TCF does RD — homebrew 7MHz SSB superhet

Last weekend was a contest double-bill with the Remembrance Day and Lighthouse contests.  I listened for a few hours Sunday late morning on the MST400 and my TCF/Super-VXO receiver strip, now with replacement audio stage using a TDA7052 instead of the prescribed LM380.

TCF-receiver-strip — SA612 mixer, 8.867MHz IF filter, SA612 product detector, TL072 op amp, 2N2222 BFO.

TCF is short for Twin Crystal Filter, a design by Drew Diamond VK3XU from 1994 that uses two separate 4-pole crystal filters, one for receive and one for transmit (as well as four NE602s, two in each of the Tx and Rx signal paths), which greatly simplifies T/R switching and general construction at the expense of parts count and space.  It is a celebration of cheap computer crystals.  I’ve built the receiver strip only, at this stage.  Peter VK3YE has a similar transceiver with a 10MHz IF and 3MHz VFO, which sounds sweet.

I am not an avid contester but I do usually join in on RD weekend.  I tuned up and down the band a few times on the MST400 (5 watts SSB) and made 10 contacts, just to exchange a few numbers. The use of ‘number of years licensed’ rather than a serial number was an interesting change. My number is 35.

Varactor-tuned Super-VXO on 16MHz with a big 10k 10 turn tuning potentiometer.

Conditions for 7MHz QRP at this time were not great, I had consistent trouble trying to work VK2s or anyone who was a bit weaker. Called a number of them with no response. The stronger VK3s and VK5s came back straight away.

It was an excellent opportunity to test out the TCF receiver with its Super-VXO while its limited segment of 40 meters was full of strong stations. Using two 16MHz crystals (and an 8.867 MHz IF) I comfortably get 35KHz swing at just the right part of the band, from 7085 to 7120 kHz.  Wishing I had a bit more coverage, I soldered in the third 16MHz crystal and got a huge increase in spread, but it pulled ever so slightly on voice peaks. This was not expected, as the VXO has a 2N2222 buffer and all stages are strongly regulated and decoupled. I removed the third rock and went back to the 35 kHz coverage which was, you could say, rock solid. I will stay with the two crystals in classic Super VXO configuration but fiddle a bit more with the series L to get the most useful spread.

The replacement audio stages from the MST400 Mk II circuitry, NE5532 pre-amp and TDA7052 audio amp, made small to fit directly over the original TCF board.

The audio from this little receiver is pleasingly crisp and clear, slightly different than the sound of the MST400 Mk1, which may be due to the higher power rating of the TDA7052 audio stage and may also be a function of where the BFO and crystal filter alignment have ended up on both receivers. Opposite sideband suppression in the TCF is not quite as good (to my ear) as in the MST400.

While the Super VXO is good, and makes for a usable simple superhet receiver over its limited coverage, it still drifts slightly, perhaps a few hundred hertz from cold, then a hundred hertz or so when sitting warmed up on the bench. When operating it, I miss the ease of the MST400’s synthesised 1kHz steps and the simple convenience of landing on discrete 1KHz increments with each ‘click’ of the detent-ed tuning encoder.

But simplicity brings charm, it is what it is, and I will stay with the Super-VXO, for the love of little receivers, and also because I think the opportunistic crystal mixing scheme (described in tutorial style by VK3YE) is just so clever!

A receiver alone does not a transceiver make.  So the transmit strip must follow soon.  I may rebuild the Super-VXO on a smaller board so that the whole thing can be boxed up to something nice and compact.  It’s bound for the summits after all.

Original article in Amareur Radio Oct 1993.