Needed: Help Diagnosing Weak R Channel on HK Citation 14 Tuner

I have a Harman Kardon Citation Fourteen Tuner... the R channel only
puts out a very weak audio.. can hear music, but only high freq and
very faint.. If anyone can give me some ideas on how and where to try
and diagnose what's causing weak R channel, would greatly appreciate..
L channel is fine and strong.

Also, pulled out the 2 blown lamps behind meters.. part # CM 1813 but
no values... anyone know values of replacement bayonet type bulbs?

Thanks for any help.. Greg in Chapel Hill, NC