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TX802 screen issue?

Posted: Sun Jan 12, 2014 10:54 pm
by SamSamba
I bought a vintage Yamaha TX802 late last year. All this time I've been wondering if the screen is faulty, or is the viewing angle really this poor?

Example here: ... screen.jpg

When mounted in a rack right next to my keyboards (in a quite cramped workspace), the screen is pretty much unreadable. There doesn't seem to be a "contrast" button or setting anywhere. The patch on screen, for example, can pretty much only be read when looking at it directly from the front.

Re: TX802 screen issue?

Posted: Mon Jan 13, 2014 4:32 am
by rhino
Welcome to the board!

AFAIK, there is no adjustment - another 'cost cutting' measure. DAMN!

The displat is connected to the motherboard by connector CN7. Locate pin #3. On the mainboard, this lead should connect to the junction of an 18K resistor and a 56 ohm resistor. Try removing the 18K and replacing it with a 10K trimmer and a 10K fixed resistor.

Re: TX802 screen issue?

Posted: Tue Jan 14, 2014 3:05 pm
by SamSamba
Hang on - so, is it faulty or by design (meaning - a poor viewing angle)? I don't think I want to start messing with resistors and trimmers.

Re: TX802 screen issue?

Posted: Tue Jan 14, 2014 5:51 pm
by rhino
I read through the manual, the schematic, and did a fast websearch. Unless somebody with more insight than I chimes in, that's how it looks. I've seen a few other rack modules like that - no adjustment. My take: It p'bly worked fine when new, but the LCD bias voltage is very critical... With no adjustment, the normal aging of componets is often enought to change the contrast.

Other members? Help/info?

Re: TX802 screen issue?

Posted: Thu Jan 16, 2014 6:57 pm
by RyanC
Not sure if this will be of any help since I don't have a TX802, but I do have both a TX16W and a QX3 that were developed at virtually the same time as the TX802 and seemingly have the same 40x2 LCD screen.

On the TX16W (also without contrast adjustment), the LCD appears the same as what is depicted in the image you posted.

The QX3, though, has a knob on its back that allows for full contrast adjustment. When rotated fully counter-clockwise, the viewing angle is just as limited as the TX16W and what I see with the TX802.

If you'd like any more details or even photos of each, feel free to let me know.