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> A few days ago I posted the below message but had not had time until
> today to dig a little deeper. I took as my starting point the loss of
> both receive and transmit as an indicator that the culprit would
> likely be something common to both and given the lack of receiver
> noise whilst the P3 panadaptor showed normal signals and band noise
> suggested it would be after the first conversion.
>
> This suggested something in the second LO as a likely possibility and
> with the first intent being to measure the oscillator signals. I used
> my SDRplay RSP1 with the Steve Andrews spectrum analyser software as
> this was handy and is easy to use. BTW, this is a great tool if you
> have any of the RSP devices. Going to the KREF3 board between the
> synthesisers and probing the 49.380MHz reference showed a healthy
> signal close to 0dBm, as expected since the P3 confirmed tuning works,
> so I moved to the output of U2 the divide by 6, hmm, nothing, nada,
> zilch. Worries of a dead 74VHC175 with no replacement on hand was the
> first thought but rapidly followed by my knowledge that this board had
> not so long ago suffered a capacitor failure causing loss of the 7
> volt transmit bias.
>
> With the KREF3 on the bench a quick measure from the 5 volt line to
> ground showed 82 ohms after L2, 87 ohms between L1 and L2 and about 91
> before L1. Way too low for sure. There are 3 components here C7, C19
> and U1. Having had a 0.1uF fail on this board already I was ready to
> bet the farm on either C7 or C19. C19 was the first to be removed and
> I hit pay dirt right off, shoulda bought that lottery ticket today!
> Another dud 0.1uF then. Measuring the 5v to ground showed it now at a
> much more respectable 1.3k ohms. Replacement was with a 1206 0.1 as I
> had done earlier with C6.
>
> Normal service is resumed. It's all back working as it should but I do
> wonder now about the possible longevity of the other 0.1uF capacitors
> both on this board and possibly in other parts of the radio. I count
> 12 more on this board alone.
>
> I should mention that my K3 is an early unit, S/N 298 so I presume
> circa 2008-ish. More than 10 years is not bad in my book and the
> faults have been fairly straight forward. It might be surface mount
> but it's eminently repairable.
>
>
> Martin, HS0ZED
>
>
> On 12/11/2019 19:48, Martin Sole wrote:
>> Hi,
>>
>> My K3 is quite an early unit, S/N 298. Recently upgraded with new
>> synths KRX3A and KXV3, I also have a P3 connected to the radio. A
>> while back it failed with a shorted smd capacitor on the K3REF board
>> causing loss of TX bias voltage. Today it has lost both receive and
>> transmit and I am thinking it might be from the same module. Here's
>> the situation.
>>
>> Rig powers on and appears to work normally except soft noise from the
>> speaker, no band noise to speak of and no signals. P3 shows plenty of
>> signals, S9 plus on FT8 freqs but nothing heard other than soft
>> noise. Tuning works as signals move in the band scope. No sub
>> receiver signals either and no TX output. Have not checked
>> transverter output but expect to find nothing there.
>>
>> P3 take off at IF output so everything is clearly working up to
>> there. DSP seems to be working as width and shift cause the expected
>> changes in audio. S meter reads dead zero, no movement.
>>
>> About the only common point I can see is the K3REF board so it would
>> appear a second failure on this module. Since this is where I had to
>> do earlier work I wouldn't put it past being a user inflicted fault,
>> but that repair was a month or more ago.
>>
>> Anyone care to have a guess or share thoughts?
>>
>> Martin, HS0ZED
>
> Earlier
>
>
> Hi,
>
> So I had some success in fixing this and thought I would write it up
> here briefly for future reference.
>
> Short version
> C6 on the K3REF module had become a 12 ohm resistor.
>
> Longer version
> Finding the 7T line only going to 3.5 volts or so was suspicious.
> Measuring the resistance to ground of that line showed about 20 ohms
> where the 7 volts is generated at U11 on the RF board, very
> suspicious. The first step then was to remove any removable item that
> has a connection to the 7T line. That's most modules in the K3.
> Checking the resistance of the 7T line each time I removed a plug in
> module showed the fault to be somewhere on the K3REF module. The 7T
> line on this board goes to Q3 via R16, R8 and R9 and also to Q2 and U3
> via L5. Measuring either side of L5 showed lower resistance on the Q2
> and U3 side, now around 12 ohms as opposed to 14 or so at pin 3 of
> P75. Focus then was directed at Q2 and U3 as likely culprits being
> active devices likely more prone to failure but removing Q2 showed no
> change nor did very carefully lifting pin8 of U3 and confirming no
> connection to L5. That left C6 as the remaining component on that line
> and its removal restored a much healthier 2.7k resistance to ground on
> that line as formed by R16, R8 and R9 around Q3 noted above.
>
> I don't have any of the microscopic 0.1uF capacitors used in this
> location, are they 0402? but I have a good selection of 1206 parts and
> being smaller than the chokes on this board I considered it a suitable
> change. The board mounts very close to the chassis so size is critical.
>
> All restored and the rig is back to normal working order. I don't know
> if these capacitors are prone generally to such failures but at least
> it was on a plug in board so easy to narrow down.
>
> So that was a fun Saturday afternoon for an hour or so, hopefully the
> radio will keep on working well now.
>
> Martin, HS0ZED
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