LSB Conversion of RT321 and RT320 HF Sets


The RT320 and 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 size and 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 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:

The pinout is:

Output Enable (Active High)Pin 1
Ground Pin 2
Output Pin 3
Supply +5V Pin 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) if interested.

Modification details

I should prefix this section by stating that I have not personally modified any sets (on the basis that mine aint broke so I will wait til I really need LSB or have need to open them for repairs). 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, 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:

R1   1K series
L1   56uH series
C1   150pF shunt
L2   180uH series
C2   150pF shunt
L3   56uH series

(This diagram needs a monospaced font like courier)

                   |       |    
From SG8002        C1      C2       To Mixer
                   |       |

A possible circuit diagram for the oscillator is:

(Click to view full size version)

Switching LSB and USB

Various switching strategies have been used including


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:

5V 7a pin 8
0V 7a pin 1
CIO ENABLE +6V 7a pin 14
1.7500MHz CIO out 7SK1

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


This page is based on earlier work by Terry G4BFS, Alain Betemps, Mike Hyers, Colin Guy, Iain M0YMK, Andy G4JAC, John Moore, G4CXT and others - apologies if anyone feels I should have credited them.
Copyright ©2010 Iain Moffat G0OZS
Last Updated 05-Oct-2010