Marantz CDR631

I have had this since new and it is still in pristine condition. It will play pre-recorded cd's but when trying to record it will only read an old Maxell CD-74R disc. It used to read Verbatim Music CD-R without issues which are easy to obtain. I get OPC error or Insert Disc after failing to read. I have cleaned the lens.
Looking through the site there was info about adjusting laser current using trimpots. I cannot find any adjustment trimpots for the laser in this CD recorder. I have looked closely under a microscope and see no way of adjusting current.
Anybody have any idea where the trimpots might be on this unit?
Thanks for any info.