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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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

Thanks Lawrence, yes I totally agree, I was able to get a little way in solving the problem, BUT, I needed the help of others to really understand what was going on. Also I have to say that using the oscilloscope and spectrum analyser really helped my see the issues, I could clearly see the voltage transitions…… so good friends and good test equipment are both essential!

Paul, G0odp

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I would like to add that I truly enjoyed this conversation.? I have learned quite a bit about our rig. It is also reassuring that there are such knowledgeable and helpful people out there.? Thank you.

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I am cheering for you and hoping the champaign is good.

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That’s a good point Nils, no I haven’t lifted the board but I have been checking voltages at both ends of the relevant tracks. I’m going to call Yaesu today to see if they have that part number, thanks for the tip. I was checking the circuit of the FT817 and I see that used similar technology with a transistor pair switch but a different part , so they could well have that part. (no….. Ive just checked, they don’t have it!)

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We’re not opening the champagne yet, but the bottle is going in the fridge to be ready!

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Do you happen to have the default settings for the jumpers around the battery, the ones used to wideband the rig? I can see the soldering around there is messy, it hasn’t been widebanded….or possibly it was and then it was reversed…. Either way the soldering looks very dodgy!

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Paul, G0odp

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Getting VERY close now.

Today we double checked the voltage on TP2002 for the VHF VCOs and as before it seemed fine for the Low VHF band (up to 139.99MHz) but clung to 7v for the high VHF band (>140MHz).

So, we then checked the switching voltage actually at both ends of the tracks, so at MO2001 and Q2015, they were fine switching at the 139/140MHz boundary. HOWEVER, the MO2002 and Q2016 pair did not show a changing voltage at the 139/140MHz boundary. So we checked the other side of Q2016, pin 4 and we could see the switching voltage there, so using our great powers of deduction (yeh right!) Q2016 has to be a bad-un!!!!

This is an FMC3A (or FMC3AT148), so we are checking suppliers now and will probably use Aliexpress (everyone else want an order of 3,000) sadly with a few weeks delay, but it’s a cheap solution and far better than having to source the VCO!

I guess our big hope is that the VCO is working fine and that it didn’t cause the switching transistor pair to blow....fingers crossed.

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Guys, thanks again so much for all your support and my local friend Tex for explaining the mysteries of the PLL circuit.

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I’ll update you when we replace the part and try the radio.

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73. Paul, G0odp

Nils, Charles, thanks again for your replies, we’ve stopped work for today and will probably not start up again until next week, we only have 2 good eyes and 1.5 steady hands between us so progress is slow!

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It’s a good job you cannot see my red embarrassed face, no I haven’t checked the battery, I’ll do that next!..... I’ve just checked it, it is 3V ok..... phew!

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We did do a service reset which didn’t make any change.

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We didn’t listen to the receiver during today’s test BUT we did listen the other day and as I recall the high end of the band (140MHz +) was very quiet as if no L.O. was being applied..... In fact I’ve just remembered there was no local oscillator signal on the output coax, we monitored this on the spectrum analyser and could quite clearly see the signal drop as we passed 139.99 MHz. I will check that more thoroughly next time. I will carry out the rest of the tests as well next time.

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As you will be aware the PLL board is buried under the chassis and beneath another circuit board so it is quite a balancing act to hold the test probes, hold the board open adjust the rx tuning and the ‘scope......?? you need octopus hands! But we are having ‘fun’ slowly tracking through the stages.

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Thanks again fellas, have a great weekend and for sure I will update you with our findings and if any of you get over here when this damned virus has gone I will buy you a pint or 3 of best English beer.

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Best regards

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Paul, G0ODP

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When it jumps to 7 the VCO is un locked

I think 2 v is locked OK

Compare with a working band

Wd8axa

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Nils, we did some checking of the voltage to the VCO on the PLL board as you suggest by looking at TP2002. On VHF band 108-139.99 the voltage varies from 2 to 6 volts measured (roughly) on my digital scope. When the radio is tuned passed 139.99 the voltage jumps to 7v and remains at that through to the end of the band and then switches back to 2v at 108MHz.

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Nils my technical knowledge is limited but I wonder what generates the variable voltage, is that done by the CPU or by the registers. I guess I am hoping that the failure is with Q2016 the switching transistor pair!

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My next test was going to be to check the voltage on pin 12 of Q2027 to see if that is varying, but before doing that I wanted to seek opinions. If Q2027 only switches the varying voltage between pins 4-7(HF) and pins 14-11(HF-VHF-VHF-UHF) then the fault could be back at the processor!

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At this point I’d really appreciate your thoughts as to the best next step.

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Sincere thanks again.

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Paul G0ODP and John G0KFM

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