Dual C 810 Service Manual page 5

Dual C 810 — electrical alignment
Necessary instruments and cassettes
2 Millivoltmeters
1 AF oscillator
1 Oscilloscope
1 Frequency counter
1 Test cassette 400 Hz —20 dB, 10 kHz —20 dB
1 Test cassette Fe703-reference medium section, CrO3-reference
medium section
1 Test cassette 400 Hz Dolby level 200 nWb/m
General notes
Before adjustment, the magnetic head and all ferrous components
in contact with the tape must be de-magnetized!
Readjustment must be carried out in the sequence stated.
Playback
Magnetic head adjustment
Remove the cover plate at the record/playback head, insert the
test cassette (section determined for gap adjustment), 10 kHz
~—20 dB.
Connect the millivoltmeters to outputs L and R (cynch connection
jacks). If only one instrument is available, then switch the outputs
in parallel.
Fe, >
Using the adjustment screw to the left side of the magnetic head,
adjust for maximum voltage. After adjustment, the adjustment
screw should be secured in position with locking lacquer.
Note: with parallel switched outputs, there will be secondary
peaks; adjust for the main peak.
The difference between channel 1 and channel 2 at 10 kHz can
be at max. 5 dB.
Playback level
Insert test cassette, 400 Hz Dolby level
Fe, Dolby NR, >
Adjust for 580 mV £0.25 GB in each case
For track 1 use VR 102 at cynch output L
For track 2 use VR 202 at cynch output R
Playback frequency response
Insert test cassette, 400 Hz —20 dB, 10 kHz —20 dB
Fe,» (Dolby: off)
Both level sections of the test cassette should have the identical
Output voltage at outputs L and R.
VU-meter
Fe, record, Dolby NR, »
AF oscillator at input, millivoltmeter (AC) at output, record level
regulators VR 101 and VR 201 fully clockwise.
Set the AF oscillator to 400 Hz and alter the input signal at the
cassette deck so that a level of 580 mV occurs at outputs L and
R.
Using potentiometers VR 103, the VU-meter left channel and
VR 203, the VU-meter right channel adjust for 0 dB record level.
Background noise
Screen amplifier, ensure that the magnetic head is free of hum.
Fe, pause, >
Maximum measured background noise at outputs L and R less
than 3 mV, measured with a 20 Hz filter (e.g. Sennheiser FO 55,
curve 2.20 Hz —3 dB) and rms rectification of meter.
HF generator
Cr, record, >
Connect frequency counter between point Z and earth and adjust
the oscillator coil (L_ 103) core to give an indicated value of 105 kHz.
HF wave traps
Connect the millivoitmeter (AC) or oscilloscope to R 122 (left
channel) and R 222 (right channel) and, using wave trap L 102
or L 202, adjust for minimum HF.
HF-magnetic bias
Insert test cassette, Cr-reference medium section.
Cr,record, >
For basic adjustment, a millivoitmeter (AC) may be inserted
between measuring point X (right channel) or Y (left channel)
and earth. For basic adjustment, a value of 55 mV is set using
VR 105 or VR 106.
More precise adjustment should be carried out in the following
manner:
Record 400 Hz and 10 kHz with —20 dB (VU-meter). Compare
the levels On reproduction. If there is no uniform level, correct
HF bias.
For alignment of track 1 (left channel) use VR 106.
For alignment of track 2 (right channel) use VR 105.
Less HF bias = treble emphasis
More HF bias = treble de-emphasis
Insert test cassette, Fe-reference medium section
Fe, record, »
Record 400 Hz and 10 kHz with —20 dB (VU-meter). Compare
the levels on reproduction. If there is no uniform level, correct
HF bias using VR 108.
Insert test cassette, FeCr-reference medium section
FeCr, record, >
Record 400 Hz and 10 kHz with —20 dB (VU-meter). If there is
no uniform level, correct HF bias using VR 107.
Record level
Insert test cassette, CrO9-reference medium section
Cr, record, Dolby NR, MPX, >
Line/DIN level contro! (P 101 and P 201) open. Record 400 Hz
with 0 dB (VU-meter) and then check playback level using the
VU-meters.
Using VR 104 (VU-meter left) and VR 204 (VU-meter right)
alter the recording current so that the playback level is equal to
the record level.
Overall frequency response
The overall frequency response for all types of tape must tie
within the specified tolerances in accordance with DIN 45 500,
sheet 4. Recording should take place at aleve! of — 26 dB as in-
dicated on the VU-meters.