ZX Spectrum +2a (black case) modifications
As you maybe know, +2a has completely new mainboard, but many 128k/+2 problems still remained there, so here are my ugly hacks to solve few of them. You can do the same on +3, concerned circuits are the same as on +2a.
Distorted picture on RF out, video out missing on RGB connector
It's incomprehensible for me, why Amstrad left there such poor video circuit design, when mainboard was completely redesigned. Poor picture quality has same source as on +2. It's C24 which has the same role as C31 in +2 - to mix sound with picture, but it brings even more noise to picture compare to +2, so I removed it.
Composite video out is completely missing on +2a. I don't understand why Amstrad removed whole video output circuit and placed +12V on same pin of RGB connector, where is video out on +2. Simply another "great" idea. You can use original composite video output circuit from +2. It's nothing wrong with it, but your TV should have 75 Ohm termination on video input (should be on the all modern TVs).
ACB stereo output (with beep mixed in)
Also in +2a is mono audio out only by default, even worse compare to +2, so I built stereo audio output, like on +2. ACB mixer is here, beep is mixed in by same way as on +2, but use two 2k2 resistors, instead of 1k, because beep is a little bit louder on +2a.
Old ROM version
Few of the +2a's came with very buggy 4.0 ROM set, so I'm recommending upgrade to version 4.1, which still has many bugs, but it's more reliable. If you don't know which one is in your computer, check last byte in PLUS3DOS bank. If it's 5Ch, your are lucky one, if it's 66h you're out of luck, so buy two 27C256 EPROMs, burn the 4.1 set and replace old one.
+2a ugly hacks