Mike,
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Here's an explanation of why D516 was added:
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If I were debugging this, I would first triple check that all the mods have been done correctly
with no unexpected shorts.? It's hard to tell from the photos.
A short could easily silence your receiver.
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Next, I might apply a receiving antenna (maybe 20 ft of magnet wire across the room)
to the bottom of L406 where it is tied to pin 5 of IC408, see if the receiver works.
If it works, we know that IC408 did not blow for some reason.
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Then try applying that receiving antenna to the drain of Q508 where it connects to C509,
see if the receiver still works.? If it does not work the problem is around Q508 somewhere.
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You might wait a few hours first, see if there are objections from others to doing this.
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A question for the group:
It looks to me like the entire receiver front end is operating at 50 ohms, being terminated?
by two effectively parallel 100 ohm resistors at R413 and R414.
So the series 1nF cap at C509 is driving a 50 ohm load.
Impedance of a cap is 1/(2*pi*f*c), so at 1.8mhz that's 1/(2*3.14*1.8e6*1e-9) = 88.5 ohms,
and likely not an issue since the low bands have so much atmospheric noise.
Would IC408 be less prone to damage if C445 were also at 1nF instead of 0.1uF?
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Jerry, KE7ER
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On Mon, Apr 21, 2025 at 10:59 AM, Jerry Gaffke wrote:
D516 prevents any large voltage spikes from from destroying the receiver front end when transmitting.
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