Epson HX-20 Faulty Display Issue

[pinko]

Hi,

I have an Epson HX-20 which has recently started experiencing problems with the LCD display.

The unit has a rotary dial which affects something called the "View Angle". It basically adjusts the contrast of the 2-line LCD display.

At first I noticed a slight flickering of the display -- getting lighter and darker as I used the device -- which meant that I had to keep adjusting the "View Angle" dial just to use the device.

I read a thread online from someone who had a similar issue, and who thought it might be the battery pack, so I opened up the device. I found out that it has nothing to do with the battery pack as I tried it with another known good one, but by opening the case it seems to have made it worse.

Sometimes the display will be off now when I turn the device on, and then will spontaneously flicker on and off after a few minutes.

I have uploaded a video below of what is happening:

Does anyone have experience with these machines, who possibly knows what the fault could be?

Or would anyone else like to take a guess?

Thanks for any help!

 

[billdeg]

i have had a similar problem. could simply be that these LCDs are not made to stay dormant for so long. Components have degraded. I would try leaving the system on for an extended period to try to wake up the stale caps,but otherwise I fear we will need to disassemble and troubleshoot.

 

[pinko]

i have had a similar problem. could simply be that these LCDs are not made to stay dormant for so long. Components have degraded. I would try leaving the system on for an extended period to try to wake up the stale caps,but otherwise I fear we will need to disassemble and troubleshoot.

I've been looking at the service manual today, and even though electronics isn't my strong point, I've pinpointed some areas that might be causing the problem.

Internally, the device LCD is connected like this:

Motherboard PCB (with power/battery) --ribbon--> Keyboard PCB (with "View Angle" control) --ribbon--> LCD PCB (LCD screen/ICs)

One of the cables from the motherboard (there are two) is responsible for supplying 7V to the Keyboard PCB.

The Keyboard PCB contains a "Voltage Regulator" circuit. This basically consists of the "View Angle" dial (a variable resistor), some additional resistors, 2 transistors, and a capacitor. This circuit is responsible for changing the 7V input to between 0V & 7V. The output voltage is sent to the LCD screen and this determines the contrast of the screen. I have a feeling that this is where the fault lies, if the output is erratic then this would explain why the contrast is flickering, fading, etc.

These two pages from the service manual describe it:



The LCD PCB contains the LCD screen itself, the ICs used to drive it, and some resistors and capacitors.



When I get time, I intend to check the following:

1. That the motherboard is outputting the 7V. This isolates the problem exclusively to the Keyboard and LCD boards.
2. That the variable resistor ("View Angle" control) in the Voltage Regulator circuit is working correctly.
3. That the entire "Voltage Regulator" circuit is working. I intend to connect a multimeter up to the output and check that the voltage output when the dial is rotated changes as expected, and is not erratic. If so, maybe it could possibly be a faulty transistor?
4. If everything above is as expected, then the fault is isolated to the LCD board. I guess, the individual resistors and capacitors will have to be checked.

There is also a troubleshooting section in the service manual, and one of the sections relates to LCD screen issues (below). It mentions here about replacing Q1 and Q2 (the transistors in the Voltage Regulator circuit)



I will try to have a look at this sometime this month, and update, but very busy at the moment!