If you are doing hand solder replacements of SMD capacitors you should
make sure you follow procedures to either pre-heat the component or take
extreme precautions to guarantee that the soldering iron does not touch
the capacitor electrodes. Thermal shock with cause the metal endcaps to
crack the ceramic which will eventually cause high leakage or shorts in
the capacitor.
Seeing that you have found two SMD caps on the same board have failed
then I am suspicious of the fabrication of this particular board.
You might want to reference
<www.kemet.com/Lists/TechnicalArticles/Attachments/49/F9207%20KEMET%20Hand%20Soldering%20Procedures.pdf>
There are other stresses that can cause the capacitor to crack and fail
such as board bending or contaminants under the part, BUT for home lab
work the primary reason is thermal shocking during hand solder.
de Dave K5MWR
On 11/14/2019 00:37, Martin Sole wrote:
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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