mW. This probably wasn’t an entirely fair test because the Tx Topper was still on the bench top, with tack-soldered connections to the BNC connectors.

Texas Topper before final install

I removed the extra N4148 from the amplifier, because I figured it probably had enough drive now to work without extra bias. Power output was measured as 2.2W, so about 6dB gain.  While I could live with that (and, in retrospect should have), I was curious what would happen if I bumped the bias back up a bit. The N4148 went back in, raising the bias voltage from about 2.05 to 2.75V. Power output was now 7.7W — 11dB gain. I measured roughly 7Vpp in, 20Vpp out, so roughly in agreement.  Part of the increase in gain might also have been due to switching from using alligator clips to apply power to using a thicker wire terminated on one end with a power pole connector and on the other end with a type N coaxial plug.

So, at this point, I was actually (hah) thinking of introducing a one or two dB attenuation pad, although the idea of burning off power in  a QRP rig feels inelegant. More inelegant, however, was trying to bend the FET forward so the heat sink would fit in the enclosure. When I pushed it forward (yes, with power applied), there was a bit of sizzle and then a bright flash from the LED. I don’t know if something arced before the LED toasted, but I was left with the acrid and no doubt carcinogenic aftertaste of stupidity wafting through the shack.

I inspected the board around the FET and couldn’t see an obvious short. The parts in that area do share some close quarters, and the heat sink is right next to both transformer coils. I took out the LED (shorted now) and yanked the FET. Good thing I had a bag of them on hand….as will soon be even more evident.

With a new FET, a new LED (not quite the same type, but close), and another diode, I was back in business. Everything was fine until I tried to stuff the heat sink into the enclosure. This time, not under power. The problem is, though, at some point, you have to apply power, or the whole thing is just a paperweight. Zot. Sizzle. Flash. Puff.

Yeah. So, at this point, I’m out of FETs and thinking that maybe I need to do something more creative regarding the strained relations between the heat sink and the enclosure. The heat sink should be applied right to the metal tab on the FET, which is the drain. The case is aluminum, and at ground potential, so that particular twain shouldn’t meet.

I tend to only order from Mouser when I get enough items on my “want” list to make the shipping work out, so it might be a bit before I replace the burnt out components, but I’m sure I will in the next couple weeks. The board has held up very well to my repeated soldering/unsoldering, and I really don’t have any complaints about the Texas Topper per se. This is more a mechanical issue at this point — all the electronic bits seem to do a fine job of amplifying. I may, in fact, order another one just to play on another band.

Even without the Texas Topper, doubling the output of the Rockmite should make it more usable. I’m looking forward to rolling both it and the TenTec 1320 out next week for the QRP to the Field Event.

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