238S Dolby S Repair

Dolby S daughtercards in Tascam 238S suffer from the same problem as the capstan - faulty electrolytic capacitors. Unlike the capstan board, the daughtercards are double-sided with a number of vias, in addition to much thinner traces. These vias and traces fail in an unpredictable pattern and requre a complete continuity test of each trace, especially around the input components (Q1-Q4, D1-D4, U2) and the Dolby chip U1. As a result, a reliable repair can be tricky.

The alternative that may work better in some cases is replacing all daughtercards with a rebuilt version. I performed reverse-engineering of the R/P board and the Dolby S daughtercard and created KiCad schematics available at https://github.com/quietcat/tascam238s. Under dolby-hic there is a PCB layout and Gerber files for the Dolby S daughtercard.

If you have a Tascam 238S with faulty Dolby S, I think the best course of action is to order a set of replacement PCBs from a low cost fab such as JLCPCB or OSH Park and transfer all components from the original boards.

Replacement electrolytic capacitors:
https://www.mouser.com/ProductDetail/667-EEE-1CA220SR - C5
https://www.mouser.com/ProductDetail/667-EEE-1CA100SR - C12
https://www.mouser.com/ProductDetail/710-865090640004 - C13
https://www.mouser.com/ProductDetail/710-865090640002 - C15, C23
https://www.mouser.com/ProductDetail/710-865090640001 - C41, C44, C48