[ICOM] IC761 problem

[Frank A. Ellis]

Hi Alan,

OK, great, I came in sort of late on this one and thought I would throw in what I knew in hopes it would help get you going in the right direction.

The PLL is partially comprised of (4) VCO's, and each VCO has it's own trimmer capacitor. If a trimmer is bad (the infamous problem) there will be usually be a couple ham bands affected. Each VCO has a frequency range, and between all (4) of them the whole range of the radio is covered. There is data in the alignment section of the manual where one could figure which VCO is bad, depending on the affected bands.

Since you are apparently having trouble on all bands, trimmers or VCO's are probably not the cause, as all of them would have to be bad.

A few key voltage measurements will reveal whether the PLL is locked or not. There are a couple known problem areas in the PLL that are listed in the troubleshooting areas of ICOM America's web site. The info for the IC-751A is also directly applicable.

I'm hurrying off to work now, so good luck with your troubleshooting, I will check the thread later in the day.


73 Frank W3UHF








On Aug 19, 2010, at 3:29 AM, zl3kr wrote:

> Hi Frank,
> many thanks for that explanation, that was the main thing I wasn't sure 
> about with my limited
> knowledge whether the PLL was involved with the read out.
> So with the symptoms that I am getting I now believe it will most likely 
> be the PLL ocillator
> that has dropped out or some component in the main oscillator has failed.
> My first thought when it was failing, after repairing the power supply 
> was that the voltage
> output of the power supply may have been too low although I was sure I 
> had adjusted it correctly, but
> I went back and rechecked that and it is very close to 13.8 v so no 
> worry on that score.
> I will have to get the manual out again and re-read about setting up the 
> PLL so as I can check
> the various voltages, especially if there is one that affects all bands 
> as it is not working on any of them.
> 
> Alan ZL3KR
> 
> 
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73, Frank A. Ellis, W3UHF