I hereby declare Thursday, April 21st was Digital Restoration Day. The moment I walked in the door after work, Lara pounced on me and told me that she had shut down her computer because a window had popped up indicating that she had fifty or so horrible viruses on it. As she guessed, it was a fake message produced by adware of some sort. Starting with the usual, I ran microsoft security essentials, which identified and quarantined the adware. I did a manual update of the malicious software removal tool and gave that a run as well. After a reboot, the machine ran normally, and Lara went back to her online projects.
My next project was my cell phone. I live on my android phone, but it had been getting quirky for the last few months — long latency when switching apps, unexplained crashes, general orneriness. Last year, I had gotten tired of waiting for T-mobile to upgrade my HTC Magic 32b (i.e., T-mobile MyTouch) from it’s original (v 1.5?) android install. Mainly, I wanted tethering ability and some of the bells and whistles of the later versions. T-mobile kept promising over the air upgrades, but after a few months, that got old — particularly when all of their new and shiny phones were being sold with the new operating system, but I was stuck with my dinosaur.
Last year, I rooted the phone and installed a then-current version of the Cyanogen Mod. The HTC Magic followed the Google G1, so it was the second android phone on the market. Although it supports a 8 GB sdcard, the onboard memory is limited, so I had to install the “tiny” version of the Google apps, and otherwise be careful about memory allocation. Over time, the phone slowed down and became buggy. It got quite annoying. Tonight, I grabbed the highest version 6 cyanogen stable release, did a factory wipe/reset, and installed the updated operating system. It booted without a problem, grabbed my settings from the cloud, and I’m back in business today with a phone that is not only more stable, but a lot perkier.
I had another go at the rockmite last night on 40 meters. I tuned the LDG tuner to 7.028 with the Kenwood B2000 and then substituted the rockmite for input; antenna was the 43 foot tree-slung vertical wire. The LDG has latching relays, so it stayed tuned even when the drive was removed. I started calling around 18:30 EDT (22:30 Z) and was answered by W2XB, Don in Lakeview, New York (a bit south of Hamburg, along the shore of Lake Erie). Don was using an Elecraft K3 and gave me a 349. His signal was weak but copiable (after I asked him to QRS down to around 15 wpm), so I gave him a 359. The rockmite is not very selective and there were signals up and down probably a khz, so most of the filtering was done between yours truly’s ears. This contact went back and forth three times, and I can’t say that I copied every last letter, but the signal was quite stable. So, that is QSO number 3 for the rockmite, 282 miles (~450 km) on 550 mW.
After listening to the rockmite for a while more, my head was beginning to swim. I craved the narrow filtering… no, make that any filtering whatsoever, so I fired up the Kenwood and sent a QRL on 40m. I was answered immediately by a “?”, so I cq’d, and was surprised when HA3OD, Arpi, came back to me. I believe he said that he was operating at 30W from a location near Pecs, Hungary. So, about 4600 mi (7400 km) on 5W — even better than the rockmite.
I’m pretty happy with QRP performance last night. Propagation conditions were not particularly great (prediction for the band was “fair”), but noise was less of a problem than usual. I am considering building some sort of audio filter to narrow the rockmite experience. I know that there was a rockmite filter, but I don’t believe it is still available. I’ll have to look around a bit.
Yesterday, my brain was mush, so I didn’t attempt anything at the workbench. I turned on the radio and tuned around the ten meter band, not expecting to hear much. However, I did hear considerable activity from South America, calling with “MM TEST”, which turned out to be the Manchester Mineira contest. Since the contest originated in Brazil, it wasn’t surprising to hear so many stations from that country. I had my first contacts with Peru and Ecuador, and logged a few with Columbia, Aruba, Cuba, Mexico and a few U.S. stations. I heard Chile and Panama, but didn’t manage to land them. I only caught the last couple hours of the contest and did not try running at any point.
Afterwards, I plugged in the 550 mW rockmite 40 and tried calling for a bit. The signal went through my LDG tuner and then my 43 foot vertical wire in the backyard. The band conditions were not great last night, but the activity level was relatively high. I tried for about an hour, and stopped around midnight. I figured that I might have more luck the next morning, with fewer competing signals (but also fewer listeners awake). I started calling around 6:30 and went about 45 minutes. Local sunrise was 06:27 EDT / 10:27 Z, so 40m was fading towards the end of that period. I didn’t get any responses, but I was picked up at reasonable levels on the reverse beacon network, by stations from Massachusetts and Georgia.
At one point this morning, my ears were nearly blasted off by a New York station operating on 7.031-something. I cranked the RF gain all the way down on the RM40, but it was still loud. The reverse beacon network registered the station at 51 dB above background in Illinois and Maryland — that’s about 10,000 times stronger than my 500mW signal, so I have to guess that the station was using either a linear amplifier or had one heck of an antenna. Even operating more than 3 khz away, he cooked me.
