John,
Look for pots R2918 and R2931 on the A5 board. These are the centering and gain. Usually the centering pot is centered and the gain is adjusted. Resistors R710 and R910 on the A1 board could be too high. The latter is for the horizontal section. These I replaced with a pair of 20meg 1% in series in my B version. The original 39 meg 1/4W carbons were almost 60 meg in mine.
Mark
