Hamfester’s Hamfest 2016

About a month ago, I attended the Hamfester’s Hamfest in Peotone, IL, south of Chicago. Sponsored by the Hamfesters club out of Crestwood, IL, we had a lovely day for it, if only a little muggy.

Hamfester’s had the most vibrant flea-market area of any of the fests I’ve been to so far. No lack of boat-anchor rigs to be sure, but also some newer ones for sale (an FT817, A TS450, a couple ICOM base stations). Lots of CW keys, and no lack of miscellany, but the “clear out the shack properly” contingent was definitely in full swing. They’re the ones who have new and interesting things, or spare rigs they don’t need, or tools.

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50 bucks for an old tube receiver. Wonder if it works?

There are several sellers that I’ve seen two or three times now – I’d be willing to bet they acquired either surplus hardware or an estate-sale or two, and now just want to be rid of all the parts. There’s one table that I make a point of picking through. It’s literally just a giant pile, but I’d found a couple little treasures in there…

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The Big Pile!

As I’ve previously mentioned, I didn’t come home with a huge haul from this swapmeet, but I did snag an unbuilt Scout Regen receiver kit that I put together. I also got a nice 30V adjustable, 500mA regulated current-limitted power supply from Elenco, in a nice case with leads, for 15 bucks. And a dual variable-cap from The Big Pile.

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See you at the Fest!

73

T/R Switching

A quick post from the day’s experiments: the transmit/receive power switching arrangement for my next project. The scheme is very much like that in the KN-Q7; I wouldn’t have stumbled across this (very simple) setup without an excellent write-up by Andrew Woodfield, Zl2PD.

Here’s the simple schematic:

The power switching arrangement for a new project.

The circuit itself is straightforward – when the “key” pad is left floating, current can flow through the 2k2 resistor attached to the 3904, providing a small base current and driving the 3904 in conduction and powering anything connected to the +12R pad. At the same time, there is nowhere for base current to flow in the 3906, so no current is provided to the +12T pad.

When the key pad is grounded (by a morse key, or other TR switching method), the base of the 3904 is pulled to ground through the small signal diode, and very little current will flow through the 3904 and into the +12R section, effectively killing receive functions. At the same time, a small current will flow through the 2k2 resistor attached to the 3906, allowing it to conduct and powering anything attached to the +12T circuit.

In short: when the key line is floating/disconnected, the circuit is powering receiver functions. When it is grounded, the circuit powers transmitter functions.

I added the little 7808 regulator to this power board, which will be powering some NE602 mixers on receive only. I put both circuits together on a little piece of copper-clad, in something like Island-Pads-meets-manhattan-construction:

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I like how the layouts ends up showing off the inherent symmetry between PNP and NPN transistors. The whole thing looks quite nifty on the bench. I left the input and output pads deliberately large, to accommodate however many connections end up being made.

I did begin construction and testing on the next project, but was stymied by my SLA-battery power source running down (it was down to 7.4 out of 12V by the time I thought to put a meter on it). Rather than switch to the bench power supply (an old computer power supply with the 12V and 5V rails brought out), I took it as a sign to call it a night.

Hear you over the sizzle of solder!

74

An Si5351-Driven QRP Amplifier

This work follows directly on from my initial thoughts on an Si5351-Based Transmitter. Check out that post for background. 

This week I finally got around to improving the transmitter/amplifier I started work on back in March. With a shipment from KitsAndParts, I replaced the J110s that had been part of this amplifier and replaced with BS170s. The differences between the parts are striking – the J110 is a general-purpose JFET (which is by definition a depletion-mode FET) with a rated power disipation of a few hundred milliwatts. The BS170 is a fast-switching enhancement-mode MOSFET with a rated disipation of 800 mW.

Here’s how the schematic looks now. Pretty similar to the last time, with a couple key changes:

CW Transmitter BS170

I stripped the heat sinks that had been on the J110s off and threw them on the BS170s. These puppies still do get hot, and even with the ability to dissipate almost a Watt, I think they’d not be pleased too much heat. The heat sinks, plus some thermal goop from Microcenter, are easing my mind a bit.

As built, the amplifier had an output impedance of about 10 ohms at 7 MHz, as measured by the method suggested by W2AEW in this Youtube Video. To bring that up to the standard 50 Ohms, I built a little L-match, using the values suggested by this L-Match calculator, about .43 uH in series with the amp and a little over 1 nH shunted to ground. Because I don’t know the voltage ratings on most of my miscellaneous caps, I put for 4.7nF caps in series to form the fun capacitance. The inductance is 12 turns on a T37-6 (I did some experimenting to find the ideal number of turns). Re-measuring the output impedance (again with the W2AEW method) showed an output impedance of around 47 ohms, which is close enough for my purposes, I figure. I

I also recently bought a little power-supply kit from Ebay, and have been using that to supply the drive current. It’s adorable:

Now, being driven by the Si5351, the amplifier puts out about 1W on a 5V supply, and about 5W on a 12V supply. Not a particularly clean signal, mind – the 5W is as measured in the power meter of an MFJ-949 – but it is an actual 5W!

Hear you on the air!

73