[Elecraft] K3 died again and fixed

[Martin Sole]

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