Troubleshooting a dead (tr)uSDX

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N1CJ
 

Posts: 31
Joined: 07 Jul 2022, 15:54

Troubleshooting a dead (tr)uSDX

Post by N1CJ »

Hello,
I recently purchased and assembled a (tr)uSDX from the Malahit kit on Amazon. Turns out even someone with mediocre soldering skills can successfully build this kit. Great job on the design.

Unfortunately, my mediocre skills in tuning the filters have resulted in a dead radio. I watched Manuel's tuning video and followed the same steps. The connectors were together, I held the boards on the opposite side to prevent them from squishing together too much. When I double-clicked the knob to switch bands, the relays clicked briefly and then everything shut off. This was using the 12v power port.

So, I unplugged power, made sure everything was connected properly and nothing looked wrong. When I plugged it back in, the screen was dim and said to check the ptt button, then displayed text turned upside down...strange stuff. After disconnecting and reconnecting power, the radio was dead.

Now, unfortunately, I not only have mediocre skills, but I also have very limited diagnostic equipment. Just a multimeter and the nanovna. Nothing looks burned out on the boards.
When plugging in 12v power without the rf board connected, the 12v pins show correct power, but the VCC pins show 0.5v. When I plug in USB power, I see about 3.5v on the VCC pins, and the radio is still dead.

Any ideas on what I may need to check and/or replace?
73 - N1CJ
N1CJ
 

Posts: 31
Joined: 07 Jul 2022, 15:54

Re: Troubleshooting a dead (tr)uSDX

Post by N1CJ »

So, I know that this radio will not operate when the battery voltage gets below 4.5v. It's no surprise that it won't power on if the 5v rail is showing less than 4v. Is it safe to assume the 78L05 voltage regulator failed? Is there a good way to determine whether this is in fact the problem?
73 - N1CJ
RichardB
 

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

Re: Troubleshooting a dead (tr)uSDX

Post by RichardB »

KN6KFR

The radio will operate from the power on the USB port. For testing and troubleshooting, I recommend using the USB port for power. Also, check the Facebook (tr)uSDX group for the schematics, I just posted updated version 'm'.

Lets start the troubleshooting by understanding the radio: The two boards - main board and rf board.


The main board can operate without the rf board connected. The main board can receive signals if an antenna is connected to the rf pin, connector J3 - pin 4. The rf board is only required for transmit. The rf board is useless without the main board.

First, separate the boards and power the main board using the USB port. Does the display work now? The PTT error is not important right now. If the display is not working then it could be a display or something else. Using a voltmeter, measure the voltage on the J3 pin 11 (there is silk-screening marking the pin as '+5V'). USB voltage passes thru diode D4 so there is a diode voltage drop - the reading should be something like 4.5V. If the voltage is below 4V then something is pulling the voltage down.

IF the display is working with the rf board disconnected, do the buttons work properly? If so, then the issue maybe the rf board. Reconnecting the rf board, the display failing would indicate the rf board is the problem.

When I am troubleshooting, I prefer the USB port for power - powered from a USB hub. This is because the USB hub limits power to 500mA. Plus, all voltages on the board would be 5V or less. If something shorts then there is less chance of damage. Using external power jack can mean higher currents to burn stuff up easier and higher voltages where a short would more likely cause damage.

So the basic question: With the main board separated from the rf board and powered from the USB port, does the display light up?

Richard
RichardB
 

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

Re: Troubleshooting a dead (tr)uSDX

Post by RichardB »

KN6KFR

Your second post, The radio will likely operate below 4.5V but the display may have issues and band switching may not work because the relays will not toggle.

The 78L05 is powered by the external power jack only. The 78L05 has built-in current limiting. The way to damage it is to apply too much voltage to the power jack. One datasheet indicates that the max input voltage to the 78L05 is 20V, another indicates 30V. It is doubtful that the regulator has failed. It is easy to hate surface mount parts as pulling the part for testing is not easy. Measuring the voltage on J3-pin 11 should be +5V. Try powering from the USB port, the voltage would be around 4.5V as it does not use the 78L05. Powering from the external jack, and you should have 5V.

What is the voltage?
N1CJ
 

Posts: 31
Joined: 07 Jul 2022, 15:54

Re: Troubleshooting a dead (tr)uSDX

Post by N1CJ »

Hi Richard. Thanks for your replies.

Like I mentioned in my first post, when I power the unit from the USB port, I get about 3.5 volts. I will get the exact voltage when I work on this again. I did forget to mention that this was the main board only, I have already separated it from the RF board.

So, 2 things lead me to believe the 78L05 is broken.
1) I only see 0.5 volts on VCC when using 12v power, and
2) Something is dragging down VCC voltage to 3.5V when using USB power.

I can confirm my USB power supply is providing 5V. If you know of a more likely culprit for this problem than a bad 78L05, please let me know.
73 - N1CJ
RichardB
 

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

Re: Troubleshooting a dead (tr)uSDX

Post by RichardB »

That is more info. When 12V is supplied, you only see 0.5V on VCC. I suggest verifying the input voltage to U2, 78L05 by measuring the voltage on the component lead, not not J1 or any other location. The idea is to verify that the component lead has not lifted and has 12V on it. It is a double check.

When powered from USB, the VCC is 3.5V - yes, I agree, something is dragging down the voltage. A double check is to locate D4 and D3. Verify that the anode side of the diodes are 5V. If not, then measure the +12V (cathode of D3) and verify that it is a reasonable voltage. It is possible that there is a problem on the +12V traces that is pulling down the 12V side. This can be verified by applying the 12V external power and verify the 12V a jack J1.

If the 12V side is 12V with external power then the 78L05 is likely bad. Then, I would suspect an additional problem. Easy check, relative: remove the 78L05, U2. Then power from USB port, if 78L05 is the only problem then the main board should work. If it doesn't work, then a second issue exists. Also, verify that both +5V and +12V lines are around 4.5V.
N1CJ
 

Posts: 31
Joined: 07 Jul 2022, 15:54

Re: Troubleshooting a dead (tr)uSDX

Post by N1CJ »

Thank you! I will check voltages at the points you mentioned and report back tomorrow.
73 - N1CJ
N1CJ
 

Posts: 31
Joined: 07 Jul 2022, 15:54

Re: Troubleshooting a dead (tr)uSDX

Post by N1CJ »

Well, I'm taking measurements on my lunch break. Unfortunately, I forgot to bring the batteries for my 12v battery pack, so I can only do the 5v USB tests.

My USB plug is providing 5.03V
VCC is at 4.36V

D3 cathode is 4.66V and anode is 4.85V
D4 cathode is 4.38V and anode is 4.85V.

When I get a chance, I will give you the 12v power supply measurements.
73 - N1CJ
N1CJ
 

Posts: 31
Joined: 07 Jul 2022, 15:54

Re: Troubleshooting a dead (tr)uSDX

Post by N1CJ »

Ok, last one. and I think I managed to short my probe across 12V and RVS this time, and let some smoke out.

My battery pack is 12.06 volts.
12V pin is 11.75 volts.
Cathode of D3 is 11.75V. Anode is 0.28V
Cathode of D4 is 0.25V, and anode is 0.28V.

So, I've fried something else, and I realize I do not have the skills or equipment to be dealing with these tiny surface mount components. So, I guess this will either sit in my desk getting dusty, or I'll have to find someone with the right skills and equipment to fix it.
73 - N1CJ
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