Recently
a fellow ham sent me a QMX rig to fix.??This is to be my 100th
QMX that I have fixed - assuming that I accomplish that with
this one.???I
wanted to share the journey.??I found this one
interesting.??This
is a Rev 2 QMX with rev 14 firmware.??Wound for 12V.??This is the first part
of a story - the diagnosis and how I arrived at it.??I wanted to share
because this is more interesting than most and it may help
someone else.??More
will be posted later.
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?My
usual routine with a new rig is to disassemble, inspect,
clean??and inspect
again.??I use a 10x
stereo microscope for everything with very bright side and
overhead lights.??I
scrape flux off, use 99% isopropyl alcohol to clean with an
extra soft toothbrush.??Then
I inspect again.??I
can fix almost all rigs this way.??There are always
solder blobs, solder balls, cold solder joints in every rig
that comes to me.
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This
rig came in very clean.??No
excess flux, no bad solder joints, no solder splashes.??It did have a few long
pins that ended up shorting to the case, adjacent boards and
wreaked a bit of havoc.??I
fixed those before proceeding.
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After
applying power at 7V with current limits, it came up only with
top row black boxes.??No
backlight, nothing on PUTTY.??Looking with a scope I
could see the 3.3V rail was good and 3.3V PWM was within
range.??But the 5V
rail was sitting at about 5.8-5.9 V and the 5V PWM was at
about 78% as measured on my scope.??Not Good!??But I knew the CPU was
running enough to produce PWM signals for the SMPS boards.??Thats good!
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I
tested the SMPS power boards on my homemade test fixture.??They were fine.??I tested for near
shorts or low resistance on the 5V rail.??It looked fine also.
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I put
the SMPS boards back into the rig and plugged in the controls
board.??I also
installed the jumper to force the QMX into firmware update
mode.??That
worked.??The QMX
came up wit the “disk drive” on my Linux desktop.??I installed rev 16
firmware knowing that Hans had fixed a bug in initializing the
EEPROM.??I thought
the forced factory reset upon installation of Rev 16 might
shed some light.??It
did.??Now the rig
came up immediately into Diagnostic mode.??It’s the only screen
that will come up in PUTTY if something is seriously wrong.??I could see that the
5V rail was at 78% duty cycle as I saw on my scope but the
reported voltage to the CPU was only 4.8 volts.??The CPU was trying its
hardest to reach 5V but it maxed out at 78% and couldn’t get
there.??I measured
the ADC_5V signal on the board and my scope read 2.4V.??That is consistent
with the diagnostic screen reporting 4.8V since there is a 2x
divider in the circuit.
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ADC_5V
only goes from the SMPS board to the CPU.??Something was clamping
it at 2.4V.??It had
to be a short on the SMPS (no, because I tested those
separately), the main board in the traces (possible, but
unlikely), or in the CPU (UGH!).??I lifted pin 32 of the
cpu and confirmed that indeed the CPU was clamping that line
at 2.4 volts.??After
lifting that pin, the board showed 2.95V for ADC_5V.??And that CPU pin 32 in
the air showed 2.4V.??Proof
enough for me.
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So this
means that replacing the CPU is the only option.??This rig has
sentimental value to its owner, so he said please proceed.??I have a few bare
boards with programmed CPUs that I have just for this kind of
occasion.
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Tomorrow
I will proceed with the operation.??I have to accomplish
that in the morning when I am at my best and after only 1 cup
of coffee.??I’ll
post my progress here.
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73 for
now
Jeff
W1NC
