OK:
Got the Black PC Engine Duo to limp me to the end of Xanadu; now the real fun begins.
I did the sound cap replacement, the 4 10mf caps, and the 1 47. I used the hi-temp caps just in case becase of the known problem with the heat synchs. It worked fine for about 2 minutes; full sound, redbook and huey music.
All of a sudden there is a heck of a lot of scratching during gameplay, crackling over the voices and occasionally cutting out. Re-booting to the main screen, the deafening scratching continued. I pulled the system apart, and cleaned it with 60/40 alcohol and a toothbrush. I -reset the laser to the origin position to see if that was the issue. Upon re-assembly, the static was really bad.
Here's the problem: there aren't any more leaky caps on the mainboard; ---
EDIT: There was a lot more wrong. I just finished a complete cap replacement, and the problems went away. At least 10 caps were leaking at the original time of this post. 4 22's, the 3.3 under the HOP-M3 (which needs replaced), 2 100's, 2 4.7's and 2 47's. (all microfarads). A good scrubbing with 70% alcohol and a toothbrush took care of the excess electrolytic fluid. Photos posted on the PCE Duo complete cap replacement thread;