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I forgot the first rule of radios - if something works, don't fix it.

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Anyway, I wanted to replace the diode that I have been using for the Rev 2 board modification with a smaller diode.? I replaced it and found that the radio would indicate high SWR every time I transmitted, so I put the original diode back in.? Now I can't hear any RF signals.? I can't even hear the "normal" birdies.? I'm attaching a picture of my "work".? Have I ruined L503?? I'm also afraid that I may have destroyed the trace at that end of L503.

Any suggestions would be helpful.? Thanks.

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73,

Mike, KL7MJ

Trying to add the image again...

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I suggest only troubleshooting this with no DC applied. Use ohmmeter.?

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See if L503 has continuity. If yes, it is likely okay. See then if you shorted something else.?

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Curt

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Curt,

Thanks. No, there’s no continuity.

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Mike

The bottom diode, smd one, looks as though you have broken off the pad that your larger diode is connected to.

As Jason pointed out, L503 needs to be replaced. ?I always add an SMD protection diode across Q508. ?Much neater and easier.

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?I also see a few questionable solder joints where you can see the copper wire poking up through the solder. ?I would go through and reflow those type of joints with some extra flux.

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good luck. ?

--

73
Jeff Moore
W1NC

I replaced L503 with a through hole 330uH inductor (I don't have any smd parts available).? It looks ugly, but I think there are solid connections to the PCB.? Unfortunately, the RF still isn't getting through. I didn't try to resolder to many joints - they were before I started messing with the diode.

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I misread the value for L503, so I replaced it with a 47uH inductor, with no change in the receiver.

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Perhaps try checking Q508.? With QMX on in receive mode, you should see close to your supply voltage on both D and S.

Mike,

Stan,

I’m seeing 0 on S and D, and 3.3 on the gate.?

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Mike,

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What you are seeing sounds like correct behavior to me for Q508 when in receive mode with RX at 3.3v.

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L502 holds the source pin at ground for DC, any received signal is in the microvolts, and very hard to detect.

With the gate pin at 3.3v, the transistor is on, so the weak pullup resistor at R510 is overpowered and the drain pin is also at ground.

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D516 prevents any large voltage spikes from from destroying the receiver front end when transmitting.

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I'm not sure how to cure your problem, but Q508 may be working correctly.

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Jerry, KE7ER

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On Mon, Apr 21, 2025 at 10:19 AM, Mike, KL7MJ wrote:

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:

Jerry,

Thanks.? I'll give it a try as soon as I can (probably over the weekend).

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73,

Mike, KL7MJ

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On Mon, Apr 21, 2025 at 10:59 AM, Jerry Gaffke wrote:

What you are seeing sounds like correct behavior to me for Q508 when in receive mode with RX at 3.3v.

Thanks for the correction, Jerry.? I mis-spoke regarding the voltages at Q508, and it does appear to be operating correctly.