A VRC 321 Side Step Modification (Removing the +2Khz Frequency Offset) by Leigton Davies, GW3FSP

Background\Purpose

In military service the clansman HF sets used an offset of +2khz while in USB, CW (W) and CW(N). mode.  In AM the dial frequency is the carrier frequency.

The purpose of this modification is to remove the 2Khz frequency shift when using SSB and the CW ( W ) and CW ( N ) modes.  This has the advantage of the operator being able to see the same frequency on the 321 as QSO partners using an amateur rig.

This article has been revised to correct wire colour errors for the CW mode select diode placement. revised 12th Jan. 2014.

LSB Mod

I am assuming that the LSB modification is the same as I have previously posted, if not you will have to provide a suitable switch to operate this modification.

Going in…

Photo 1

Firstly you remove the set from the case , remove the 4 set screws holding the front panel in place and open up the front panel, looking at the rear of the front panel on the left hand side of the PA module is the Miscellaneous function PCB.

The mode switch wiring is routed to this in order to operate the mode settings.

In photo 1 above you see the DPDT switch I fitted to operate the LSB board, one section of the switch is unused, this section will be used to switch the side step on and off, connect 2 wires to the NORMALLY CLOSED contacts of the switch, feed the wires between the PA module and the front panel and pull the ends through to the side of the Misc function PCB where the loom wires connect to the PCB.

 Function Switch Connections

According to the circuit details in the EMER the function switch connects to the following terminals on the PCB:

  • AM – Pin 16.
  • SSB – pin 19
  • CW (N) and (W) – pin 20.

The CW mode switch positions are linked together at the switch.

However the numbering of these pins is ambiguous, so I removed the PA module and chased the wires from the mode switch to the PCB while noting the wire colour codes which are:

  • AM – White with Red trace.
  • SSB – Red with Black trace.
  • CW modes – Green with Brown trace.

 

Photo 2

Photo 2 – This shows the 2 diodes connected to the SSB and CW mode wire terminals, the negative end of the diodes connected to the PCB. I cut the wires to about ¼ inch from the body of the diodes, one diode to the Red/Black wire the other to the Green/Brown wire,  then twist the unsoldered diode wires together tightly and cut off about ½ inch from the diodes, solder them together.

Photo 3

Photo 3, right, shows the 2 diodes connected ( the black band of the diode to the switch wire ) to one wire from the switch, the other wire from the switch is connected to the White/Red wire for the AM mode.

 

 

 

Photo 4

Photo 4, Left, shows the completed wiring with sleeving pushed over the diodes.

Closing Up

Position the diodes and wiring to make sure they are not trapped when you close the front panel, replace the 4 set screws and tighten. 

 

Testing the Set

Tune to a convenient frequency on AM, monitor your transmission on a separate set switched to AM on the same frequency, you should be spot on frequency, switch both sets to USB. You should not need to adjust tuning on either set to hear your transmission, switch the 321 to CW (W)  and press the pressel, you should hear a 2Khz tone ( this is the 2Khz tone used to generate the CW signal ) switch the 321 to CW(N) and repeat the test, again you should hear the 2Khz tone.( The 321 uses USB for CW modes ). Set the 321 to SSB and switch to LSB ( if fitted ) and LSB on the monitor set,  when transmitting on the 321 you should be about 1Khz off frequency as the LSB mod needs to be 1Khz off frequency, tune the monitor set to resolve the signal.

As a further test, tune the 321 on SSB ( USB ) to a known transmission ( I used Shannon Volmet on 5.505Mhz )  you should hear a clear transmission, now switch to CW ( W )  you should still hear a clear transmission without needing to retune.

Mod done, box up the set and relax!.

Leighton. GW3FSP.