Looking Back

So I was flipping though the pages of the latest issue of QST magazine when I came upon the section "25, 50, and 75 years ago in QST"  (Actually it's also 100 years ago, but a century ago this date we were in the middle of the first world war and amateur radio, along with the ARRL was shut down for the duration).

The cover of the magazine for April 1968 looked familiar.  It was in fact the very first issue of QST that I had ever bought from a new stand, about two years before I finally got my Novice license.  I think  I had been visiting what remained of NY's Radio Row, probably Harrison Electronics, or maybe Barry Electronics when I bought that magazine, along with a copy of the 1967 edition of the Radio Amateur's Handbook (I probably bought the previous years version at half price after seeing that it wasn't that much different from the then current version).

Surprisingly, the technology represented in that 50 year old magazine didn't feel like it was a half century old.  The lead article on the front page was a construction project for an electronic keyer using three integrated circuits, after all.   They were however RTL logic chips, now obsolete and probably no longer obtainable, except perhaps on Ebay from Chinese vendors with a history of selling counterfeit parts.

The other major construction article was for a 40 meter QRP transceiver using a mix of silicon and germanium transistors. 

Cute looking little rig isn't it?  After looking at the parts list for this rig, and comparing it to the contents of my junque box, I'm giving serious thought of actually building a copy of this little rig.  Of course I'll probably make a few parts substitutions along the way, mostly to replace the few germanium transistors with silicon ones. 

Now this WAS the April issue, and QST has had this nasty habit of putting a few articles in such that were meant to be jokes.  While I doubt that the description of this project was intended as an April Fool's laugh, there certainly were a number of mistakes in the parts list that would have tripped up a constructor.  For example, 2N3905 NPN transistors were specified, and I know damn well that this part is a PNP transistor, a lower spec version of the 2N3906, which is also called out for in the article, correctly as a PNP unit.  I think the author meant 2N3904, but the mistake was made across the entire article.  A 2N1305 transistor is also called for, in two places as a PNP (correct), and in one place as an NPN (oops!).  Now there IS an NPN version of this germanium device with the part number of 2N1306, perhaps that's what he meant.  Finally, the 15uh inductance value for the VFO coil was wrong, although the number of turns on the coil was correct.  A quick bit of math showed that coils needed to be about 5uh to cover the required frequency spread.

One of the PNP germanium units is the audio output stage (driving  2000 ohm magnetic headphones), the other is a keying switch for the transmitting mixer and pre-driver stages.  The NPN germanium device is used in the VFO.   I'll probably look at adjusting the bias resistors in the VFO and switch to a silicon device here, and I'll substitute a 2N2905 silicon transistor for the keyer switch (again adjusting the bias resistors).  I'll leave the audio output stage as is, but maybe I'll add an output transformer to drive lower impedance headsets (like my David Clark aviation headphones).

Here is a image of the inside of this little rig.  Note the use of compression mica trimmer capacitors, transistor sockets, and half watt resistors.  The one "modern" construction feature is the use of toroid core inductors.  (Actually toroids have been around a LONG time, and were 'rediscovered' in the middle 1960's).

The receiver has an rf amplifier stage using a complementary circuit with  2N3904 NPN and 2N3906 PNP transistors.  The complementary circuit is similar to the cascode amplifier, however the devices are NOT connected in series for DC.  Like the cascode circuit, the complementary amplifier has the first stage operating in the common emitter mode, and the second in grounded base mode.  The IF amplifier uses the identical topology, and the gain of both stages are controlled by varying the bias on the second stage's base via a panel mounted control.  There is no audio gain control in the receiver.

A single NPN transistor is used as the receiver's first mixer, and a simple two crystal IF filter is used.

Here you can see the rf stage, mixer, IF filter and half of the IF stage.  A search in my junque box discovered a half dozen 5.068 mhz crystals, close enough to the 5.010 mhz called for by the author.  They will work by changing the adjustment of the VFO slightly.

That's all for now, I'll try to post an update later in the month.