This is the continuation of fixing the FT847 VHF second channel fault that we’d tracked down to a faulty switching transistor pair, Q2016, that provides the supply rail for the VCO, MO2002.

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John received a few of the FMC3 transistor pairs from AliExpress and with much profanity and considerable magnification I managed to replace the duff one. I found it best to simply crush the old part in-situ then de-solder the remaining fragments, clean up the board and put the new one in place.

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It seemed we’d solved the problem as we could see the voltage switching at the output pin 2 of the 5 pin switching device. Unfortunately the voltage never reached the VCO....mystery.... there is only a length of track and 100 Ohm smd resistor between the two parts! The track checked out ok as did the resistor, so where the heck did the voltage go! There had to be a short at the VCO end grounding that supply line. There are only 3 components that could be causing this (I think), the VCO, C2061 10uF smd cap and C2063 .001uF.

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Well, at this stage we gave up for the day, it was time to put the board back and take the wife out. We’ll continue the fault finding later this week.

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I’m just hoping it’s not the VCO, but I fear the worst.

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Paul, G0odp

Well guys, we’ve cracked it, with all your help and suggestions.

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The culprit was indeed the filter electrolytic C2061 connected to VCC on Mo2002 VCO. It had gone short circuit. I reckon that had dragged down the switching pair Q2016 which in turn had failed.

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Anyway, I’ve managed to work John on 2mtrs across the shack..... it’ll never be a candidate for the million-miles-per-watt award but we are dead happy.

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Many thanks again all for your help.

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Paul, G0odp and John, G0KFM

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