Jun 22, 2008

HB0- summer 2008 preparation; IC746 Mods

The Icom IC746 which we will use on 50Mhz during our trip to Liechtenstein (HB0) next july, needs some work:
  • Cleanup
  • IC151 modification
  • Master LO modification
  • Re-alignment

Yeah sure, but let's wait untill we return from our HB0 trip. If you can find the dials and read the screen you can make qso's right? Its a bit dusty, nothing to worry about now.

IC151 Modification
The IC-746 suffers from potential thermal failure of IC151 resulting in no RF output. During normal operation this ic dissipates some 250mW continuously, where it is rated at 330mW maximum @25deg C. Its tiny legs become the heatsink in practise.
See the small MMIC red marked in the lower left side.
5 pins (3 left and 2 on the right) are GND connections.

Just Google on IC151 and you will find many ways to rework this issue.
I found a small thin piece of copper which served as an internal cabinet ground contact of some other rig. After a small modification and carefull cleaning it was ready for its new job; act as a heatsink.
I resoldered all pins of IC151 and carefully soldered the copper plate to the 3 GND pins, making sure its flat bottom was seated against the cleaned topside.
At this moment I'm testing the rig and it works fine. After some 10min now I can feel that this copper plate gets warm, that means it acts as a heatsink. One job done.

The small solder 'flag' was removed after the picture.

Master LO Modification
The reference Oscillator has its crystal mounted in the middle of an open (!) metal cabinet.
Any quick temperature change (like during TX, when the large internal fan accellarates) will potentially result in a short-term frequency offset. Sure any (optional) TCXO will handle that, but there is a cheaper solution. Just stuff it with wadding from a first-aid kit. That is all.

Alignment can better be done when more time is available and the rig & measuring equipment are warmed up properly.

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