Thanks you very much for this "hard to find" service manual.
Will help a lot in repairing this tuner.
Text excerpt from page 11 (click to view)
BM-23MK2
Tape Speed 2.4cm/s Adjustment Switch position TAPE SPEED Switch : 2.4 cm Procedure:
test WS-48A (3 kHz, 0 dB) 18 � set J102 frequency counter
1. Play back WS-48A (tape center portion) in FWD mode. Adjust the RV602 so that the frequency counter reads 1,490± 15 Hz. 2. Play back WS-48A (tape center portion) in REV mode. Confirm that the reading of frequency counter is within 2.5 % from the reading in step 1. Adjustment Parts Location Diagram :