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
