In this post I'll try to make the LCD problem to disappear.
I'll be talking only about 16x2 HD44780 compatible LCDs.
1) HD44780 compatible LCDs are divided into two types: with back light (backlit) and without back light.
They may come either with 14 or 16 pins. The LCDs with 14 pins are guaranteed not to have back light. At the same time, if an LCD has 16 pins it does NOT guarantee it has back light.
A backlit LCD when connected to the board must show some back light, even if everything else is broken in the LCD. If it does, but it's very dim, you need to replace the resistors on the Digital Board (or Synthex's all-in-one board) going to the pins 15 and 16 of the LCD with smaller value resistors. They should be between 470R and 33R (these are Ohms and NOT kOhm!). If it doesn't (even a slightest bit), measure voltage between pins 15 and 16 of the LCD. It should be around 3-5V. If it's the case, then either the back light LED is broken or your LCD doesn't have back light. If it is 0V, then check wires/connectivity to the board. Also some people reported that in some LCDs polarity of the back light LED is reversed so try to swap wires to pins 15 and 16.
2) Let's assume you sorted out the back light problem. Even without back light, with exterior light you should see something on the LCD. If you don't see anything it could be for a few reasons. First, and most often, the contrast is not right. Contrast is controlled by voltage on pin 3 of the LCD. The lower voltage between ground (pin 1) and pin 3 of the LCD the higher is the contrast. The higher voltage between ground and pin 3 of the LCD the lower the contrast. The contrast may go down so low that you won't be able to see anything on the LCD. Usually the voltage on the pin 3 must be between 0.5V (high contrast) and 1.5V (low contrast). On my development LCD the best contrast is achieved with voltage of 0.9V.
Before going further, make sure you have +5V between pin 1 and pin 2 of the LCD. If it isn't, check connectivity between the LCD and the board.
In the old type of MegaDrum schematic the contrast voltage on pin 3 of the LCD was controlled by a pot where side pins of the pot were connected to ground and +5 and the wiper of the pot was connected to pin 3 of the LCD so you had to adjust the contrast with this pot. This pot might be a precision type pot (as far as I know, Synthex used this type of pots in his old pre version 2.8 kits) and may require several turns (or even a few dozen turns) to set correct voltage on pin 3 of the LCD.
With the current version of the MegaDrum schematic, including the Synthex's kit 2.8, the contrast is controlled by MegaDrum firmware without any pots. To change the contrast (provided Atmega is programmed, working correctly and there is proper data communication between Atmega and the LCD) , you have to enter MegaDrum menu and navigate to the first point of the menu where it shows "<MegaDrum.info > (c)D.Skachkov". There you have to press UP to raise the LCD contrast or press DOWN to lower the LCD contrast. If the contrast doesn't change, check the connectivity between pin 3 of the LCD and the Digital board. Also check correct values of C9, R19 and R21 (see http://www.megadrum.info/sites/default/ ... _kicad.png) and that they're properly soldered and don't have breaks/short circuits.
Even if data communication between Atmega/Digital board and the LCD is out of order (but Atmega is working properly), i.e. you don't see VU meters or the MegaDrum menu, you still should be able to adjust the contrast. Only in this case when you adjust the contrast you should see a block of black boxes in the top raw of the LCD. If you adjust the contrast with a pot but still not able to see black boxes, than your LCD is faulty. If you adjust the contrast from the firmware (blindly, since the data communication between Atmega and the LCD is presumably broken, by pressing LEFT for a few seconds to guarantee you're in the first point of the menu and then pressing UP/DOWN) but still not able to see black boxes, than either the LCD is faulty or Atmega is not working properly. In that case temporarily connect pin 3 of the LCD to pin 1 ground. DON'T ACCIDENTALLY CONNECT pins 1 and 2 of the LCD - you may damage USB port or power supply. When you connect pin 3 of the LCD to the ground you set LCD contrast to the maximum and black boxes in the top raw should stand out straight away. If they don't, your LCD is faulty. If they do, your Atmega is not working properly.
3. Let's assume you sorted out the contrast and the LCD is not faulty but you're still not able to see VU meters (without any input activity VU meters look like just like underscore symbols in all character positions of the LCD) or the MegaDrum menu. This means one of or a combination of some of the following:
- Atmega is faulty. Replace Atmega.
- Atmega is not programmed. Program Atmega.
- A faulty Atmega crystal. Replace the crystal.
- Incorrect capacitors connected to the crystal. Replace capacitors.
- Atmega is working properly, e.g. tested with MIDI activity, but data communication with LCD is broken. Check connectivity between pins 4-14 of the LCD and the board right to the pins of Atmega and that there are no breaks/short circuits on those data lines. If you're sure that the connections are ok, than your LCD is faulty.
Hope the LCD problems are now gone forever:)
