G4HUP/DL4MUP Modifications

GPS Locking

IC706 MkII

Fan Drive Modification - Frequency Stability
Spectran plot of IC706 before modification - scale tic marks on left of screen are at 30 sec intervals
Same signal - after modification

I had known for many years that the IC706 had a temperature related frequency stability problem in CW and SSB. This is not a major problem - the drift cycles by about +/- 30Hz over a 2 minute period as the fan is switched in Rx mode, as you can see from the 'before' plot above. In normal usage this drift is only just enough for the ear to detect - if you know what you are listening for - and would not be an issue for CW/SSB QSO's. I found this when using FFTDSP to monitor GB3MHS when I was building it in 1998.

The two comparison pictures above were captured using Spectran while monitoring the DB0AAT beacon on 23cm.

Once identified and understood,I didn't really worry about it - however, with an increasing interest in JT44/65, I decided to do something about it!

An internet search lead me to IC 706 mods , where I found the problem described and a solution given for the 706MkIIG. Instead of having the fan switched, it is run at low level permanently while on Rx - this is enough air-flow to prevent the need for it to be switched on at normal temparatures. A 220R resistor bleeds enough current to run the fan - I used a 5W type because it was available - I would not recommend going below 1W, although calculation says a 0.6W should handle the power

However, on opening the bottom cover of the rig, I discovered that the MkII has a different location of components on the PA board as compared to the MKIIG, so the pictures referenced were not a lot of help! For the MkII the correct point to pick off the +12v switched is shown in the picture below - the same circuit point as for the MkIIG, but a different physical location on the board.

One end of the resistor is connected to the +12v switched line, picked up from the top of the ferrite choke. The other end is attached to the + line to the fan - a small white two pin connector is visible in the left hand picture below - just to the side of the fan. Carefully form and solder the resistor lead to the tab corresponding to the + (red!) connection of the socket. There is not much space to bring the lead up from the connection point, so I would recommend insulating it to avoid any risk of shorting the +12v to the chassis due to vibration etc. Be careful about where you position the body of the resistor, so that any heat generated does not cause the insulation on other cables to be damaged.
View into PA compartment of IC706, showing resistor position under the wires
Another view into PA compartment of IC706, showing where the switched +12v is picked up

IC706 Mk2 GPS Locking modification

The master oscillator in the IC706 series rigs is 30MHz. By replacing with a GPS derived signal, the stability and accuracy of the rig is greatly increased. The modification is simple, but requires a bit of care.

You will need:

Follow the IC706 manual instructions as if you are installing the High Stability oscillator option. Cut the leads to L35 and R135 as instructed - this removes the supply voltage from the standard oscillator, and isolates the output. If your rig already has the High Stability oscillator installed you will need to carefully remove it. You will see a row of four pads across the oscillator location on the PCB. The 30MHz signal must be connected to the right hand pad, with the ground from the coax on the next adjacent pad. See the picture below, and note that there are two ground vias very close to the signal pad (only about 1mm away!), so care is needed in making the fixing to avoid shorting out the signal.

Prepare the free end of the cable to attach to the pads. Feed the coax in through the rear panel vent hole, as shown below and lead it to the front of the rig. Solder the cable ends to the pads. Replace the bottom cover and test.

Note that this modification, as described, is not switchable - you will need an external source of 30MHZ to use the rig at all times. A switchable modification would be feasible, by using a small relay, and putting DC on the coax to operate the relay using bias tees at each end - have a play!

Caveat Emptor This information is provided in good faith, and has worked successfully on the IC706Mk2 sample I was loaned to carry out the tests and validation on (thanks to Martin, M0BCT, for being brave!). Whilst the mod is not difficult, it does require working in a confined space, and there is potential to damage other parts if care is not taken - if you do not feel competent to perform this modification, then seek assistance from a friend!

All variants of the IC706 (Mk1, Mk2 and Mk2g) use 30MHz as the standard master oscillator, so the principle of the mod should be equally applicable to all - however, there may be variations in physical component locations between the versions. If so, follow the principles and use the manual and service manual to guide you.
High Stability oscillator mounting pads - note the 2 ground vias, very close to the signal pad
Routing the 30MHz cable out through the vent hole - an alternative would be to bring it out under the Tuner 4 way connector, but this would need a notch filing in the case

Results

Before modifying the the rig, I set it up to receive GB3VHF, which is a GPS locked beacon on 2m (144.430MHz). Using JT65B I observed the rigs stability and performance overnight. To receive the signal correctly, the rig was tuned to 144.42901MHz, and this gave a initial DF error on JT65 of -11Hz. Over the period of measurement, as the temperature varied, the DF error increased to -89Hz at around 0500, and started to converge towards the initial error again as temperatures climbed.

The modified rig was set up to repeat the test - this time the frequency indication of the rig was 144.428490MHz, and the JT DF error shown was 3Hz. Left overnight the error remained constant.

Sources of Error

Of course, the rig is not the only frequency sensitive item in the chain. The measurments were carried out with a PC and Joe's WSJT software. The accuracy of the PC clock, both short term and long term, since these tests were conducted overnight, will have an impact on the results. My shack PC, on which these measurements were made is time controlled by D4 internet software from Thinking Man, and is set to register every 30 seconds, so I believe that the result given here is valid, and that we really are talking about the variations in the IC706 being the only significant ones. The GB3VHF beacon is GPS controlled and my 30MHz source is also GPS locked.
Initial JT65 reception - note variation in DF
JT65 reception of GB3VHF with external GPS 30MHz - note DF is now constant
lower part of screen - GPS controlled, upper part reversion to internal oscillator

page created 08.02.04 page last updated: 07.07.08