No bias in Cary-SLI-80

Please help - my recent tube-rolling attempt was frustrating. I replaced the RFT EL-34 output tubes by GEC KT-88 and wanted to make a bias adjustment, but the left channel showed no current. When I insert the multimeter plug into the left bias adjustment socket, the battery powered meter shows "-", while the right channel is OK. I replaced the tubes from right to left - no effect. The fuses are OK (last week I experienced the same problem, but it was a blown fuse at that time). I thought perhaps an output tube was dead and replaced them all with no effect.
There is no authorized maintenance shop here and it will take some time to find a technician.
I know it's difficult to identify the problem this way, but I would appreciate any inputs and advice of Cary users who experienced the same problem.

Last month I installed the Jensen coupling caps (with the help of a technician) in the SLI-80. Is there any chance it could cause this problem?

Thanks in advance

Alexander