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You need a frequency meter to be connected on blue and brown wires coming from tacho head on the capstan speed board. There are two versions of capstan speed board, maybe you have the old one with discriminator coil and one trimmer. The fast speed is adjustable with the coil to read 1600 Hz at the tacho head with a tape in place and during play. The lower speed must be 800 Hz and is set with the trimmer. Check also the voltage on the tacho at the lowest speed, must be between 35 mV and 50 mV (you can re-set the tacho distance between it and the capstan motor, if it is necessary). If you have the new capstan speed board with NE555 there is only one trimmer and you have to set 1600 Hz at highest speed (the lower speed will be automatically 800 Hz, check for).
Please download here the A77 service manual ftp://ftp.studer.ch/public/products/...x_A77_Serv.pdf, at the pages 28, 43 and 44 and you will find all the informations you need!
P.S. your pinch roller must be in perfect conditions and also his pressure force, measure 1,2 Kg for sandblasted shafts and 1,5 for polished shafts, see at page 16 of SM!
The speed problem on the A77 is sometimes due to aging of the pull-up spring for the pinch roller arm assembly. This is adjustable.
Usually what happens is that the pinch roller doesn't grab the capstan tightly enough and thus the tape speed comes under the influence of the takeup reel drive torque.
Also, check motor phasing caps.
Closely examine the rectangular .47uF caps associated with the relay bank at the left, lower, rear section of the set. These leak air, experience dialectric breakdown, and then short!