THORENS PC 650 - Control Systems; Logic Control Board VM 17

14. Calibration of the Dolby Playback Filter
15.
The Dolby playback filter generally requires recalibration only after repairs on the boards VM 6/A or
VM 10 have been carried out or the erase head has been replaced.
a) Insert an unrecorded cassette, set the BIAS/EO selector to the appropriate position, turn the Dolby
switch to ON, and press the REC and PLAY buttons simultaneously.
b) Feed in a 2 KHz sinewave signal; set the leve! for an indication of about ’’-10 dB” on the signal level
meters.
c) Adjust L 604 (L) and L 603 (R) on the Dolby playback board VM 6/W for the minimum voltage at
MP 3 (L) and MP 4 (R).
d) The difference in voltage level between Dolby ON and OFF must be less than 1 dB, measured at MP 3
and MP 4.
Calibration of the Dolby Record Filter
The Dolby record filter generally requires recalibration only after repairs on the board VM 6/W have
been carried out.
a) Insert an unrecorded cassette and press the REC button.
b) Feed in a 333 Hz sinewave signal; set the level for a visible indication (about ’-20 dB’’) on the signal
level meters.
c) Switch the signal generator frequency to 19 KHz, retaining the same signal level.
d) Adjust L 604 (L) and L 603 (R) on the Dolby record board VM 6/A for the minimum voltage (de-
termined with an oscilloscope) at MP 1 (L) and MP 2 (R).
TRANSPORT FUNCTIONS
16.
The control commands, which determine the tape transport functions, are produced on the Logic
Control Board VM 17 and delivered to the Power Control Board VM 14 for execution.
Logic Control! Board VM 17
When one of the function selector buttons is pressed, the corresponding logic control input is set to
logical 0 (0 V)..The reception of commands can be verified with DC voltage measurements on the input
connections (connector pins):
Connection Function Button Pressed Not Pressed
S$ 1701 STOP OV app. +12 V (1)
S 1702 RECORD OV app. +12 V,0 V w/PLAY
S 1704 PAUSE OV app. +12 V
S$ 1705 PLAY OV app. +12 V
S 1706 F. Rew. Ov app. +12 V
S 1707 F. Fwd. OV app. +12 V
(1}: If S 1701 remains at O V, the clamp connector on the orange wire of the cassette
compartment contact has not been removed from the pin on VM 1.
A further input is connection S 1714, which receives voltage pulses from VM 14 whenever the reel
motor is running. The DC voltage value on S 1714 lies between +1 V and +3 V when the tape is in
motion.
An input command 0 causes the logic circuit to produce a control command of logical 71 at the appro-
priate output connection.
Connection Function Function ON Function OFF
S 1716 RECORD app. +12 V app. OV
$1717 PLAY app. +12 V app. OV
S$ 1719 F. Fwd. app. +12 V app. 0 V
S 1720 F. Rwd. app. +12 V app. OV
When
the
function
STOP
has
been
initiated,
all
commands
of
logical
1
(+12
V)
are
absent.
If the voltage pulses at S 1714 disappear, a control command 0 is produced at connection S 1718 to
cause the holding magnet to be released by VM 14 during operation in the PLAY mode.