(This article appeared in "Six News" Issue 59 and "Radio & Communications" April 1999)
Is your 706 not transmitting on FM properly? This maybe why…
Having taken my Icom IC-706 back for service twice before for the same fault, I was not prepared to be inconvenienced a third time! As it turns out IC-706s all over the World have been exhibiting the same fault. I’ve had e-mail from other unhappy IC-706 owners as far away as the U.S.A. and the U.K.
Upon checking the previous Fault Reports I noticed that the FM VCO had been tweaked.
Hmmm… There is a 9MHz VCO active during FM mode only which is modulated from the mic amp.
Fault condition symptoms are;
- Very low Output power (enough to be heard on a nearby receiver on the shack bench but that's about it).
- Carrier frequency is NOT what the dial indicates, typically 5 - 30kHz on the low side.
- Carrier tends to wander and sounds spurious.
The culprit
I discovered that C267 a miniature SMT trimmer capacitor located – see picture - 9mm to the right of the 9.0115 MHz IF Filter on the Main Unit was not re-flowed to the PCB to a satisfactory standard. There was insufficient solder paste applied during the printing process and the end result being a poor solder joint and an intermittent 9MHz VCO. After re-touching both pins of C267 with fresh solder it's performed faultlessly since.
Although access to the offending area is easy you will need a very fine tip on your iron to get to the trimmer pin on the filter side without damaging the capacitor itself and disturbing two nearby 0603 chip capacitors.
I cannot stress enough to successfully carry this repair out you need a very very fine tipped soldering iron, and ensure the tip is in good order.
Equipment Required
You’ll require the following tools;
- Small Phillips-head screwdriver.
- Accurate Frequency Counter.
- (Another receiver will do).
- Dummy Load, Power Meter & coax patch leads.
- Soldering iron with very fine tip.
- Fine solder 0.7 or 0.5mm.
- <![endif]--> De-solder wick if required.
NEVER EVER use a plunger type solder sucker on SMT boards. The re-coil action can damage boards and parts.
Procedure;
- Turn transceiver off and disconnect all leads. (Obvious?)
- Using a Phillips-head driver remove top cover only.
- With radio upright & front facing towards you (ie; normal operating position) the MAIN Unit PBA is the front board.
- Locate the 9.0115 MHz IF filter and then C267 (identified on silkscreen) 9mm to the right of the filter.
- Using fresh solder, carefully re-touch up solder joints to both pins of the trimmer cap.
The filter side is difficult to solder due to nearby capacitors and the fact that the ground plane will sink a lot of heat. This is the pin that is usually not soldered properly ex the factory.
- Inspect solder joints carefully, particularly the ground plane side.
- Re-connect DC Power lead.
- Connect Dummy Load via a PWR meter.
- Turn the transceiver on, select FM mode, and select a convenient test frequency, ie; 29.6MHz.
- Confirm TX carrier frequency is correct against Frequency Counter (or another receiver).
- Check Power O/P is appropriate to the PWR setting in the menu.
- If all ok, turn off and replace the top cover.
It is most unlikely you will need to tweak C267.
The setting should still be correct, otherwise alignment is not that critical, just set it for centre of lock range.
Needless to say, any work performed on your transceiver other than by an authorised Icom service agent may void your warranty, if that is still applicable.
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