I just noticed you're looking for a place in Chiang Mai. Sorry, the shop I mentioned above, A2J, is in Bangkok.
There’s a good service called A2J in the Palladium Shopping Center at the corner of Ratchadamri and Rajprarop Roads, Pratunam area. The phone number of owner Jack Sakol Sae-Tia is 086-600-3561. His email is email@example.com He has successfully recovered music for me from old cassette tapes, so I would think he could recover material from reel-to-reel tapes. You can tell him that Ajarn Bob recommended his service, if you’d like.