Sunday, 21 June 2020

Yamaha TX7 Backlit LCD Upgrade

There was a time when 7-segment displays were in fashion:


They were followed by LCDs, which consumed less power and provided more detail in a smaller space. But these early LCDs had a problem - they used ambient light because they didn't have any internal illumination. So they weren't very easy to read in the dark - unlike 7-segment displays. (and for guitar pedals and some other 'need to be visible in the dark' applications, then 7-segment displays are back in fashion!)

The second generation of LCDs had LED backlights, which were often green in the early days, although yellow and red were also possible (red looks very distinctive!). Nowadays you can get green, yellow, red, blue and even white, although LCDs have been, to some extent, superseded by OLEDs, although there is a tendency for OLEDs to be very small, maybe even tiny, in size.

But back to the 1980s, and my TX7. Here's my TX7 as it was:


The TX7 is an interesting device to photograph, because the sloping front panel means that it almost always has a reflection of a window or a ceiling light in it. In the photo above, the only way to avoid this is to take the photo from an angle which is so low that the LCD is being viewed from the wrong angle, and so looks worse that it actually is. But a photo where the LCD is readable also has windows and ceiling lights reflected in it! Modern house building trends, like large windows, skylights and arrays of small LED ceiling lights, just make it worse. Even taken from a crazy angle, there's still a reflection - you can see an edge between the T and the X.

So years of struggling with the TX7's display finally persuaded me to do something about it - and I bought a backlit LCD. There are two types of backlight for LCDs: electro-luminescent panels (several different variants), and LEDs, Electroluminescent panels use a high frequency, high voltage oscillator to light up a panel behind the LCD, and the pale blue 'EL Foil' variety seem to have lives of a few years - so for example, my Kawai K5, Korg Wavestation and Yamaha SY99 all have electroluminescent backlights that no longer work. Replacing these displays (or the inverter electronics, which seem to contain the parts that fail) buys you another few years, and then they die again (my K5, for example). On gear which is already vintage, then the strain of major surgery to replace a display every few years seems like a dangerous gamble...

Notable 'classic' electroluminescent panels were used in the HH IC100 amplifiers in the 1970s. Curiously, the green-blue backlight, which illuminates all of the markings on the rotary controls on the front panels, seems to have a very long life, with many still working...


(Photo of an HH IC100 in the dark by James Davies,  https://www.flickr.com/photos/jamesdavies/14764665183 )

Anyway, I bought a replacement LCD (with a green LED backlight) for the TX7 from Chris at CPMagneticMedia on ebay.co.uk. It arrived very quickly, and was well packaged, bubble-wrapped on the outside, and then that dark reflective anti-static plastic inside. Fitting it was straight-forward: the TX7 needs only a few screws removing to open it up, and the LCD is easily accessible. The small black plastic 'pop rivet' connectors that are used to fasten the LCD onto the mounting brackets are quite fiddly, so here's a quick guide to releasing and installing them if you aren't familiar with how they work.


Releasing the pop rivet is kind of counter-intuitive. You can try to pull out the top of the mushroom part, but it is much easier to just push at the stalk with a small screwdriver and the mushroom part will just pop out.


Installing is the fiddly bit. First, you push the rivet in place, to hold the LC module onto the bracket.


Then you push the mushroom part into place in the round hole side of the rivet:


The mushroom part will lock into place and the sides of the rivet are pushed out slightly, holding the LCD module securely to the metal bracket.


Once the LCD is securely held in the brackets, there's a temptation to over-tighten the screws on the brackets, which the detailed instruction PDF warns you not to do! It was interesting to see that Yamaha had put an empty header with +5 and Ground on the main PCB, which made the backlight power easy to hook up, and I did wonder if Yamaha intended it to be used for a backlit LCD upgrade...

The end result tones with the cyan buttons on the TX7 rather nicely, and it is consistent with many of the 1990s Yamaha LCDs that also used green backlights:


Most importantly, it is now easily readable in high ambient lighting, with reflections, with ceiling lights, and in the dark. Much better, and I wish I had done it a long time ago. If it wasn't for my bad experience with the K5, then I might consider doing it for some of my other vintage synths. (although in general, many 80s synths require quite a lot of disassembly in order to get to the display and replace it, and that can be a lot of work...) Maybe one day...

However, I have to say that recent 'hardware' excursions have rather whetted my appetite for minor updates, and after the TR-505 and TX7, there may soon be additional posts on simple updates to some more of my vintage gear...

Links

HH Electronics on Wikipedia
CPMagneticMedia on ebay.co.uk
Yamaha TX7 on Encyclotronic

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