Advice on repairing Radio Transmitter

[Amcv495]

I am having an issue with a radio transmitter, when power is on, everything else is working except the LCD. I've replaced the screen however I've had no luck with the new screen either. I'm thinking there must be an issue with power supply to the screen but I'm not sure how to test it. The only thing i can see is a capacitor which is slightly discoloured compared to the rest but I don't know if this has any effect.

I have some basic circuit knowledge and a multimeter - would anyone have any advice on the best way forward?

[Shonky]

Measure 3V3 to GND on the debug header with the meter on DC volts.

If you have USB to UART module I'd listen on TX line and see if the CPU is spitting anything out. NB: nothing doesn't necessarily mean it's broken but if it is outputting it might provide some info and at least lets you know if the basics of the CPU are OK.

[Amcv495]

Thank you, I'll try that - I don't have anything to measure TX line but I'll put the multimeter over the GND and 3V3 and post the data.

[MathWizard]

Does the screen do anything at all? Does it light up at all ? You can look up all the chips and make sure they have Vcc.

I'm guessing the big chip runs everything including the video, or maybe there's a smaller separate MCU that's the brains. Look up the chips coming from the ribbon cable, see if they are doing anything.

Are there any chips or PCB on the actual LCD panel? You replaced the screen, but anything on there still could give some indication of what's wrong.

I haven't had much luck with my old broken monitors, and I had to throw them out to save space.

[Amcv495]

I am fixing this for someone else and they had mentioned that the screen was flickering before it failed. No IC on the screen or mount, and that is the only chip related to the screen. There is another chip bit that's for the radio I think.

What would be my best way of testing the connections from the chip? Is it a diode test. Makes things difficult without a schematic lol

[Shonky]

If the screen was flickering check the power supply. Have you done that yet? Is there a battery pack and another board somewhere?

No a diode test is not what you you need to test connections and it's somewhat unlikely that has failed. Even then a visual inspection should give you a pretty good idea. You won't really be able to check continuity to the screen. Once it leaves the FFC (the connector) it's much harder to probe anything. You said you replaced the LCD anyway.

[Amcv495]

I thought it was the power supply, it is 8 AA batteries connected together with 2 fly leads. I will try it maybe with a Lipo battery. The LEDs and Buzzer are still working fine and all buttons seem to work. Just the screen is not

[fzabkar]

FWIW, here is a datasheet for the Inanbo-T35C-ILI9488-V1 display:

https://www.micros.com.pl/mediaserver/info-o%20tft-3.5tp-ips.pdf

The recommended supply voltage for the logic is 2.8V.

The LED forward voltage is 3.2V, and there are 6 LEDs.

This is the manufacturer's web site:

http://www.inanbo.com/products.php

Datasheet for MCU:

https://www.st.com/resource/en/datasheet/stm32f103rc.pdf
https://www.st.com/resource/en/datasheet/stm32f103c8.pdf

Other Inanbo datasheets:

https://hadibanelectronic.com/wp-content/uploads/2022/03/INANBO-T28-ILI9325-ILI9328-V11.pdf
http://aitendo3.sakura.ne.jp/aitendo_data/product_img/lcd/tft2/T43/INANBO-T43-V1.pdf
http://manager3.dotname.kr/OHM_DATA/lE060408D/manual/T32-HX8347A-V12-ETC.pdf

[Amcv495]

This is great thank you!

[MathWizard]

Ok so the big chip is a common MCU breed, can they actually be programmed to drive a small screen, or would they still need other chips for that ? I see a lot of traces going right to the ribbon cable, but IDK. Maybe there's no chips on the bottom of the PCB either.

[Amcv495]

Is there a way to test the chip and what's coming from it? (I have little to no knowledge of programming) I know UART module was mentioned above

[Shonky]

You could put a logic analyzer (or an oscilliscope) on the data lines. Unfortunately the display (if the datasheet linked is correct) is a parallel interface so means it's not as easy as it would be if was I2C/SPI/UART

Maybe it's the LED backlight only? You should be able to see if the display is doing something by shining a light at it.

Measured the power supply yet? If you don't check the basics it's kind of pointless checking the more complicated bits.

[Amcv495]

I can't seem to see anything from the screen when I shine a light on it. I was expecting to see some sort of a line or something but nothing. But you're right I need to make sure the power supply is sufficient - I have no way to charge the power supply but taking someones word for it that it is fully charged. I'll put the multimeter over it this evening

[Amcv495]

Tested the battery and there is 10.5 Volts across it. Should be closer to 12V. But surely that is enough to power the LCD?

[Shonky]

See post #1. Measure 3.3V

Check power is on the LCD pins per the data sheet. Look for a cap nearby since the pads are small.