Iain Moffat's Radio Website

view edit history print

only moffatig.plus.com

Site Menu


Military Vehicles

Photo Library
Google Search
Update Log

[ Contact ] [ Privacy ]
[ Cookies ] [ Security ]

The owner of this site is not responsible for the security, privacy and suitability of pages you reach by clicking on links to other sites found in these pages. These are the responsibility the owner and contributors of the respective sites.

Main » LSB

LSB Conversion of RT321 and RT320 HF Sets


The UK/PRC-320 aka RT320 and UK/PRC-321? aka RT321 Clansman HF sets are designed for Upper Side Band (USB) operation only. This conflicts with the Amateur radio convention which is to use LSB on bands below 10MHz. In both radios the IF used for SSB generation is 1.7500 MHz and there is a sideband inversion between the IF and the antenna. So LSB at the IF comes out as USB and USB at the IF comes out as LSB. The IF filter passband for SSB is roughly 0.2 to 3.0KHz below the IF frequency so the 1.7500MHz carrier is blocked leaving pure LSB.

To get LSB at the output it would be necessary either to change the filter to a USB one or to move the carrier below the filter passband. The latter approach is what was first documented by Dr. Andrew Smith G4OEP in his excellent article now available on the VMARS Manuals web site:


A crystal at 3.2KHz below the original 1.7500MHz = 1.7468MHz is needed for the G4OEP LSB modification. Unfortunately the one-off cost of a 1.7468MHz crystal is now somewhat of a problem and other cheaper solutions have been sought. One is to use a programmable oscillator comprising a VHF crystal and a PLL to generate programmed arbitary frequencies. Such an oscillator can replace the transistor CIO in the G4OEP circuit provided a suitable (5 V TTL) power supply is available and the output is adequately filtered and at a level compatible with the radio.

The SG8002

A suitable oscillator is the Sieko-Epson SG8002-JAPTB which covers 1 to 125MHz and has a 0 to 70 degrees C tolerance of 50ppm (so would be up to 87.5Hz off - not a problem for SSB but would affect narrow data modes). At room temperature samples have measured within 5ppm of the programmed frequency. Physically it is a J-Leaded plastic package about the size of an 8 pin DIP op-amp. The data sheet is at


The programming details are as follows:

CaseJA(Plastic J-Lead)
Pin 4POutput Enable

The pinout is:

Output Enable (Active High)Pin 1
GroundPin 2
OutputPin 3
Supply +5VPin 4

Important Note: The Output Enable is active high - I misread the data sheet when preparing the original suggested circuit diagram. This has been pointed out by G4CXT and John Moore.

Last time I bought some they were £1.36 each for 50, with a programming charge of £25 and carriage of £10 making an effective price of £2.06 plus postage for programmed units.

I still have some left - please inquire at (my casllsign) @btconnect.com if interested.

Modification details

I should prefix this section by stating that I have not personally modified any UK/VRC-321 sets (one of mine was converted when I bought it and the other is not, so I will wait til I need to convert the spare). I have done UK/PRC320s using the G4IQE/G3TPJ board.

I have coordinated a bulk purchase of SG8002s and several people have had success with the conversion on both RT320 and RT321. Critical factors for success are to inhibit the 1.7500MHz built in CIO when using LSB or have a changeover relay with good isolation (otherwise it appears as a 3.2KHz tone on the transmitted LSB signal) and to set the output level of the 1.7468MHz signal to match the original 1.75MHz carrier.


The RT320 modification is as described by G4OEP except that the 1.7468MHz transistor CIO is replaced by the SG8002. A small 5V voltage regulator is required which should be fed from the 12V supply or the 24V raw battery feed available in the "Battery Check" switch position, since no 5V logic supply exists in this set. The 5V TTL output of the SG8002 must be filtered to get a sine wave output. A suitable filter design is:

R11K series
L156uH series
C1150pF shunt
L2180uH series
C2150pF shunt
L356uH series
(This diagram needs a monospaced font like courier)

                   |       |    
From SG8002        C1      C2       To Mixer
                   |       |

Ian M0YMK found a 22K series resistor between the oscillator output and the mixer achieved suitable levels and there was sufficient pre-existing filtering.

A possible circuit diagram for the oscillator is:

(Click to view full size version)

This is similar to the board designed by Ollie G3TPJ and marketed by Dave G4IQE. My article on fitting this board using the "Battery Check" switch to enable LSB is at http://www.g0ozs.org/clansman/rt320/lsb/subalbum_1.html until I get round to importing it to the wiki.

Switching LSB and USB

Various switching strategies have been used including

  • The use of one headphone output as described by G4OEP
  • Use of the batt check or freq check front panel switch positions
  • Use of a reed switch inside the 320 case operated by an external magnet as used by G8LBS(SK)


The RT321 1.75MHz CIO appears on SMB socket 7SK1 at the bottom of module 7 (1st LO) and suitable power and control signals also exist on pins of this module. The circuit internally to module 7 is a TTL output with 10nF and 1K in series so the same components in series with the SG8002 output should be satisfactory.

The other relevant pins on module 7 are:

5V7a pin 8
0V7a pin 1
CIO ENABLE +6V7a pin 14
1.7500MHz CIO out7SK1

There is a version of the G4IQE/G3TPJ board for the UK/VRC-321 and Leighton GW3FSP has published an article on its installation for the UK/PRC-321? at http://5820-99-114.com/TCRU/?p=423.

Switching LSB and USB

As with the RT320 a number of switching approaches are possible including

  • A push or toggle switch in the Elapsed Time Indicator position on the front panel
  • Decoding the frequency switch inputs to module 7 to select LSB for 3 and 7MHz - these are on 7a pins 16, 15, 24, 23, 21 and 22 in order. I don't have a decode for these to MHz so further investigation is needed.
  • Use of the left or right headphone output on one audio socket
  • Use of harness or control socket pins for an external switch

Because the ETI contained hazardous material (mercury) and was (or should have been) removed prior to release from the army I can see no reason not to use the ETI socket - for those who want perfect cosmetics the ETI socket was a fuse holder and simply fitting or removing the fuse can enable or disable LSB if the internal connections are used to power the oscillator.


This page is based on earlier work by Dave G4IQE, Ollie G3TPJ, Leighton GW3FSP, Terry G4BFS, Alain Betemps, Mike Hyers, Colin Guy G4DDI , Ian M0YMK, Andy G4JAC, John Moore, G4CXT all based on the article by Dr. Andrew Smith G4OEP, and others - apologies if anyone feels I should have credited them.

Copyright ©2010-2017 Iain Moffat G0OZS First Published 05-Oct-2010; Last Updated 16-Dec-2017.

[ Radios ] [ Antennas? ] [ Audio? ] [ Batteries? ] [ Connectors? ] [ Harness? ] [ Other? ] [ Links? ]

Page last modified on January 24, 2018, at 10:56 PM