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PostPosted: Mon Sep 14, 2009 20:04 
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Joined: Mon Sep 14, 2009 15:42
Posts: 4
Hi,

New member here, and I'm also new to using LCDs. I'm having some trouble replacing a dead display in a synthesizer of mine, and would really appreciate some advice from anyone with more experience of these matters.

The original display which I'm trying to replace had burnt out LEDs, and lines of pixels running across it. It is a Varitronix MGLS24064-67C 240 x 64 graphic LCD, using Toshiba T6963C controller. The contrast voltage is 9V, generated by a +5V input (also used for control logic supply) and a -4V input. Direct link to data sheet is below:

Varitronix MGLS24064-67C

As a replacement, I bought this Orient Display AMG24064AR-M-B6WTDW from eio.com. This also uses a Toshiba T6963C controller, so I have assumed it should be compatible. It requires a contrast voltage of 12V, so I need to supply about -7V to it, along with the +5V from the existing loom. A direct link for the data sheet for the AMG24064A is below:

Orient Display AMG24064A

The synth has +12V, +5V, 0V, -12V available from the PSU. To generate the -7V, I made a simple op-amp circuit using (one amp of 4 from) an LM348, which is configured as a non inverting buffer, with a trim pot providing -ve voltage at the input.

The circuit works fine under open circuit conditions. However, once connected to the new LCD, the op-amp output seems to clamp at -1.8V. The op-amp is rated to provide up to 20mA, and the display is rated to require no more than about 2mA on that line.

I did a couple of tests, while trouble shooting:

I connected my op-amp circuit to the old display, and my circuit functions correctly. The image is still visible in the old display, despite having lines running through it, and no back light.

I then connected the synth's built in contrast drive (about -4V) to the new display, and again the voltage was brought down to -1.8V. This contrast voltage was designed to drive the Varitronix display, which is rated as requiring 2.4mA nominal, which is greater than the rated requirements of the replacement.

So, I'm really not sure what is happening in my new display. The contrast voltage appears to be clamping at -1.8V. I'm hesitant to just build a -ve supply with greater drive capability, as this shouldn't be required, and I'm not sure that it is really the issue here.

Can anyone shed any light on this matter? I'm somewhat discouraged and confused.


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PostPosted: Tue Sep 15, 2009 13:13 
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Joined: Mon Sep 14, 2009 15:42
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Hi,

I've been trying to work through the circuit with a multimeter. The -ve contrast voltage Vo connects to a transistor Q1 (not sure which electrode EBC, DGS ?). One of the other two pads of Q1 go to a set of resistors (looks like 5 resistors in series). The third pad of Q1 goes to pin 20 of the interface. However, pin 20 is listed as NC in the data sheet. I wondered if maybe pin 20 should be connected to something.

There are also quite a few solder pad jumpers, and unpopulated component pads on the board. Is it often necessary to configure these things before use? There is no mention of any of this in the (rather sparse) data sheet.

I'll try calling eio.com, and see if they can offer any support. I'm not sure if they have much in the way of technical people, or if they are mainly just a shop front.

Cheers,

Oli


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PostPosted: Tue Sep 15, 2009 15:17 
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Joined: Mon Sep 14, 2009 15:42
Posts: 4
Hi,

Just let people know, I have sorted this one out now. I had left pin 20 connected to the original loom of the Varitronix display which I was replacing. The Varitronix data sheet also specifies pin 20 as NC, so I had assumed I wouldn't need to change anything from the original connection. After removing pin 20 from the header on my Orient Display (leaving it floating) the contrast voltage was as required, and the display functions correctly.

So happy I can now use this synth that has been sitting broken since I bought it from eBay.

Apologies if I have wasted anyone's time. I was somewhat at a loss. This is actually the second LCD module which I have tried (I think I may have killed the first one, by plugging in the loom backwards?).

Cheers,

Oli


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PostPosted: Tue Sep 15, 2009 22:28 
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Joined: Mon Sep 14, 2009 15:42
Posts: 4
Oli wrote:
... This is actually the second LCD module which I have tried (I think I may have killed the first one, by plugging in the loom backwards?).


Hi, just a quick follow up. The first display which I purchased is fine, and I have now installed it as my replacement.

The reason I'm posting here, is just to let people know that this module is extremely tolerant of contrast voltage. The display is this Brilliant Crystal module from Futurlec. The spec requires 12V contrast (+5V, -7V), though I swept it through a range of about 5V to 15V with almost no perceptible difference in the display.

Thought some people may be interested in this.

Cheers,

Oli


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