Fading LCD screen  - Page 1
Forum section: electronics / repair

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 14, 2023, 01:03:31 pm
content: Thought I would ask, before giving up. I have a Roberts dab radio RD 27 model, it has an LCD dot matrix display. The radio is quite old when I acquired it, and it has fading dots in the matrix. When the radio is cold, and first switched on the display is fine, no visible fading, but when it's warmed up, the display shows some fading on the matrix segment dots. I was wandering if it's repairable. The one anomaly that puzzle's me is when it's switch on from cold, the display matrix is totally fine. I would have thought if it's a failing display, it would be degraded segment matrix dots from start up. Could this be a capacitor problem with the LCD display. Any ideas to look at appreciated. And Thank you for reading.

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2023, 01:31:58 pm
content: It looks like thermal drift in the contrast 'setting'. It would be worth checking the manual to see if display contrast can be adjusted using the radio settings. Whether it is just due to the display getting heated up or circuit drift, it's hard to say.If not s/w adjustable, then there is probably a resistive divider setting the contrast. It looks as if the display could be a standard HD44780 style display (16 pin interface including backlight. In that case, the contrast adjustment will be on pin 3...https://en.wikipedia.org/wiki/Hitachi_HD44780_LCD_controller

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 14, 2023, 05:05:08 pm
content: Thank you for your help, I did think it strange that powered on from cold shows a perfect display. But once warmed up the fading starts in different segments, most notably if I scroll the info display. I did a complete reset, but that didn't fix it. I've two things to look for now, the resistor network for the display, and try and establish the model make of the LCD. Still some hope there. Thanks. I will pop back once I've opened it up to have a look.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 14, 2023, 05:28:33 pm
content: Those 16 by 2 displays are now a generic item and widely available in different colours. The 16 contacts include 8 data bits though the interface protocol supports both 8 bit and 4 bit mode. Some are supplied with a piggyback board to give an I2C interface though I doubt your radio uses that type.

author: Gyro
author_rank: Super Contributor
timestamp: May 15, 2023, 09:08:40 am
content: It also means that, worst case, you ought to be able to find an off the shelf replacement in the same form factor - the slightly uneven fading across the display does look a bit funny (unless there is also a significant temperature gradient, eg. heat dissipating components just behind it). Turning the contrast up will hopefully mask it though.Edit: What he said IV

author: Fraser
author_rank: Super Contributor
timestamp: May 15, 2023, 09:19:25 am
content: When observing a non uniform change in an LCD display panel over a period of time, you should consider the possibility of a thermal stimulus close to the display panel. That stimulus can range from the LCD panels own microprocessor in distress to a component or module adjacent to ithe panel that is overheating. It certainly is not ‘normal’ for this to occur.In your position I would open the radio and look for correlation between the low contrast area of the display and a possible source of heat behind it. If it is the panels microprocessor on the rear of the PCB, it is either faulty or is receiving too high a supply voltage for some reason. If the thermal source is an adjacent component, you need to investigate why that component has started to get hot when it did not when Roberts designed the radio. A simple piece of card as a heat shield will prove the case of an external thermal source causing the problem.In the attached photos of a generic LCD module, note the positions of the processor and ancillary IC.As has been stated. If the LCD panel module is faulty, these are often available in a generic form. You need a replacement with the same microprocessor and pinout though so get the data sheet for the original fitted by Roberts Radio. Hopefully the panel will have a part number on its PCB.Fraser

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 15, 2023, 12:37:06 pm
content: Thanks for your replies, hopefully it's an adjacent component getting hot. And maybe I can shield it from the LCD, I've never tried replacing a microprocessor before, it's the solder removal that would be challenging as it's probably a quad sided component. It is just the left side of the display that shows fading. The radio functions just fine, I have it on in the kitchen most days. I can still read the display, it's just faded on the first two 8 segments of the display. There is no way to adjust the display contrast from the radios settings. I will see what it looks like when I open it up. It was an eBay find, looking back there's significant ware on the top of the power button, you can see that power switch has been used a lot, the paint has come off and the button top looks polished through heavy switch operations. It's something I will be mindful in the future.

author: Fraser
author_rank: Super Contributor
timestamp: May 15, 2023, 01:30:34 pm
content: No need to change a processor. The processor is part of the LCD display module so you basically remove the module by unsoldering its relatively few connections and fit the replacement. If you look at the pictures of the modules I posted, you will see the module connections along one side in gold through hole formatThe processor in those images is one of the black resin "blobs" and it cannot be repaired of changed on its own. The reference to the "processor" relates only to buying a compatible LCD display module that uses the same processor type to ensure compatibility.Fraser

author: wasedadoc
author_rank: Super Contributor
timestamp: May 15, 2023, 02:25:52 pm
content: Quote from: Fraser on May 15, 2023, 01:30:34 pmThe processor type on the display module is irrelevant as long as the module implements the de-facto standard interface protocol. I've yet to encounter one of the type in the second photo with 1602A written on the back which does not. Furthermore there is no way to visually identify what processor is under one of those black blobs.Type '16x2 lcd display' into the search box on ebay UK to get an idea of colours and pricing.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 19, 2023, 05:22:03 pm
content: Haven't been able to post for a few days. I've taken the radio apart, hoping to find quick molex disconnects. Which there is, but there's a far few point to point soldered wires. To access the LCD PCB screws these all have to be disconnected to get a screwdriver on those screws. There is a heat shield behind the LCD, to protect the display I'd imagine. It's certainly not an easy accessable repair. I would have to take some notes before desoldering these point to point wires. To make it more fun the manufacturer has kept these wires as short as possible, making it incredibly difficult to reach the wire solder pads. The risk is burning other wire installation if it's not done extremely carefully. I haven't started that yet, but will do in the next few days. There was a pinched wire where the case came together, it's not broken thankfully. I've put it back together for now, and no faded display yet. I wander if just reseating the molex plugs has done something. If it goes faded again I will commit to doing properly.