Eddystone User Group

S640 Identity Crisis

January 15, 2024 By: chris Category: News

Way back in September 2023 Chris G0EYO (our beloved mentor) put out a request from Jon MI6XGZ to see if anybody would repair his deceased father’s (callsign GI3ZX) S640 as he would like it to be working so he could use it for old times sake. As MI6XGZ obviously lives in N.I. and me living just over the water in Southern Scotland, I responded and arrangements were made for a friend visiting MI6XGZ living in Central Scotland to drop it off as he passed the door from the Stranraer ferry. It was duly dropped off in January and put in the workshop for attention.

Now MI6XGZ’s deceased father, Desmond, has an interesting history in his own right. Licensed well before 1939 he was conscripted into the VI service at the start of WW2 and formally moved to the RSS secret listening station in N.I. GILNAHIRK where he stayed until de-listed in 1947. He passed away in 1984 and Jon MI6XGZ kindly scanned the final pages of his father’s log book from 1937 showing who was on the air then and the final entry before receiving official orders to go QRT. The log pages are shown below.


As soon as the set was received the bench was cleared and the transport box was opened and yes there was an Eddystone S640. However, it had a long large umbilical cord coming out the back with an industrial size plug on the end. Ok, could be a link to a transmitter or something.

Lifted it onto the bench and the front was from a 640 but looked very modified and the obvious give away should have been a microphone socket on the front panel!!. Eventually opening the lid (seized due to rust) sitting looking up at me was a pair of TT21 PA valves. Strange???

Both tuning capacitors had been removed and a small 30pf variable installed in their place. The coil pack instead of having the two valves (EF39 and 6K8) had 7 B9A valves and the I.F panel had its top

chassis cut away and  removed and in its place was a series of relays and what looked like filters of some sort or other.

This S640 was obviously a transmitter built into a 640 case and chassis. The construction was a masterpiece of ingenuity to squeeze a 13 valve transmitter into a 640 case. So why the umbilical cord then?. Was this for the power supply and modulator for AM? Could be.

Removing it from the case caused more intrigue. The coil pack had been stripped of all its previous coils and poking up were a couple of crystals 8998 and 9001khz. Turning the set back over and cleaning a filthy small box revealed a crystal SSB filter at 9MHz so we must have an SSB transmitter,

It gets more amazing! There’s a loudspeaker on the back of the front panel, chasing the modules through not only do we have a transmitter made out of a 640 but we also have a receiver built in as well. A quick look about confirms that we have a 80/20metre SSB transceiver built into and on a S640 chassis and case.

I decided not to put power onto it as; a) I would have had to build a power unit and b) as it must be over 50, probably 60 years old, heavens knows what would have blown up!!.

So there you have it. Not only a wonderful piece of engineering expertise but craftsmanship as well. Pity we don’t have people like GI3ZX about these days when it’s so easy to buy a transceiver. Hope you have enjoyed this little story.