An implementation of The Lower Sideband Modification to the UK\RT 321

By Leighton Davies – GW3FSP – July 2013

The mod kits for this job were obtained from Oliver Tillet G3TPJ, they are a small double sided PCB with pins for connection to the 321 set wiring, included is an instruction sheet with diagrams of the various works which are required and wiring instructions, these were read very carefully referring to the internals of the 321 in order to make very sure I would know exactly where and what I needed to do.

321 LSB Mod PCB

Switch Mod

The first problem anybody faces when doing this mod is how to provide an LSB switch. I decided to use the Elapsed Time Indicator (ETI) socket which has long been a dormant feature of the 321.

To achieve this I ran in the extra wiring required, the 2 red supply wires from where I was going to put my switch in place of the fuse holder originally for the elapsed time capsule which I removed after snipping the wires and insulating them, you may also have 2 small capacitors to remove, one across the ‘fuse holder’ one from the fuse holder to ground.


Lower Socket in location…

Wiring up

I then fed the red supply wire to the PCB location using the loom as a guide feeding the wire through the lacing cord leaving about 6 ” or so spare at the PCB end.

Complete PCB Showing Wiring

I then removed the wire from tag 14 of the tag strip, soldered on a length of white wire and sleeved the join, an extra white wire then from tag 14 to the PCB location again leaving an extra 6″ or so.

Siting the PCB

The lacing needs to be cut from the loom where the PCB is going to be fitted , I marked the coax from SK1 where I was going to cut it where the cable clip is.


Bracket and Clip…

…removed and ready to take PCB

Making Space

I then removed the cable clip and bracket, 1 screw for the cable clip and one for the bracket from the side of the chassis, and the nut and washers holding the bracket to the main chassis, once the bracket was removed I decided to put 1 washer and the nut back onto the screw and tighten down, I then snapped off the rest of the screw with pliers, I decided I did not want the rest of the screw moving about if I just snapped it off where it was.

I then removed the coax cable from SK1 to the next module from the set, cut the coax where I marked it , trimmed the sleeve back and connected the inner and outer to the required pins on the PCB.

Connected the 2 white wires to the required pins and then a red wire I had used for the 24v supply to the pin on the PCB, trimming the length of these wires to leave about 2″ of slack, I did not connect the ground wire as the coax outer braid does that and I’m suspicious of earth loops. I then checked all the wiring I’d done to make sure I had the correct wire going to the correct pin on the PCB and trimmed the pins back as far as I could.

Fitting the Board

I then used a small component bag opened out to completely wrap the PCB, this was then taped in place with ordinary insulating tape, I fed the PCB under the loom, secured it in place with a cable tie, re-fitted the coaxial cable and also fitted several ties to secure the loom where I’d removed the lacing cord.


That Switch…

Lower socket out...

Lower socket out…

On the original modification I fitted the switch straight into the hole using a thin washer inside the panel and another washer on the outside as the hole is about 1/8″ larger then the switch barrel, however I then tried the following method on the second set as an experiment and as it worked I then re-worked the first set to copy this switch fitting method.

I then placed the set so I could get at the back of the front panel with the panel flat on the bench, moving the wires gently out of the way I then stuck a small square of masking tape over the hole (about 1″ sq )

And pushed it firmly into place making sure the tape was flat and not pushed into the hole, I used a wad of tissue paper folded over repeatedly and pushed that on top of the masking tape, with the front panel closed this will push against the tape stopping the tape falling away from the panel.

Closing the front panel I then put the set on it’s end with the front panel uppermost.

I then mixed the Araldite I was going to use to fill the hole, I used just a bit more hardener than adhesive, mixed it well and poured it gently into the hole filling the hole very slightly proud of the panel.

Then left it to set over-night.

The base for the switch ready…

And believe it or not it had! (does what it says on the tin(ed)).  I opened up the front panel and removed the tissue paper and masking tape from inside the panel, grabbing my battery powered BnQ drill and put a small pilot hole through the centre of the filled hole first ( 3mm drill) VERY GENTLY !, then a 6mm, then 8mm finally the 12 mm.

The switch I decided to use was from Maplin Electronics and is a sealed IP67 spec switch, ( N25KA ) and I also fitted a switch cover ( YL01) which is a water/dust proof cover, done up tightly.

l Switch front

LSB Toggle Switch

 Final Wiring

The red supply wire to the PCB was then connected as was the feed to the switch, I took the supply from the tag strip sticking out of the metal screening can over the rear of the 2 audio sockets on the front panel, One tag strip has the 2 audio screened leads, the other has the PTT and audio out wiring, the 3rd connection from the bottom of the panel (the hinged end ) has 24 volts on it ( check to make sure ! ) the other red lead feeding the switch was attached there.

Running Up

I then closed and secured the front panel with one of the top bolts.  Connected power, headset, Turf unit and dummy load and switched on (fingers crossed), no bang !.  I dialled up on 20 metres – I think I used 14.202 Mhz , and tuned up, set to low power then set my 847 to 14.200 and tried USB to and from the 847, yes it worked, switched to LSB on both and dialled up 14.198 on the 321 and it worked.

Later that evening I tested this set with a neighbour Amateur and all worked ok on AM, USB and LSB.

The modification was then repeated on the other set with the same procedure and all ok.


The photo’s were captured using an old web-cam which has proved far more usable to produce small but quite good quality pictures, these I have up-loaded to the 321 Yahoo Group site for your perusal.

(Editors note: I hear the iphone has become almost the defacto standard for photographers due to its image quality and portability.  These shots are not too bad and convey the meaning.)

Have fun guys!


This entry was posted in Mods, UK/RT321 by Leighton Davies. Bookmark the permalink.

About Leighton Davies

Licensed July 1971 as GW8EYG, mostly on 2M SSB, Always into radio and electronics thanks to my father GW3FSP, Managed to re-activate his callsign when Ofcom removed the morse requirment. Since then been active all bands up to 13cm.

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