Curing the RT 321 Sticky Coil Drive Mechanism with a Selector Unit Coil Assembly Replacement

In this article Leighton Davies, GW3FSP discusses the ultimate option for the repair of a sticky coil drive mechanism in the UK\RT 321.

Caution.

USE STATIC PROTECTION !, DO NOT HANDLE THE PCB’S UNLESS YOU ARE USING A CONDUCTIVE MAT AND GROUNDING LEADS, REMEMBER TO EARTH YOURSELF WITH A WRIST STRAP TO THE MAT AND ONE LEAD FROM THE MAT TO THE SELECTOR UNIT PCB EARTH.

Purpose

I decided to carry out this repair in order to re-furbish the Selector Unit due to tuning problems caused by the coil drive mechanism in the set sticking and giving the characteristic continuous un-ready bleeps (often ‘cured’ by a sharp tap with a blunt object in the right place!).

There are several levels of repair and this one can be carried out if my ‘fix’ of lubricating the bearings, gears and coil tuning guides and screw drive does not cure the problem.

Testing the Replacement Unit

Before Stripping

Before Stripping

This can be achieved before replacing the coil pack by applying 12 volts across the motor terminals to drive the slugs in and out of the coils to check operation.  Also VERY lightly oil the sliding guides, the gears, bearings and the screw drive system.

Out with the old….

Dismantled Unit

Dismantled Unit

Removing the Selector unit as before (2 screws top and bottom, unplug the 4 coax leads, unsolder the link wires under the coax leads, and unsolder the tag strip connections to the PSU – (Or remove the PSU and tag strip from the chassis and move them to one side).

 

 

Another View of the Dismantled Unit

Another View of the Dismantled Unit

 

 

 

 

 

 

 

Remove the outer case completely from the internals and removed the small PCB with the relays fixed to the unit.

Connections to the Coil Assembly

Link Wire

Link Wire

Removing Wires.  The coil assembly is connected by very few wires;  2 to the motor ( note the + and – connections), 4 to the coils themselves (note which wire goes where) and 2 wires to the limit switches, one by the motor, the other via a feed-though terminal to the PCB.  When this link wire is removed from the coil pack, fit it to the new coil pack ( see photo).

 

In with the new…

New Coil Pack

New Coil Pack

Refitting.  Refit the coil wires, motor wires, switch wires and the small PCB with the relays, and the main PCB to the Coil Assembly.  GENTLY refit the Ledex Drive and Coil Assembly into the bottom case.  Be careful with the 4 wires to the coils, tweak them gently into position.

 

 

Screw the Coil Assembly and Ledex Unit back into the case.

Coil Pack Installed

Coil Pack Installed

Check all wires you removed have been replaced where they should be and have not come adrift as they are all single core and quite fragile.

Refit the top case and check all screws are tight.

 

 

 

Optional Test Rig

Test Rig

Test Rig

This bit is optional – I made up an extension cable using some flat ribbon cable from an old floppy drive cable from a scrap PC, there are 12 wires from the Selector unit to the tag strip, see photo, I then connected the selector unit to the tag strip using the ribbon, plugged in the 4 coax connectors, I used a mouse mat to insulate the selector unit from the chassis and checked everything.

 

I then powered up the set and checked that the Mhz tuning operated the Ledex and the Coil Assembly from 1.5 to 29.5 Mhz in 1 Mhz steps, it worked ! – end of optional test.

Re-Assembly

Refit the selector unit in the chassis, re-make the 12 way tag strip wires, plug in the 4 coax cables and re-fit the PSU.

Check the 12 wires AGAIN to make sure they are in the correct sequence.

Switch on and test the full Mhz tuning range 1.5 to 29.5 Mhz in 1 Mhz steps (slowly !).

When I carried out this repair I was happy to say everything worked.

General notes:

Hopefully if I need to repeat this with the other set that will be somewhat easier than this set, it took me several days to complete, as I had problems with a dirty Mhz switch which was not sending the correct signals to the Ledex De-coder to tune correctly.  This took a day to sort out by itself.

Leighton.  GW3FSP