Spontaneous sending of an endless series of dahs

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edwalker
 

Posts: 4
Joined: 25 Jul 2022, 15:10

Spontaneous sending of an endless series of dahs

Post by edwalker »

Just assembled my radio, experienced kit builder with several QCXs under my belt, nothing out of the ordinary on this one. Powered up just fine, and everything seems normal except one odd behavior.

When I change the keyer in the menu to Iambic A or B, with no paddles connected, the radio starts to emit a series of dahs which cannot be turned off unless the radio is rebooted. If the paddles are connected at power on this does not occur. Also, Straight Key mode does not seem to do this. So my workaround so far is to either keep the unit in Straight Key mode or make sure that paddles are connected at power on. This behavior can also be silenced by partially inserting the key plug (or a toothpick) so that it engages the first jack switch, disconnecting mic (SN) from (S). Insertion of a normally wired paddle seems to work without a problem.

I checked the schematic and traced the key/mic circuit all the way back from J7 (the key/mic jack) to the processor, and I can find no shorts. A polarity reversed mic could cause this, but the mic is not reversed. The mic works correctly on SSB, and the paddles work correctly on CW.

I'm quite honestly stumped at this point and wondering if it might be a firmware bug of some sort since the key circuit seems to work properly under most conditions and exhibits these anomalies only under a limited set of circumstances.

Anybody else seen this? Thanks in advance for any ideas.

73, Ed AG7DT
edwalker
 

Posts: 4
Joined: 25 Jul 2022, 15:10

Re: Spontaneous sending of an endless series of dahs

Post by edwalker »

I have some more data on this problem that may be helpful. Sorry, I tried to paste a copy of the schematic so you might follow along, but I couldn't get the image to post. There are at least 3 other posts that seem to have the same problem in a slightly different presentation.

If you look at the Mic/Key J7 jack you'll see that the mic is normally connected when there is no key present, and that the insertion of the key disconnects the mic through a small switch within J7, allowing the key to connect. Importantly, the switch does two things: 1) it disconnects the mic and 2) makes contact with the ring of the plug, the connection for the dahs.

When the key plug is in place the mic is out of the circuit, and the key works normally, acting as a normally open switch until a dah is selected by the operator. If the key is removed the mic is put back in the circuit and the series of dahs starts, as if the operator is holding the key closed. Even a toothpick activating the J7 switch to remove the mic will stop the series of dahs (it opens the switch).

The only thing that changes when the dah series starts is that the mic is reconnected. Note that the other side of the mic goes to ground. Since the problem starts when the mic is inserted, it has to be a bad mic (there's nothing else in the circuit). DL2MAN has already pointed out that a reversed polarity mic insertion will cause this symptom, and even though I have repeatedly demonstrated correct polarity, and that the mic works on SSB, I have to assume the component is damaged in some way that allows it to work partially for voice but not in keeping the line from going to ground.

Since the vast majority of you of you haven't complained of this so far on this board I have to assume that this isn't a firmware issue, and that it hast to be a component failure. The one test I can do next is to simply replace the mic. I'll order a few and try replacing them to see what happens. I'll let you know in a few weeks what I find out.

73 Ed AG7DT
DL2MAN
 

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Joined: 30 Dec 2021, 19:18
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Re: Spontaneous sending of an endless series of dahs

Post by DL2MAN »

What is the source of your Kit ?
Maybe they have chosen the wrong type of mic ? Or maybe there are tolerance variations of the mic ?
I suggest: Remove mic, to exclude it. If behavior is still the same: It was not the mic -> search coninues.
If it stops, find replacement. Electret capsules are cheap....
Some mics provide extremely low resistance, and can act, as it you´ve connected a paddle.

73 Manuel; DL2MAN
edwalker
 

Posts: 4
Joined: 25 Jul 2022, 15:10

Re: Spontaneous sending of an endless series of dahs

Post by edwalker »

Hi, Manuel -

Didn't expect such prompt, personal customer service! Thank you! You've been a great inspiration in helping to standardize this delightful radio. Appreciate your (and Guido's) vision and efforts.

Removed the mic this morning and the problem seems to have resolved. The mic was part of a kit from uellyqy on eBay. I reinserted and removed the mic multiple times and the problem clearly follows the defective component.

I don't know how to test the mic for defects, however the ground pin and case have continuity and the + terminal shows 800 k ohms against the ground. There are no part numbers for the mic printed or stamped on the case for identification.

I ordered several from LCSC using the approved Mic parts link on the website. One of them has to be good!

By the way, for those of you who are getting the !!Check PTT/Key message on startup or key withdrawal, this was happening to me as well and has resolved by pulling the mic.

This is what's great about amateur radio - we get to go on these detective missions and learn more every day. :)

73, Ed AG7DT
edwalker
 

Posts: 4
Joined: 25 Jul 2022, 15:10

Re: Spontaneous sending of an endless series of dahs

Post by edwalker »

Replacing the mic resolved the issue of the endless repeating dahs as well as the !!Check PTT/Key warning. The radio appears to be working normally now.
RichardB
 

Posts: 12
Joined: 15 May 2022, 01:37

Re: Spontaneous sending of an endless series of dahs

Post by RichardB »

One other thing that can happen, is the PCB pad for the + side of the mike is large enough that it could short to the case of the mic, thus grounding the mic input. I noticed this during assembly. I have a sheet of thin Teflon and so cut a disk to go under the mike to prevent a possible short.
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