Mesa Boogie Lone Star Special

[bsco]

Hi there. Got this Lone Star Special amp and there is no sound on the 5 watt mode...the 15 and 30 watt modes work fine...The owner had purchased and replaced the rectifier tube but still no sound in 5 watt mode. I opened it up, can't see anything obivious...all tubes test ok....so far..I don't have a schematic and so far I have been unable to find one...Any suggestions??
Cheers,
Bernie