Here’s my MC-505 as it arrived from the seller on Reverb:
The LCD display (the orange rectangle top centre) shows the common problem that these units suffer: many pixels are inoperative. A closer look:
The first thing to try as a fix is to re-heat the LCD ribbon cable, to re-make the electrical contacts. We need to extract the LCD from the unit first. Start by removing all knobs from the front of the unit (they just pull off) and opening the case:
The white ribbon cable attached to the upper main board is for the LCD – it just pulls out. To extract the LCD itself we need to get access to the screws which attach it: these are underneath the plastic screen at the front of the MC-505. Some people use a knife and pry it off from the front: a bit risky, as the screen could bend and break. Instead, remove the mainboard, which involves detaching all the ribbon cables, all the jack sockets, and several screws. Take photos of everything as you gut the machine:
Cut any cable ties as necessary, such as the one in the photo above (and remember to replace them on final reassembly). Here we see the interior after removal of the main board;
We can see the rear of the LCD display top centre. Now remove the large board that holds all the keys and switches. No need to detach the ribbon cables from it. There are many small screws that attach this board to the case. Here’s the key switch board removed:
If you are going to clean the switches etc., remove them from the board – the plastic piano key switches have small latches that hook them to the board, but are not hard to remove. Be very careful, as they are quite flimsy. Here’s a view of the case, without the boards, but with the various switches still in position. Remove them for cleaning if desired. With access to the rear of the plastic screen, it can be gently pushed outwards by applying firm pressure through the aperture – it’s only held in by double sided tape, so little effort is required.
With the screen removed, the LCD display unit can be accessed and removed from the front of the unit – it’s held in place by four small screws. Here’s the case with all switches, knobs etc. and the LCD display removed.
With the LCD display removed, re-assemble the unit’s circuit boards, including the power supply board, but leave the back of the unit’s case off so that the innards can be accessed. From the front, feed the LCD display’s ribbon cable through the aperture and attach the ribbon cable to the main board. Now the machine can be powered up again.
If you are lucky, you can fix the missing LCD pixels by heating. There is a wide ribbon cable that connects the LCD display at the front to the LCD circuit board at the back. This ribbon cable is poorly connected. Using a soldering iron, and while the machine is powered on, so you can see the results, run the tip of the iron slowly backwards and forwards along the ribbon cable where it attaches to the circuit board. You should see pixels reappear as the connections get re-made.
In my case, I could get most of the display back using this method, but parts were still missing no matter how many times I applied heat.
I ordered the following:
Arducam 1602 16×2 LCD Display Module Based on HD44780 Controller Character White on Blue with Backlight for Arduino
for $5.99 on Amazon. I then removed the ribbon cable from the old LCD display by de-soldering it, opened up the individual wires in the cable, and soldered it to the pinouts on the Arducam unit, referring to the Arducam datasheet and the MC-505_groovebox_SM .
I added a 680 Ohm resistor between the VDD (+V) pin and the A pin, and a connection between the K pin and VSS (ground) which adjusted the brightness of the backlight on the LCD to a good level. The display worked:
Now all that remained was to position the new display in the aperture, drill a couple of new holes at the left for mounting purposes, and fix it into position.
Finally, I reattached the plastic cover using some double sided tape. The LCD unit stands a little higher than the original, so the cover sits a bit proud of the unit – not a big deal. It certainly looks a lot better than the original, and is fully readable!