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Hello again,

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today my QMX went partly deaf again.

I checked the bias voltage at R401/402 and it was just 0.5V. Grabbing Chipquik and an fresh 74BT3253 solved the issue within 15 minutes.

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This is now the second time that I had to replace this IC and I searched a little bit in the forum. There are some comments that the MUX might be damaged in combination with the 50W PA, which I'm also using. Is there a confirmation for this? Are the both devices not compatible?

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I just would like to avoid to change the IC every second week and I'm also curious if and why this could be related (or what esle could be the reason).

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73

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Dirk? -? DK7CVD

Hi Stan,

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thanks for your comment.

I do have a newer revision with D516 already installed as SMD.

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KR

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Dirk

Good to know it's blowing that IC with normal operation.?

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I also have the revision with the SMD diode in it and it too has seen that IC blown by the 50W PA, although when I blew it I also blew my finals in the amp as well, so that was probably part of the problem.?

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But I did get a clue when I blew the finals on the 50W PA. The finals weren't the only thing that blew. When I replaced the finals, I was no longer getting any bias to the power transistors. I suspect that the 5 volt regulator was blown when the finals blew. That regulator runs off the same rail as the biasing circuit for the 1n4007 diodes being used for the TX switch.?

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I don't have any definitive answer, but I strongly suspect that all the inductance and capacitance along that whole loop (I'm suspecting those 47u inductors and the 1uF caps as possible culprits) could get "loaded up" with the right combination of keying and once there's a good charge on that 1uF cap or in that 47uH inductor it discharges backwards into the transmitter. Theoretically, any voltage on C2 ought to discharge to ground through R2 and R4 or through T1, but I can imagine a scenario where if the reactances all line up just right *wrong*, then it zaps the QMX. Because of the relatively large values of those parts, I'm suspicious that this is happening at the frequency of keying, not RF.?

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Just my inexpert opinion.?

On Tue, Sep 24, 2024 at 07:15 AM, Dirk wrote:

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Likely its the DC wiring and there is a ground bounce, a ground loop

that shows when there is a step current change.

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This is not happening often and and I'm suspect of wiring including the

coax braid continuity from Q-X to AMP.

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Ground lead to power sources for both should be heavy and if

two sources their common ground need to be robust.

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The the 78xx failure is excess dc greater than 5V applied to its

output pin, likely cause is IRF510 internal short gate to drain.

I'd expect R5 to limit the current to safe level.

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--

Allison
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direct email will go to a bit bucket.

This is a much likelier scenario than my speculation. I don’t know why I didn’t think of the simpler/more obvious explanation first.?

In any case, it’s relevant to reiterate that when I blew IC-402, it was when I blew the finals, and it certainly wasn’t during normal operation!?

I didn’t see any other posts in the forum about the 50W amp blowing the QMX. It’s nice that IC is easy to replace!?