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When debugging the receiver, I believe we could safely inject a signal up to 3.3v pk-to-pk
without doing any damage. ? The op amps have a gain of around 10, so they would saturate,
feeding the PCM1804 with 5v square waves.
Safer might be 1v pk-to-pk or lower, which is still easily seen on a cheap scope.
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Injecting an appropriate S9+ signal into the BNC for receiver tests as you suggest
is the best first step for debugging the receiver on a completed rig.
But a completed rig could have any number of errors in the build,
a progressive build would make finding those errors much easier.
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If we drive at 100khz or less into RX_IN and short across a receiver BPF
with a cap,? debug could be done with a $30 scope: ?
https://www.amazon.com/Oscilloscope-Handheld-Portable-Automotive-Bandwidth/dp/B0CD1WKP33
This assumes we can get sufficient energy through T401 at such a low frequency.
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I would like to create a complete set of debug instructions for a progressive build:
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To expand on that for the receiver:
First step might be to take the raw QMX+ board as delivered, use alligator clips to supply
a current limited 7vdc to the holes for the power connector, a USB connection to a host
to control the rig and catch the receiver audio, check out Vdd and Vcc.
Inject RF into IC403 pins 7 and/or 9, verify results on the USB host.
Next step would be to add T401, short across one of the BPF's,
inject a signal into RF_IN, should now get single signal recepetion.
Third step is install one of the BPF's and feed RX_IN with an appropriate signal.
From there, move on to the transmitter, and then the front panel.
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At each step the builder can verify it works, and if things suddenly fail
they can inspect only what they just worked on and hopefully find an error.
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I hope the forum didn't find my rant on dB and dBm annoying.
It helps me understand stuff like that if I explain it in full,
and once I write it up I figure I may as well post it.?
Might be of use to somebody.
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Stan wrote:
"I was referred to this article in the Wiki /g/QRPLabs/wiki/37111"
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As that article mentions, there've been a number of failures of the PCM1804.??
Would be good to know if that is still happening since the last firmware upgrade improved the SMPS's.
Easy to imagine that the PCM1804 might fail before the 5.6v zener at D108 manages to clamp Vcc.
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Jerry, KE7ER
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On Sat, Nov 9, 2024 at 10:56 PM, Stan Dye wrote:
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