I have two of these, one permanently in use, the second I used briefly as a headphone amp but didn't really use it so it's been in a box for years. Recently, I started a refurb of a Leak Stereo 20 and thought a B4 would make an ideal preamp for that. Ok, so I found it, dusted it off and switched it on. It worked perfectly. I changed the input caps for 0.1uF Ruskie PIOs and shorted the output caps. I must have done something bad because now LED 101 and 102 are not lit. Anyone got any ideas? I'm sure I saw something years ago about troubleshooting this preamp but I can't find anything with google, so apols if it's blindingly obvious.