Re: QCX-mini failure on new build

Timothy Freeze

Hi Mont,

I have a cheap logic analyzer I bought on a whim.  No idea how to use it! 🙂

I just received a complete 80 M kit, built the display, and put the header pin in upside down.  I'm so dumb.  I had to remove the 10 pin header and resolder it in place properly.  Some of the thru hole copper came out, butit appears to work ok.  I finished putting it together and tried it in the QCX, well it gives the same result.  Sometimes ???or other characters and a flashing cursor to the right.  So I'd guess that means the main board is bad?

I'm thinking of just sucking it up and order another 20 M kit,  trying again and hoping for the best.  I loaded the complete firmware to another CPU and it didn't make any difference.   I haven't tried the new kit's cpu but I suspect I'd get the same result.

The twisted tabs on the LCD are not bent over.  They don't appear to be touching anything.

Here's the two displays, one connected with jumpers to the 10 pin header.  The other just on the QCX.

I'm sorry to be taking up so much of your time. 

I'd be fine with testing the more complicated display on breadboard rails if that the thing to do.

Kindest regards,


From: <> on behalf of Mont Pierce KM6WT <de.km6wt@...>
Sent: Wednesday, June 23, 2021 12:34 PM
To: <>
Subject: Re: [QRPLabs] QCX-mini failure on new build
On Wed, Jun 23, 2021 at 06:19 AM, Timothy Freeze wrote:
I have a couple of blue 16x2 displays from my Aurdino collection.  Should I replace the display with one of them?
The 2nd one, with the 16 pin header will do nicely.

BEFORE we go on, there is one possibility that should be considered...  in constructing the mini, one of the twisted tabs under the LCD that hold it together has to be bent over to avoid hitting the alignment trimmer pots...  sometimes doing this causes the LCD issues, as it has internal contacts held together by pressure...  usually it causes missing rows of dots.  But, just wanted to mention it here.  Not sure what would happen if the tab broke off, or create too much pressure...

One test I would do, but it's just because I'm crazy... 
I would hold the LCD with the 16 pin header up against the display board's LCD, with all the pins touching the corresponding display board LCD pads.  Then power on the mini.  Two display boards can work in parallel.
(Ok ok, bad idea... don't do this crazy stunt...)

Another alternative would be to use dupont breadboard jumpers, male-to-female, tack solder the male ends to the LCD pads on the display board, and plug the female ends onto the LCD with the 16 pin header.  Important:  you only need to do the 6 pins from each end.  The middle 4 pins are not used.  The two LCDs will work in parallel. 
        If you see same data on both, then data is corrupted (missing bits) before it gets to the LCD. 
        Could be weak or bad pins on the processor.

        If you see good data on the attached LCD, then the display board's LCD must is probably bad...

A third approach, which is probably more reliable, but a bit more complicated, is to use a small breadboard with power rails.

Plug the LCD with 16 pin header into the breadboard.
Use jumpers to attach ground and VCC to the breadboard power rails.
Use jumpers to attach the LCD D7-D4, EN, RS, pins to their corresponding header pins on main board.
then for the other pins, duplicate the circuitry from schematic onto the breadboard.
    LCD pin 16 can just be tied to ground.
    LCD pin 15 needs R48, a 560 (+/- 200ohms, value not super critical, determines brightness of backlight)
    LCD pin 1 and 5 also tied to ground.
    LCD pin 3 needs a 3.3K to ground
    LCD pin 3 also uses the wiper on 22k Pot between ground and VCC to control contrast
         you can try different values of fixed resistors between VCC and pin 3

This would be a lot simpler I guess if you had another mini, or a friend close by with a mini, and could swap components back and forth to identify where it's failing... 

This puzzle is a definite challenge....

You don't by chance have a Logic Analyzer?  One like this from Amazon (click here) would do.


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