About 2 weeks ago I finished building a high band Rev 3 QMX.? I was able to flash the firmware and almost everything worked:
- I connected to WSJT-X and used the CAT functionality to change frequency and decode FT8 message traffic
- I went into CW mode, turned on CW decode and the radio decoded my key input
- I was able to tune around in DIGI mode and listen to SSB QSOs
- All the menus, buttons and diagnostics work with the exception of ZERO power out
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If I held down the ¡°t¡± key in diagnostics mode on the terminal program the 4 PA (Power Amplifier) MOSFETs and T501 started getting hot.
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I opened a thread on this forum and went through a number of troubleshooting steps:
#119725??
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I don¡¯t want to replicate everything that went on with that thread, but here are the highlights.
1) I suspected T501 and after rewiring it a couple of times I am pretty certain I have it correctly wired and installed.
2) I suspected one or more of the BS170 PA MOSFETs (Q503 - Q506) had failed but when I pulled them out of circuit they all passed on a component tester.
3) High resolution pictures of both sides of the board didn¡¯t help me find any bridged solder pads or missing SMD components.
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I recently purchased a very modest, but functional Oscilloscope and was trying to measure the signal at the power transformer (T501).? I must have held the ¡°t¡± key down a little bit too long on the Diagnostics utility and Q505 smoked.
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After I replaced Q505 I followed an idea John Z suggested on the earlier thread which was to pull one leg of L502 out and do some troubleshooting without applying power to the PA.
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I will give some details of my current findings in the hope that I can get some additional assistance with this challenge.? Thank you in advance for any suggestions on next steps.?
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1) O-Scope wave forms at Gate of Q504 and Q505 during transmit on 14.074 MHZ:
2) All 3 legs of Q507 measure 11.5 Volts RMS in both transmit and receive states.
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John Z suggested that I might introduce a small voltage to the PA MOSFETs and use that to trace the signal through the rest of the circuit.? As I look at the schematic, would this be a small positive voltage applied to the tap of T501?? I have limited equipment (don¡¯t have a current limiting power supply) but I thought I could wire up a regular AA battery?
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I am so sorry for this extra long post, but I feel like I needed to provide some context to where I am on this troubleshooting journey.? Any ideas for next steps would be much appreciated.
Thank you.
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Mike
KC2EHR
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