I assembled 2 devices on the received boards. Due to my lack of experience in mixing digital and analogue signals on the same board and the usage of Eagle CAD's autorouter, the sound output suffered from digital noise. During debugging with our group member Pavel (Ptero) we managed to reduce the interference to the minimum, so it's not audible during audio playback. However the existing boards need minor modifications:
1. The output of 78L05 has to be disconnected from ROM's power supply pin
2. The ROM has to be connected to external +5v power
3. A ceramic capacitor in 0805 form factor has to be soldered between pins 1 and 2 of the 4558 amplifier
The ceramic capacitor's value is from 470pF to 1nF. It fits perferctly between the pins. If a socket is used for the ROM, then a wire needs to be soldered to a via (marked with red circle on the pic) and the wire must be secured with a hot glue to avoid damaging the board. If no socket it used, a wire can be soldered directly to ROM chip.
At this picture you can see the installed board and 7 wires that are needed to connect it to the board. The computer is Yamaha YIS503III converted to MSX2+ (now it's true MSX2+ with FMPAC).
At the below pictures you can see the necessary modifications to the existing boards.
I will be contacting people who pre-ordered the boards soon. Due to the need to modify the board to improve audio output, those who preordered the boards can cancel their orders. The available boards will be sold on "first come - first served" basis. Please contact me if you are interested.
The new version of the board has been created. All previously found issues were taken into account when designing this version. Now digital and analogue power and ground are separated, blocking capacitors were added, audio circuits are moved as far as possible from digital ones, additional capacitor was added on the amplifier's output.
BTW, the C5 polarity issue is now fixed.
The new boards have arrived. I assembled 2 units and they work perfectly! The new board is on the right at the picture:
I am selling the 2 assembled boards of the older design (fully assembled or without z80, 4558 and YM2413). If someone also needs the newly designed boards, please contact me. The e-mail is in my msx.org profile.
The new board (verion 1.3) is ready. It has a bit better layout, more blocking capacitors, extra noise suppressing capacitor, different (more easy to get) ROM chip, selectable by jumper (internal/external) ROM image. And of course the error that was on the previous board version has been corrected.
I am going to order at least 10 boards. If someone is interested to buy the new board, please let me know by mid-March.
I have 1 board of the last order ^^
It's difficult to solve this error in this board? Thanks Alexey 