Need help diagnosing a preamp problem

I have a Shindo  Monbrison preamp with a MM/MC phono section.  It is about 12 years old, maybe more. The phono section contains a 12AU7 and 12AT7 in both right and left channels.  No other tubes in the phono section.

Two days ago  the right channel stopped producing output when playing a record.  Other source inputs are working fine.

I replaced all 4 tubes with known working tubes of the same type and vintage.

Problem remains.

I have a standalone phono preamp available if that would help.