So, no cigar so far on the higher-power RM, but I am sure that the signal is getting out. Maybe I need to keep an eye on the QRPspot site.
Actually, let’s start on a bright note, and then we’ll get to the part involving smoke.
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.
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.
I seem to be doing well enough running the Kenwood B2000 at 5W or the TenTec 1320 at around 4 watts, but I haven’t made many contacts with my Rockmite, which on a good day puts out around 250+ mW, but less when the battery runs down a bit. A while back, I had ordered the Texas Topper (a.k.a. Tuna Topper) amplifier (nominally 5W out) from www.QRPme.com. For $25 it’s a good deal. The design lends itself to flexibility and experimentation, allowing the user to choose whether to use onboard/offboard options, a tuna-shaped round or altoids-shaped rectangular form factor, a range of input powers (using fixed or variable attenuator, if necessary), fast/slow switching, and transceiver or xmtr/rcvr configuration.
I put the kit together last week, made an enclosure, stuck it in, connected everything up, and … nothing. Well, not quite nothing. My WM-2 wattmeter read about 50mW output. Not good — the amplifier was doing something, but not in the direction that I had hoped.
The amplifier is well-documented on Chuck Carpenter’s website, which provides a parts list, pictures of the board, schematics and helpful advice. From the circuit diagram, it’s apparent that there are two halves to the pc board — one that controls switching through a relay, and the business end of the amplifier that centers on a MOSFET followed by a filter network. As far as I could tell, nothing was shorted. In the “receive” state, signals fed straight through from input to output connectors. When I keyed down the rockmite, I could hear the relay click in, and I was able to verify that the input signal was being appropriately routed over to a 4:1 transformer to feed to the MOSFET.
I checked DC voltages with a DVM, and verified that the bias voltage (determined by the forward voltage drop across an LED that conveniently also serves to let you know power is applied) was 2.05V. The voltage on the MOSFET’s metal tab (the drain) was about the same as battery voltage, as it should be. Unfortunately, I didn’t have an RF probe on hand for tracing of RF voltages — the probe was lost in the last move. It would be easy enough to build one (see nice plans on W5ESE‘s site), but I didn’t have a suitable diode on hand and apparently Radio Shack no longer carries the 1N34 in stock. No problem — I have something better, although not quite as portable: an oscilloscope.
The incoming signal was about 8Vpp, and 4Vpp after the 4:1 input transformer. I expected to detect something on the drain of the MOSFET, but all I got was hum (maybe just background). Probing beyond the MOSFET, I didn’t get much. I was stumped at this point, and starting wondering if I had done something wrong during construction.
It seemed to me that there were two likely places that I could have screwed up — in winding the two bifilar transformers (which, I recall I did while watching an episode of “No Ordinary Family”, so maybe I was distracted), or maybe in installing the MOSFET. I had placed a mouser order at the same time as the kit order, so I had a couple extra MOSFETs to play with. Using static-free everything (mat, wrist band, soldering tip, etc) and low heat, I replaced the MOSFET. No change. The kit comes with 22 and 26 gauge red magnet wire for winding toroids. To be extra careful, when I rewound the two bifilar transformers, I used on strand of red, and a strand of another color. Radio Shack does carry a magnet wire set, which conveniently includes 22, 26 and 30 Ga lacquered wire, and the 22 is gold and the 26 is green. The transformers look much better when wound with two color wire, and it’s easy to verify at a glance that the correct wires are tied together and that they all end up where they should. Again, though, no change.
I tried replacing the MOSFET one more time, as I thought that perhaps I had not had the right load on the amplifier when I tested it the first time, but again, no change.
After I looked at the data sheet for the FET and noted that the gate threshold voltage is listed as a minimum of 2v and max of 4v. The transfer function graph showed the drain current picking up sharply above 4v. My rockmite has lower output than most, and it occurred to me that I might be at the lower end of this amplifier’s design — not enough umph to drive the FET’s gate. To up the bias voltage, I stuck a 1N4148 diode between the stock LED and ground. This bumped the bias from about 2V up to 2.75V. Result: 1.5W output. On the oscilloscope, the waveform was a bit distorted on the FET’s drain, but smoothed out in the filter and was well formed at output. Going from 250 mW to 1.5W is somewhere around 7dB gain — not quite the 10 dB gain typical for this amp, but a huge improvement over my rockmite’s usual output.
So, now I am playing around a little to see what happens when I run this system with a fully charged battery and play a bit with the bias voltage. Hopefully, I’ll have a chance to try out the rockmite-on-steroids this weekend.